Sistering Floor Joists

Sistering Floor Joists

Homeowners can notice several symptoms, such as a gap between the baseboard and the floor, a slope notice by feeling high and low sectors when walking across the living quarters.

A floor joist is a piece of hardwood resembling a plank. When this board or plank is utilized in framing for structural and framing purposes, it is called a joist.

Older homes often have floor or ceiling beams that were damaged during the life of the structure. Some newer homes also have designed floors. Adding an extra joist beside some or all of the existing joists is one way to reinforce these structures. The second joist is often called a sister joist

Joist sistering is adding an extra identical floor joist, to a damaged or inadueqate floor joist, and tieing the two together with screws or nails. It is a very effective way of adding the additional strength needed to hold up a sagging floor.

“Sistering” the joists means to fasten the joists together through the face to double the thickness of the framing.

One of the most common ways to fix a sagging floor joist is to pair it with another piece of dimensional lumber of the same length and width. This additional piece of lumber is then mechanically fastened with nails or bolts to the sagging joist. Pairing the old joist with a “new” joist is called “sistering” a joist.

There are many methods for sistering a joist, and not all involve dimensional lumber – but using the same sized lumber is the most common method. Plywood, OSB, and even metal flitch plates can also be used as a “sister” to an existing wood joist.

Sistering floor joists is an extremely effective method for fixing a broken floor joist of any kind. Still, it can also be a fairly laborious process as you may have to remove a finished ceiling, wiring, plumbing, HVAC, or other utilities that pass through the problem joist.

A sagging floor is the biggest telltale sign that there are issues within the structure of your home. Doors that stick or drag on the floor, cracked walls or ceilings, beams that bow, and cracks in the foundation are also signs that can indicate structural damage within a house.

When you sister a joist, you secure a complementary joist to the original one. You can use dimensional lumber or 3/4-inch plywood. Either material must be as wide as the original joist. For example, select two-by-10 dimensional lumber if the current joists are two-by-10s, or cut sheets of plywood into strips that are the width of the current joist. Cover the joist span with either material. Add a bead of construction adhesive to the existing joist before nailing the sister into place. The adhesive adds strength to the bond between joists, and it prevents squeaking between the two joist members as the floor moves.

Sistering is a more extensive version of scabbing. Instead of attaching an extra piece to the side of one part of the joist, in this method you add another entire joist beside the joist that is having problems. The effect is the same as replacing the original joist, but you are spared the hassle of removing the original. Sistering can be used when the floor above the joist is sagging because the house was built with inadequate specs, or when the joist has deteriorated due to insect damage or rot.

When a joist is weight-bearing, meaning it holds up a wall or has extra weight attached to it, it’s sometimes necessary to sister multiple pieces of new material to the old one. This is usually done by “sandwiching” the old joist in the middle of the two new ones. Doing this gives additional support to the joist.

Joist Span Tables Useful For Sistering Floor Joists

Sistering Floor Joists Help Table Yellow Pine,
Douglas Fir
Hemlock, Spruce
 Western red cedar,
Eastern white pine
Dimensional Lumber SizingSpacing at 16″ on centerSpacing at 24″ on center Spacing at 16″ on center Spacing at 24″ on center Spacing at 16″ on center Spacing at 24″ on center
2×69′ 9″8′ 3″8′ 8″7′ 6″7′ 6″6′ 3″
2×812′ 8″10′ 8″11′ 0″10′ 2″10′ 5″8′ 6″
2×1016′ 0″13′ 0″14′ 6″12′ 4″12′ 9″10′ 5″
2×1218′ 6″15′ 0″17′ 6″14′ 4″14′ 9″13′ 0″

DIY Sistering Floor Joists

I want to provide to you step-by-step instructions on how to make structural repairs by sistering floor joists alongside weak joists. If you have sagging, cracked or twisted joists, which can happen in older houses, this project will provide the extra support the floor needs.

You can do a visual inspection of your floor joists and find that some are rotten in places, or have soft spots due to moisture damage. It is important to repair those joists as they’ve lost some ability to handle a proper load. In those instances, sistering the damaged joist is required.

Duration is a full working day and the cost goes from $133 to $277 unless the older joists are 2×6 or 2×4 where I have a maximum of $305.

Tools Required

  • Caulk gun
  • Circular saw
  • corded drill and bits
  • Hammer
  • Hearing protection
  • Safety glasses
  • Tape measure
  • Safety gloves

Materials Required

  • 16d nails
  • Construction adhesive
  • Hydraulic jack
  • Joists

1) Optional Structural Engineer Inspection

If you’re concerned about the structural integrity of sagging, cracked or twisted joists in your house, first call in a structural engineer to inspect the floor and recommend strengthening measures that’ll take out the sags and bring the framing up to code.

The inspection ($300 to $500) will tell if you have structural problems, such as undersized joists, that will require professional repairs. However, you can usually remedy sagging problems yourself by “sistering” new joists alongside the existing ones (this also works for stiffening bouncy floors).


2) Disconnection Of The Utilities

First remove electrical cables, pipes and other obstructions. If this is difficult, consult your local building inspector about notching the new joists. Notches will compromise the strength of the new joists, but some notches are acceptable, depending on their location. As a general rule, no notching in the middle third of a joist, no nothing in the bottom chords and upper chords.

All wiring, plumbing, HVAC, and other impediments will have to be removed. This usually means removing wires from the breaker box, detaching pipe from valves, and removing ductwork.

For many, removing so many pipes, wires, and ducts leads people to partially sister a joist. This is an option, but know that for optimal support of your subfloor, running a sistered joist the entire length of the original joist is recommended.

3) Clear And Clean The Workplace

Often electrical, plumbing, and heating utilities easily hinder the positioning of the joists. Some of these obstacles may need to be moved or removed temporarily.

I had a few obstructions in my way; there were electrical wires, thermostat cables, a refrigerator water line, electrical conduit and heating pipes in the area. I also found old nails and pipe brackets, left in place from years past—I hate that.

The first step was to temporarily remove or relocate the electrical cables and pipes. This necessitated shutting off the electricity and disconnecting a smoke alarm and some thermostat wiring. I also had to pull wire staples and unscrew conduit and heating pipe supports. These items would all need to be replaced after sistering the joist.

But clearing the workspace for sistering floor joists can be difficult when it is not an unfinished basement flooring.

If the home has an unfinished basement, that’s not a problem. But if it is post and beam construction, there may not be enough space under the floor to work. In that case, the area under the home may need to be excavated; an excruciating project that must be done by hand, or the flooring may need to be pulled up to gain access.

3) Measurement And Cutting Of The Sister Joist

Determine how you will slide the joist into place.You can sometimes use joists slightly shorter than the total range by attaching them securely to the existing joist. More on that below, after the step by step process explanation.

When installing a new joist on a foundation wall you may need to notch the bottom edge so it can fit. Each situation is different. Follow IRC and your local building code guidelines for notching framing material before doing this repair.

Apply abundant construction adhesive on the older joist until it is covered completely in the lateral area wherein the new joist will be sistered. When the sagging joists are level, apply a generous bead of construction adhesive to the existing joist. You must be wearing your safety glasses because you are working with wood and to avoid the gravitational effects of the utilization of the adhesive over your head.

Set new joists usually of the same height as existing ones (with exceptions if your old joists are 2×4 (not allowed any more) or 2×6 as I explain here) alongside the older joist.

If you are running the sistered joist the same length as the old joist, then your job is easy – keep it the exact dimension as the old joist. If you are cutting a partial joist, use the ⅔ rule if possible. That means if your joist is 12’, then a sistered joist of 8’ minimum is adequate.

Ensure your new joist has at least 3’ on either end of the point of deflection – the point where the sag is greatest – to properly support the old joist.

Ideally, the sister joint should be both glued with construction adhesive and bolted through with at least 3/8” diameter bolts, forming a “W” pattern, with the bolts spaced about 8” apart. This should extend through the entire three feet of wood that is on either side of the damaged section. Screws can be used in place of bolts, but if they are, then use three times as many screws as bolts.

4) Locate The Pattern Far From The End Of the Joist

Before predrilling, what I consider necessary here when working with hardwood, we have to define a pattern. We can utilize lag screws for sistering floor joists.

You can utilize screws or nails. Screws are better for holding power and nails are instead, better for shear strength.

Framing specialists and woodworking doctrine, generally prefers nails instead of screws when sistering floor joists, in virtue of its superior shear strength.

The pattern, using nails, will be three 16d common nails driven every sixteen inches, but this is just a recommendation.

Of course, the nail pattern must be towards the center of the wood what means that you should not screw closer to the end of the wood. This is very important.

The closer to the end of the wood you are, the less binding forces you have in the fibers that will hold the would together, in the side towards the end, and the pressure of the screw going in can create a shearing force exceeding the biding force, causing the split.

Farther down the board the binding forces are greater along the wood as there is more area and length on each side of the hole. This is also why its easier to split thin wood in the center of the board with a small screw that won’t split thicker wood.

This is a general principle because the definition above is also influenced by the dryness of the wood, the run of the grain, type of wood, and grade.

5) Predrill Lag Screws According To A Defined Pattern

Predrill and drive 3/8-in. x 3-in. lag screws: Our next step will be driving 16d common nails in old, hardwood is difficult. So in this step we will predrill and drive 3/8-in. x 3-in. lag screws as I consider that we will have some issues nailing in the hardwood.

Sister the new joist to the old joist and nail both joists in a nail pattern of three 16d common nails driven every sixteen inches. Use also safety gloves.

It is very convenient to predrill. With the predrilling suggestion, I want to avoid a stress fracture in the joists. The larger diameter fastener, the larger amount of wood that gets displaced. Thus, the wood has no where to go but out away from the point where the pressure is being applied versus instead, removing the wood with a bit by pre drilling, lessening the chance of a stress fracture.

6) Lodge The Sister Joist Alongside The Original Joist

Shim either end to ensure the sistered joist sits snugly against the old joist. You may have difficulty getting a full-sized sister joist to fit on either bearing end against the old joist. It is likely your old joist has shrunk slightly or cupped, resulting in a narrower space between the beam or bearing wall and the subfloor above.

If you cannot fit the sistered joist against the old due to tight fit, you can slightly – and carefully – jack up the old joist at either bearing end to slide the new joist into place. Be very careful, as you can damage your finished walls above if you jack the old joist too much.

Use a jack post or a floor jack with a piece of lumber placed directly below the old joist next to the point of bearing on the beam or wall it rests on. This should allow you to fit your sister joist up against the old one.

If installing a partial joist, you’ll also need to jack up the old joist to level. One tactic is to bold the sister joist to the new joist. Install a bolt on one end and the middle, then jack up the other end of the sistered joist to fit the old joist, install the bolt, and use a level to check everything is snug and proper.

Tack a beam under the sagging joists. Nailing two 2x4s together will work to span about three joists, unless the sag is under a weight-bearing wall. Set a hydraulic jack and post under the beam, and jack up the joists about 1/8 in. a day until they’re level. Jacking them up too fast may cause cracks in the walls and floors overhead.

7) Joining And Sistering Floor Joists

Screws are better for holding power and nails are better for shear strength. In this article I went briefly through both options.

When sistering, 16d spiral nails are best. A 16d nail is 3.5” long and will adequately fasten two 1.5” thick joists to one another. The ends will protrude through the other side – nail them off, bending them back into the wood.

When nailing a sistered joist, alternate top, middle, bottom every 6”. This will look like a zig-zag of nails across the sistered joist. On either end of a partial sistered joist, a couple of inches from the end of each side of the sistered joist, put 3 nails – top, middle, and bottom.

If you use bolts in place of nails, use 2 3” lag bolts – one on the top and one on the bottom – every 24”. Alternatively, you can use 4” carriage bolts at the same spacing. Bolts should be spaced 2” from the edge of the joist to prevent cracking or splitting.

Finally, when nailing or bolting – or both – be sure to also use a very liberal amount of construction adhesive. Fasteners are the most critical component of this job, but the glue will help keep the spaces between the fasteners adhered and improve the overall connection of the sistered joists.

8) Reconnection Of Utilities

After fastening and installing, it is time to reinstall your utilities. Before doing so, check for level on your new sistered joist, as well as any creaking when you walk on the floor from above. If there are any issues, you want to solve them before you re-install all your wiring and plumbing.

Building Code For Sistering Floor Joists

There is not a building code for sistering joists as it is a reinforcement of an existing structure while codification refers to the primary design and construction standards.

Nevertheless, industry standards apply, establishing that the nailing pattern must suffice to transfer the load from one joist member to the new joist ensuring structural stability and that the joist blocking or bracing is maintained at a maximum of 4ft increments.

The non-written principle in building code for sistering joists, mostly an industry standard I guess because there is no codification, is that the new joists are required to fulfill the support requirements, so that the nails must be sufficient to transfer the total load from one member to the other, from one joist to the other joist.

Because sistering joists are obviously not used in new residential construction, there is no section on them in the residential building code for that purpose. Houses are designed to be constructed on single joists with predetermined spacing.

Sistering is only necessary if the house has been wrecked or if incorrect construction methods were employed, which the building code doesn’t concern itself with in this instance.

The code does require that all floor joists carry a minimum of three inches on concrete and at least 1.5 inches on wood. This also applies to sistered joists. Even if the original joist is incapable of bearing that load, the replacement joist must.

Additionally, you must have sistered joists if the joist is connected to a perpendicular header that is longer than 4′. This is often seen when joists are unable to run the whole length from bearing wall to beam owing to the presence of stairs. In these instances, a header is attached to both sides of the joists. Sistered joists must bear on both ends against a wall or beam.

I have a complete article to discuss the building code for sistering floor joists right here.

A building code (3) for sistering joists, or any building code in general, specifies minimum standards (2) in the design and construction of floor systems (1) and not their maintenance or reinforcement. For this reason, there is not a building code for sistering joists as this activity is considered as a reinforcement of a framing structure. Therefore, only industry standards of the structural engineering practice (10) apply in regards to sistering joists. Building codes, such as the Residential Building Code, that does not apply in Wisconsin (11) (12), do not refer to floor joists. Likewise, some states consider them directly incorporated by law and adopted by reference (4). The US remains as one of the few advanced countries without a uniform building code (5), but finally now all states utilize the International Building Code. This is different from the European Union (6), whose codification is being adopted also outside the EU (7), such as EFTA countries (8) and has overwhelming adoption in Asia Pacific (9).

Furthermore, joist blocking should preferably be kept at maximums of 4ft. Joist blocking refers to solid, lateral supports installed between floor joists to evenly distribute loads placed atop floor joists. Blocking uses lumber the same size as the floor joists and is fastened either in a staggered or straight line mid-span or every 8′ depending on the length of the joist.

Therefore, usually 1 – 1/2″ bearing wherein the nailing is therefore required to be sufficient to transfer the full load to the other joist. In general 16d nails will carry about 100#. Most joists carry 66#/ft so 16″ nail spacing works out.

A 1-1/2″ bearing may be acceptable in many cases, but for long spans it is required to perform the necessary calculations in order to determine how much PSI (pounds per square inch) you are introducing on that segment as these calculations will inform you about the ability to carry loads accordingly.

Building Code for Sistering Deck Joists

Despite some differences between floor joists and deck joists, there is no building code for sistering deck joists as this procedure is a strengthening of a present framing structure whereas building codification entails directives for design and construction standards.

Nevertheless, industry standards for sistering deck joists exists and they have to be followed and are subject to inspection as well.

Requirements From Homeowners For Sistering Floor Joists

Evaluate your floor or ceiling system. If you have weak or damaged joists or are adding significant weight like a hot tub, adding beams next to running joists is an excellent way to reinforce damaged areas or reinforce the system to support the additional load

Which are the goals we want to achieve when we engage on a sistering floor joists project? Let´s see.

The most common case I see is that homeowners will utilize an area for living or sleeping quarters so they will add furniture and the weight of the people itself. Therefore, it is this change of destination their previously underutilized rooms that obliges them to consider the reinforcement of the framing and this is the most common requirement in my own experience.

But there are many more reasons, please let´s see.

Sistering is used to fix wobbly floors, sagging floors, or rotten joists. In most homes, a wobbly floor is caused by an oversized joist span, joists spaced too far apart, or joists that are failing. A sistered joist in any of these instances will correct a wobbly floor. The reasons for sistering a joist include:

  • Reducing the wobbling and the bouncing felt when walking.
  • Reinforcement of the framing due to a change of destination of previously not utilized living quarters. The solution here is usually to reinforce older and weaker joists of dimensions 2×4 and 2×6 with stronger joists, sistering 2×8 or 2×10 to the original weaker joists even if they are in a good condition.
  • Leveling the flooring and the reduction of the sagging effect and achieve this via sistering floor joists.
  • Original floor joists that are damaged due to an improper nailing pattern, notching or, frequently, they are rot. Therefore, sistering floor joists enables to transfer the structural load to the new sister joist.

A common reason for sistering a joist is to fix a sagging floor. Over time, joists can sag due to being undersized or rotten. They may also be spaced too far apart for the span they support. In those cases, sistering one or several joists after jacking up the old joists to the desired height will fix the sag permanently.

And finally, if you can do a visual inspection of your floor joists and find that some are rotten in places, or have soft spots due to moisture damage. It is important to repair those joists as they’ve lost some ability to handle a proper load. In those instances, sistering the damaged joist is required.

Causes Of Structural Failure Of Floor Joists

I would like to visit now briefly which are the causes of structural failure that generate the requirement for sistering floor joists

In residential homes, most floor joists come from natural or engineered wood. These joists are sometimes doubled or even tripled to give extra load capacity depending on where in the house and what the building codes require. So why do they fail sometimes?

Loss Of Integral Strength

The first reason is the loss of integral strength due to crawl space moisture. Oftentimes, crawl spaces contain moisture from exposed damp ground, groundwater flooding, and outside humidity. This dampens and weakens the wood, leading to fungus and wood rot.

Water Damage

Another reason joists fail is due to water damage. If a house has a plumbing leak, it can cause wood to deteriorate more rapidly. And to compound it, if a joist already sags, water will naturally expedite the damage and movement.

The last common reason for floor joist failure is termites. Termite infestations in homes can cause significant damage to the structure of homes, leading to sagging floors. 

Sagging Floors

It is not an error to say that sagging floors are not the cause, but the consequence of the aforementioned causes, such as loss of structural strength, and water damage, moisture in the crawl space, and hydrostatic pressure in the basement.

The other most common reason for sistering a joist is to fix a sagging floor. Over time, joists can sag due to being undersized or rotten.

They may also be spaced too far apart for the span they support. In those cases, sistering one or several joists after jacking up the old joists to the desired height will fix the sag permanently.

What I refer here, is that the floor is wobbly or sagging because the joists are very separated. There are span tables for each type of wood, grade, and dimensional lumber code.

Here is where the requirement of sistering floor joists appear in order to transfer the loads from the originary floor joists, to the new joists that are sistered to the older ones with a defined nail pattern.

Sistering floor joists is the addition of an equivalent floor joist to an older and inadequate floor joist that is sistered through a nail or screw pattern with the objective of transferring the structural loads from the originary floor joists to the sistered joists.

Common Mistakes When Sistering Floor Joists

There are a lot of mistakes that you can make when sistering floor joists, and I think I made them all already myself. So I come frequently to update this least with the typical failures that we can provoke.

Most of the issues are related to an inadequate sistering that does not enable the structural load to be transferred properly to the new strong floor joist.

Original Requirement Remains Unattended

Most of the time, sistering floor joists is the solution decided to address a previous requirement from the homeowner: Damage caused by termites in the original joists, water damage originated by leaks in the pipes, rotting caused by excess of moisture, or rotting caused by condensation and excess of humidity.

So the homeowner performs the sistering floor joists process thoroughly, but the original cause was never attended: There are still termites, there is still leaking in some pipes, the PVC piping is still broken, there is still water damage, there is still sewage contamination, there is still a very high moisture percentage because the vapor barrier is still punctured because it had only a 6 mil thickness, and we can continue.

So, I was that person, and if this is you now, solve that problem first: solve the water damage issue, then dry and clean the foundation area affected, then encapsulate your crawl space, then put the dehumidifier.

Only at the end of the aforementioned process, you should attend the problem of the rotten joists affected by that excess of moisture, and do it through the association of planks in that procedure that the framing industry knows as sistering floor joists.

  • Issues when sistering 2×4 and 2×6 floor joists: I explain in this article the issues when sistering 2×6 floor joists and why you should sister them with a 2×8 despite the dimensional differences.
  • Errors During Notching Joists: Joists that have been incorrectly notched cannot have their maximum depth exceeded by more than one-quarter of the joist’s length. The maximum amount of notching allowed in the outer third of the joists is one-sixth of the joist depth, and there is no notching allowed in the center third of a joist. Incorrect notching along any length of the board may have a negative impact on the structural integrity of a joist. The sister joist is a part of the floor joist construction and needs to follow the same notching rule. What would be the point of adding a sister and then weakening it by notching it wrong?
  • Incorrect Joist Leveling: Floor leveling is a time-consuming procedure that may take many days. The reason for this is that raising one joist may cause the remainder of the home to settle in ways that were not anticipated. This is not related with the lifting or raising of other floors, as we are talking about framing here.
  • The New Board Is Not Leveled With The Sistered One: When sistering a new board to an existing joist, it is common for it to take several persons to hold it up until it is level. The objective of the work will be defeated if the new board is not leveled with the old joist.
  • The Boards Are Not Correctly Fastened: Not understanding how to properly secure sistering joists together may result in structural issues in the future.
  • Overlap Of Boards Is Not Sufficient And Does Not Transfer Loads:  When determining how much the overlapping of the boards must be, it is essential to consider the weight loads on the boards. Failure to properly overlapping both boards might ensue in a subsequent sagging of the structure.
  • Lack Of Weight Support Due To An Insufficient Analysis Of Loads: This occurs often in the case of load-bearing walls and other special situations when additional support is required to hold up a joist.
  • The joist with the notch around the armored cable is useless. It has been compromised and is just adding weight to its sister, not helping to strengthen it. At 15 feet, the 2×6 joists are way over spanned as well and I explain the issues of sistering 2×6 floor joists right here.
  • The joist bays must be free of any obstructions, such as electrical cables, ductwork, and plumbing, while you are doing the work.
  • The nails must be sufficient to transfer the total load from one member to the other. Usually 1 – 1/2″ bearing wherein the nailing is therefore required to be sufficient to transfer the full load to the other joist. In general 16d nails will carry about 100#. Most joists carry 66#/ft so 16″ nail spacing works out.
  • Building permits can be required accorsing to the entity, and with disregards of the fact that there is no actual building code for sistering joists being enforced.

Incorrectly Notching Floor Joists

Notches can compromise the strength of floor joists, although some notches are acceptable, depending on their location as dictated by the International Residential Code [IRC]. Here’s a good rule of thumb:

  1. Notches should be no deeper than 1/6 the depth of the joist.
  2. Notches at the end of the joist should be no deeper than 1/4 the depth.
  3. Limit the length of notches to 1/3 of the joist’s depth.

An aggressive nothing pattern may alter negatively the structural integrity of the flooring upward.

So in simple words, I can explain it like this.

Maximum notching at the end of the joist cannot exceed 1/4 of the joist depth. Maximum notching in the outer third of the joists is 1/6 of the joist depth, and no notching in the middle third of the joist. Incorrect notching at any length of the board can seriously affect the integrity of a joist.

A severely, cracked or sagging floor joist can get worse over time, causing the floor above it to slope or become unleveled.

Failing to Level Joists

Generally, when a beam becomes a sister, the new wood does not cover the old beam’s entire length. Because of this, the end of the new wood does not get support. It is essential to set this level to prevent the floor from falling and prevent the sleepers from chipping later.

Leveling a floor is a process that sometimes takes days. This is because jacking up a joist may cause the rest of the house to settle in ways that weren’t expected

Sistered Board Is Not Correctly Leveled

Sistering a new board to an old joist often takes multiple people to hold up in order to get level. Not leveling the new board to the old joist will defeat the purpose of the job.

Lift The Damaged Area So The New Board Does Not Require Carving

This paragraph is related with other problems here too. It is not very common but can be a mistake as well.

Generally, you can make newer floor beams using 2x8s or new I-beams. Older houses, however, can have 2x6s. Although it is a good idea to replace the rotten material with 2x8s, a fit may be necessary between the sub-floor and the ledge plate. It is straightforward to cut the notches and remove debris in two quick cuts, but cutting the notches in this way weakens the structural integrity of the replacement wood. In situations where joining damaged floor joists with the wood of similar dimensions, it can be easy to carve and snap the sister board into place. However, this does not correct the slope the floor has taken since the old joists were damaged. Take some extra time and lift the damaged area so that the new wood does not need any carving.

Failure to Properly Secure Boards Together

The amount of weight that each floor beam handles is quite surprising. When putting these pieces together, it is easy to put just a few nails on the boards and move on. However, displacement and settlement can work these open areas and separate the plates. Ideally, use nails to fix the part temporarily and then lag screws or, more preferably, use transport screws to lock them together.

Rotting rarely occurs only in the center of the beam, but occasionally the most significant damage occurs there, and it may seem more straightforward to cover the rot. But, breaking into a winery, where the new wood does not get supported by the central beam or the accounting panel, often worsens the problem. Make sure to start a sister joist where it is supported.

Insufficient Over-lapping

There is no need for sister joists to overlap for more than 6 inches. Many overlaps are also wrong because if you place the load in the joist’s center, the end of the overlap may sag slightly.

Knowing weight loads is important when figuring out how much to over-lap boards. Failure to over-lap boards enough may cause future sagging to reoccur.

Overlapping the new wood with good wood from the existing joist with just a few centimeters can be pretty straightforward. By doing this, you save wood but do nothing to protect the pieces. Overlapping boards by two feet or more is better.

Not Using Enough Support

This happens a lot in the case of load-bearing walls and other unique circumstances when more support is needed to hold up a joist.

Not having the necessary build permits

I discuss here the building code for sistering joists: not much to see there, but there are many regulation for floor joists therein.

Not using the correct joist dimensions

If you have problems frequently with 2×6 joists, you would have to sister them with 2×8 and have spans of not more than 9 ft. But this depends also on the species and grade of the wood.

Sistering Floor Joists Not For Concrete Slabs

In this article, I only refer to sistering floor joists for crawlspace and basements. If you have a concrete slab foundation that you consider has an inadequate slope or grading or it is sagging, your solutions will be related to slabjacking or helical piers.

Therefore, all the solutions herein proposed related to sistering floor joists are not applicable for you in this article, so I refer you to my articles about slabjacking and helical piers.

sistering floor joists
“Flooring” here is the subfloor (normally plywood that is across the joists and you do not “see it”) and the superfloor or just “floor”. This floor is “what we see” and there you can have, tiles, carpets, hardwood, and so on.  

Partial Joist Sistering And Horizontal Shear: Length Of The Sister Joist

Whereas a full sistering joist will provide more structural strength according to most loading conditions, it is feasible to perform a partial joist sistering following the correct nailing pattern or bolt pattern that will occur when the selected pattern transfers completely the required horizontal shear developed by the load between the two joists.

The purpose of sistering a joist is to increase the moment of inertia of the joist at the location of maximum stress, which is usually, but not always with wood, in the middle.

Therefore, sistering a joist only in the middle is a good solution if the problem you are trying to solve is for example, an excess deflection at the middle of the joist, or a joist which is failing near the middle, but not failing on the ends.

The aforementioned issue is frequent, you can look at an overspanned joist and often the joist is fine near the walls where it is adequately supported, but not fine in the middle.

The key to making a partial sistering joist work is to ensure that the nailing or bolt pattern transfers a hundred percent of the required horizontal shear developed by the load between the two joists.

This horizontal shear should preferably be calculated by a structural engineer, whereas carpenters arrangements would be 10d or 16d nails 6 inches OC, staggered top and bottom.

A full sister provides more strength, but interesting not much more than sistering about half the joist, centered over the middle, for most loading conditions. So a partial sister is fine, it would be similar to scabbing, if you cannot execute the entire linear length of the affected joist.

Just make sure you use a good nailing pattern or bolt pattern as I have described briefly above.

sistering floor joists
via Shutterstock

Length Definition For The Sister Joist

All right then, sistered floor joists can be shorter than the original joist they are being sistered to. But how long it should be, how long it can be to accomplish its final goal that is to receive the structural load transferred by the original joist.

While no specific rule exists for how long a sistered joist should be, a commonly held rule is that a sistered joist ⅔ the length of the original is adequate – only if you cannot run the joist the entire length. A non-full sized sister joist will still fix sagging in a joist, which is commonly referred to as deflection.

If space is limited and you need to fix a sagging or damaged joist, you can partially sister a joist, ensuring you get at least 3’ from the damaged area on either end. The further away from the point of deflection you can get the sistered joist, the better. Sistering a joist with one end at the point of deflection will not result in any significant reduction in the sag.

When partially sistering a joist, remember that the way you fasten the joists together is of utmost importance as the fasteners will carry the load to the original joist. In particular, the fasteners at either end of the sistered joist, provided they are not bearing, will experience the most force and have more fasteners than the center of the sistered joist.

Length Required For The Sister Joist

Sistering floor joists do not need to go from bearing point to bearing point as it is not necessary for all damaged framing. It is not coded or mandatory at all that the sister joist has the exact same length as the older joist.

Sistered joists do not have to run the length of the original joist. While no specific rule exists for how long a sistered joist should be, a commonly held rule is that a sistered joist ⅔ the length of the original is adequate – only if you cannot run the joist the entire length. A non-full sized sister joist will still fix sagging in a joist, which is commonly referred to as deflection. Also it is called scabbing.

If space is limited and you need to fix a sagging or damaged joist, you can partially sister a joist, ensuring you get at least 3’ from the damaged area on either end. The further away from the point of deflection you can get the sistered joist, the better. Sistering a joist with one end at the point of deflection will not result in any significant reduction in the sag.

When partially sistering a joist, remember that the way you fasten the joists together is of utmost importance as the fasteners will carry the load to the original joist. In particular, the fasteners at either end of the sistered joist, provided they are not bearing, will experience the most force and have more fasteners than the center of the sistered joist.

So the important thing if the sister joist does not cover the complete linear footage of the original joist, is that the structural load of the old joist is transferred to the new sister joist adequately.

It is like in scabbing, an alternative to sistering floor joists.

After all, a scab, the process called scabbing, works always in the same way, a piece of lumber utilized to join to adjacent pieces of timber together to increase their overall structural strength.

Scabbing involves taking a piece of wood and permanently affixing it on the side of the joist over the problem area. Scabs can be attached on one or both sides of the joist, the latter option being preferable for greater strength and durability. Scabs can be attached with glue and screws to create a very strong bond. This method may be suitable when a joist has a small rotted area but has not yet deteriorated to the point where it must be replaced.

To repair floor joists, the damaged joist is sistered together with a new piece of wood. Usually, the new piece of lumber spans the length of the old one. At times when this isn’t possible, shorter pieces are used instead.

So it is not wrong to say that scabbing is like doing a partial sistering, in a way.

However, the most structurally sound approach, going from bearing point to bearing point, just as the original one did. And this applies whether we are discussing about a block wall, a pier wall, or a stem wall.

Sistering Floor Joists With Plywood

While the use of dimensional lumber to sister joists is common, it is not actually necessary. If you look at modern floor joists, they are rarely solid dimensional lumber anymore. That’s actually a very inefficient design, using more lumber than necessary. Today’s floor joists are usually either wood I-beams or open web floor trusses. These options not only are cheaper than solid lumber joists, but they provide less deflection, an important factor for ceramic tile floors, granite countertops and other modern building materials.

Scabbing ½” CDX construction plywood to both sides of the damaged floor joist can actually produce a stronger joist than using solid dimensional lumber. This should be attached with construction adhesive and either screw or bolts, just as if it were dimensional lumber. If bolts are going to be used, then a few screws to hold the plywood in place, while the bolt holes are drilled and the bolts are installed, can help considerably.

Another alternative, especially if both sides of the existing joist are not readily accessible, is to use a single layer of ¾” CDX plywood on the side that is more accessible. As with either the dimensional lumber sister or the dual ½” plywood sisters, both construction adhesive and 3/8” bolts should be used to connect the sister to the original joist.

Jacking And Sistering Joists

It is not necessary to jack up the first joist to get the benefit of sistering. The first joist is stressed because of the load on it, hence takes a deflected shape. This deflected shape is adopted because all stressed structural elements tend to bend due to the stress. If you jack the first joist back to level before you proceed to the sistering, so before you sister joists, there can be some structural issues to face.

First off, the first joist has probably taken a permanent deflection, since wood is an inelastic material which takes a permanent set after it has been exposed to load for a year or more. You can see this in any old house, the floors are out of level, because the joists have permanently deflected due to the load. Even if you remove the load, the joists will not spring back to their original shape.

If you, nevertheless, force them back to their original shape by jacking, you are effectively prestressing the joists in the opposite sense of their deflection, so you may end up in a situation where the top of the joist is no longer in compression, it could be in tension while the jack is in place.

Then when you nail on a sister, you are locking in the reverse stress in the original joist, and you can get a wide range of stress results when you release the jack and reload the joist pair.

If you, on the other hand, do not jack the joist back up but simply nail on the sister, the two joists will share the load. They will share the load approximately evenly as long as the two joists are fastened together sufficiently well that the horizontal shear which develops when the two joists are loaded gets shared.

The mechanism for horizontal shear transfer is the nails, bolts, or glue that holds the two together. Thus, horizontal shear depends on the adherence arrangement used (nails, bolts or glue)

If you forget to nail the two joists together, The highest joist will carry all the load, until it deflects far enough so the second lower joist begins to pick up load. This is not a compliant structural arrangement, since the first joist could conceivably crack before the second joist picks up load, then the second joist can crack some time later too since the first joist is no longer contributing much strength.

This type of failure occurs too in industrial and commercial projects where two structural elements were not properly connected, and become loaded, overloaded I would say, before the fastening is completed.

Rotting Risks For Sistered Joists: Using PT Lumber (Problem Mostly For Decks)

This situation applies mostly to decks. You can put insulation in the joists, such as the Fiberglass R21 batts. There are other options, but I have used that one and works fine.

The problem is that moisture gets in between the two joists and doesn’t dry out. Rot quickly sets in, and the side-by-side joists soon turn to mush. Every time I’ve repaired a deck with sistered joists, they have been rotted, while single joists have typically fared much better.

I hear home owners talking about “dry rot”, that they have “dry rot” in a joist and that is why they require sistering floor joists. I tell them that that cause is always moisture. The joists are looking dry now, but the joist became rotted when it was damp and humid.

Caulking the top of sistered joists may delay decay but probably will not ensure long-term protection. Inserting new joists halfway between the existing joists may work. The use of pressure-treated joists is a good solution, but good-quality, straight PT lumber stock is not always available. Decks should be framed with PT lumber, pressured treated lumber.

Pressure treated lumber is wood that has been infused with chemical preservatives to protect the wood from rot and insects. The wood is placed in a depressurized holding tank that removes the air and replaces it with a preservative.

If you must sister joists, I recommend using 1/2-inch spacers to separate them (1/2-inch nuts on staggered carriage bolts work well), creating an air space that allows drying between the joists.

Is PT lumber the only option for decks? No, let´s see other possibilities.

The treated southern yellow pine generally accepts chemical treatment far better than does the incised and treated hem fir and Douglas fir used in the West.

Nevertheless, tightly sistering untreated deck joists, or those whose cores are untreated, will lead to decay.

If the decking boards are being removed as part of the job, then the problem is easily avoided by capping the sistered joists with a waterproofing layer such as Grace Vycor Deck Protector, YorkShield 106, or strips of #30 tar paper (tar paper is not only used in roofing) to keep water out of the joint.

In fact, I often space joists with 1/2-inch to 3/4-inch strips of PT wood ripped from scrap. I space them vertically 12 inches to 16 inches apart and through-nail. The lumber contact issue isn’t just limited to sister joists, either; a built-up beam should also be capped to avoid rot problems when using suspect treated lumber, or if you want a bullet-proof job.

Alternatives To Sistering Floor Joists

Sistering floor joists is not the only possibility or technique that is viable to solve the issue of sagging floors, or unleveled floors. Let´s see briefly some alternatives we have.

Scabbing As An Alternative To Sistering Floor Joists

Scabbing applies when the problem is minor.

Scabbing involves taking a piece of wood and permanently affixing it on the side of the joist over the problem area. Scabs can be attached on one or both sides of the joist, the latter option being preferable for greater strength and durability.

Scabs can be attached with glue and screws to create a very strong bond. This method may be suitable when a joist has a small rotted area but has not yet deteriorated to the point where it must be replaced.

Sistering floor joists is very similar. In sistering, rather than adding an extra piece to the side of the joist that is having trouble, you add a whole new joist next to the one that is having problems.

If you have to reinforce the entire joist, we would be talking about sistering joists. However, you will notice that some contractors and some other colleagues in other websites define scabbing as a partial sistering and the definition is fine.

Jack Posts: Lifting Floor Joists using Jacks

Damaged joists usually mean sagging floors—right.  Well, to make a floor level again, it needs lifting.

In order to lift a sagging floor, install a screw jack and beam underneath to push the damaged sagging joist(s) up. A screw jack is essentially a metal support post that raises or lowers by the turning of a threaded steel pipe that’s inside of it (hydraulic jacks are often used for this purpose as well).

If there are significant sags in multiple joists spanning the floor, then professionals will use multiple jacks to do the heavy lifting. Holding a 4ft level up to the joists will show how much a floor could need to be jacked up. 

Lifting a house always include the risk of damage. Lifting floor joists can cause cosmetic damage to paint, drywall, flooring, counters, trim, etc. It’s important to lift carefully but some damage cannot be avoided if lift is necessary.

Jack posts are heavy-duty metal pipes with a threaded platform at the top that can be raised by turning it with a wrench. You can use a jack post to raise and stabilize the center of a sagging joist by placing the jack post underneath the joist’s center and rotating the upper platform so that it is forced up underneath the joist. Jack posts are designed to be left permanently in place, although a permanent post can also be put in their place so the jack posts can be used again.

Replacement Of The Affected Floor Joists

A joist can be replaced when its problems are severe. Replacement is preferable to sistering when the joist is in a location that doesn’t offer enough room for a sistered joist, or when leaving the original joist would not solve the problem, for example, if the joist is infested with carpenter ants.

The most difficult part of replacing a joist is often getting the ends of the new joist into the space where the original joist was. Be sure to fully support the surrounding floor with jack posts before removing the damaged joist.

In practice, you may not need to replace an entire affected joist in cases that are different than the ones described above. This is because with the process of sistering floor joists, you can simply make the old joists to transfer the structural loads to the new joists and the objective is accomplished.

Duration Of Alternatives To Sistering Floor Joists

Jacking up a joist and raising a sagging floor can be a relatively quick process. An expert repair technician will know the size board to install and how to perform the repair efficiently. 

If lifting the floors more than 1/2″ is your goal, it should should be done over time to avoid damage. Sometimes this means that a screw jack is only raised once or twice a month, moving up only 1/4″ at a time. If your sag is 1″ or 2″, then you can see how this process would take a little longer. Expect new cracks to form on walls and ceilings after they’ve been jacked up.

Engineered Wood For Sistering Floor Joists

Engineered wood joists are slowly replacing 2×10 dimensional lumber in framing projects and thus, in sistering floor joists initiatives. They are easy to utilize and relatively light, although more expensive than dimensional lumber 2×10 boards. We already discuss engineered woods for structural framing projects in other articles, so in this one, I just want to analyze how to handle engineered wood in the frame of sistering floor joists approaches.

I-Joists Of Engineered Wood For Sistering Floor Joists

We have already studied in other articles what are the I-beams and analyzed their structural stability, the configuration of the web and the flanges. Therefore, now we will describe how I-beams work in sistering floor joists.

Prefabricated wood I-joists are structural, load-bearing products. I-joists are typically available in long lengths and because they’re lightweight, have the advantage of being easily handled at the jobsite without the need for costly handling equipment. Their “I” configuration provides high bending strength and stiffness characteristics. 

That “I” configuration consists of an upper and lower horizontal component—flange—and a vertical section—web—between the flanges. Flanges are made from laminated veneer lumber or laminated structural lumber and range in size from 1 5/16 to 1 1/2 inches thick and from 1 1/2 inches to 3 1/2 inches wide. The web is typically made of OSB (oriented strand board) and is either 3/8 inch or 7/16 inch thick, depending on its application in a residential installation.

Sistering an engineered I-beam with a standard piece of dimensional lumber can be tricky, as the widths of I-beams do not always correspond to the actual widths of dimensional lumber.

As well, I-beams have a flange – the top and bottom – with a narrower web between. This means that in addition to sistering a joist, you have to also include a filler piece to fit between the web and the sistered joist.

Since I-beams are difficult to match with dimensional lumber width-wise, it is more common to sister engineered I-beams with plywood or OSB. In these cases, it is still advantageous to sister the I-beam the length of the original joist. If not, spanning the sistered joist as long as possible is ideal.

Most I-beams will have a 1” difference between the web and the outer edge of the flange. You’ll have to fill that in ½” plywood or OSB in two layers. When doing so, stagger the pieces, so gaps do not overlap. Once you’ve filled in the web, use ¾” plywood as the sister joist, being sure to continue to stagger your plywood over the top of one another.

Use of the same nailing patterns as mentioned above is, except that you’ll be using shorter nails – most likely 8d – as the thickness would be less than if you were sistering two standard pieces of dimensional lumber.

Laminated Veneer Lumber Floor Joists

Laminated veneer lumber (LVL) is an engineered wood product that uses multiple layers of thin wood assembled with adhesives that can be successfully utilized in sistering floor joists projects.

If you happen to have LVL floor joists, which are uncommon due to their cost, then the process of sistering them would be similar to sistering dimensional lumber. If you are able, find lumber that is the same width as your LVL joist.

Likely, you will only be able to partially sister the joist, as LVLs are used primarily in longer spans that dimensional lumber cannot reach. In that case, follow the same procedure as you would when sistering dimensional lumber.

Floor Joists Vs Band Joists: Differences With Sistering

Do sistering floor joists concepts referred in this article apply also to band joists? Let´s see how they are related.

A floor joist is one of the boards that runs under your subfloor, from sill plate to sill plate. They are usually supported in the center of the home by the main beam.  If you have damaged floor joists, you will normally have dips in your floor, or high and low spots.  The low spots are where the joist is failing, and the high spots are where the loads are not as heavy and the floor joists are not yet damaged. 

Floor joists should be repaired like a band joist.  While removing a floor joist won’t compromise the overall flooring structure, it’s still quite likely to damage the subfloor and flooring.  Often, builders both nail AND glue the subfloor down, so separating a joist from the subfloor is still a bad idea. (Repairing tile or hardwood floor is an added expense no one needs!)

However, it’s far easier to sister a floor joist than it is to sister a band joist.  The joists need fewer jacks to secure the flooring (since there are more of them), and they are much easier to access. 

Sistering Floor Joists Costs

Sistering floor joists costs are from $107 to $386 per joist.

Sources And References

  1. Ching, Francis D. K.; Winkel, Steven R. (2016-03-22). Building Codes Illustrated: A Guide to Understanding the 2015 International Building Code. John Wiley & Sons. ISBN 978-1-119-15095-4.
  2. “About ICC”. Retrieved 2013-12-08. Search was building code for sistering joists.
  3.  Canada, Government of Canada. National Research Council. “Codes Canada – National Research Council Canada” Retrieved 1 October 2021. Building code for sistering joists
  4. Northampton Borough CouncilBuilding Control – regularisation charges, accessed 15 March 2021

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