Building Code for Sistering Joists

Building Code for Sistering Joists

building code for sistering joists

A building code (3) 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 apply in regards to sistering joists. Building codes, such as the Residential Building Code, that does not apply in Wisconsin, 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, unlike the European Union (6), whose codification is being adopted also outside the EU (7), such as EFTA countries (8) and overwhelming adoption in Asia Pacific (9).

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 of the code, 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.

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.


Sistering is reinforcing an existing joist; it isn’t altering the structure as long as joist blocking or bracing is maintained at a maximum of 4-foot increments. Doubling joists is known as an alternate building practice. These may be the reasons your local code doesn’t address it. Having said that, here are some examples of how different local ordinances address sistering.

Some Building Codes require sistering to extend a minimum of 4-feet on either side of the affected area and have ½” bolts in a ‘W’ pattern for a stronger connection. Other Codes identify that the only acceptable way to sister a joist is to double it full-length with both ends of the new joist supported on bearing points.

In some municipalities, the local Code is more specific regarding sistering. For a badly deteriorated joist, full removal and replacement is required. Another requires the removal of the affected section and replacing it with a new piece of matching dimensions. The new and old sections of the joist must then be sandwiched between two matching dimensional boards that extend 2-feet or 4-feet beyond both ends of the repaired section.

building code for sistering joists

Working on your house yourself can save you money as well as give you a sense of pride and ownership. If you have the tools and the time, you might be tempted to take on the job and try floor joist repair sistering yourself. Below are some issues you might run into along the way.

  • 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.
  • Incorrectly Notching Floor Joists: 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.
  • Failing to Level Joists: 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. 
  • Sister Board Not Level: 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.
  • Failure to Properly Secure Boards Together: Not knowing how to secure sistering joists together can cause structural problems in the future.
  • Insufficient Over-lapping: 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.
  • 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.
  • 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?
  • The joist with the notch around the armored cable is useless. It has been compromised and is just adding weight to it’s 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’re doing the work.
  • As, explained above with more details, 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 code for sistering joists is also important to know when making joist repairs. Not knowing these will lead to problems if you ever plan to sell your house. Sometimes building permits are needed as well.

Assesment Of A Structural Engineer

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 (search online or for “Engineers, Structural” in the yellow pages). 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).

First remove electrical cables, pipes and other obstructions. If this is difficult (it often is!), 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.

Then set new joists (same height as existing ones) alongside the old. 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.

If a crack or sag is isolated to one area, the sister joist should extend at least 3 ft. on both sides of the problem area. But it’s usually best to run the sister joist over the entire span. When the sagging joists are level, apply a generous bead of construction adhesive to the existing joist. Then attach the sister joist using three 16d common nails driven every 16 in. Driving 16d common nails in old, hard wood is difficult. Predrill and drive 3/8-in. x 3-in. lag screws if you’re having trouble nailing.

Framing Details For Joists In The International Building Code

2308.8.2 Framing details.
Joists shall be supported laterally at the ends and at each support by solid blocking
except where the ends of the joists are nailed to a header, band or rim joist or to an
adjoining stud or by other means. Solid blocking shall not be less than 2 inches (51mm)
in thickness and the full depth of the joist. Notches on the ends of joists shall not
exceed one-fourth the joist depth. Holes bored in joists shall not be within 2 inches (51
mm) of the top or bottom of the joist, and the diameter of any such hole shall not exceed
one-third the depth of the joist. Notches in the top or bottom of joists shall not exceed
one-sixth the depth and shall not be located in the middle third of the span.
Joist framing from opposite sides of a beam, girder or partition shall be lapped at least 3
inches (76 mm) or the opposing joists shall be tied together in an approved manner.
Joists framing into the side of a wood girder shall be supported by framing anchors or
on ledger strips not less than 2 inches by 2 inches (51 mm by 51 mm).

Building Code Application For Decks And Floors

A traditional deck has posts cemented below ground level, it has a foundation, making the deck a permanent structure and subject to local building codes just as the framing, joists and sister joists within the living quarters.

A permanent structure, for this purpose, is any outdoor structure that is fixed in place and unable to be moved about a property. Examples of permanent structures include homes, garages, barns, and sheds that are cemented to where it stands

Joist Lap At A Load Bearing Wall

There is codification about joist lap at a load bearing well. However, take into account that the joist lap is something different, and that there is no building code for sistering joists.

Joist lap at a load bearing wall would be three inches overlap minimum and three nails minimum per IRC 502.6.1. If there was nothing under them, then that is surely an issue.

You would be required to have at least utilize the cantilevered joist rules for something even close to this situation, about a 2/3 length overlap, could be the case, unless there is a metal flitch plate.

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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