Building Code for Sistering Joists

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 do not apply in Wisconsin ¹¹ but a hybrid variation 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 the Asia Pacific region 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, 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.

Industry Standards For Sistering Joists Outside A Building Code

Unlike sistering, which alters the structure, sistering reinforces an existent joist and does not change the structure as long as joist blocking or reinforcement is sustained at a maximal limit of four ft. increment. 

The technique of sistering floor joists is referred to as an alternate construction practice. It is possible that these are the reasons why the local regulations do not your local code does not consider them. 

However, in order to illustrate how various municipal laws handle sistering, the following instances are provided.

A minimum of four feet on each side of the concerned section is required by certain building codes, and ½”  bolts in a ‘W’ pattern are required to make the connection stronger. Several other codifications specify that the only permitted method of sistering a joist is to double it full-length and support both ends of the new joist on bearing points at both ends of the old joist.

Depending on the municipality, the local codification may be more accurate in terms of sistering. Removal and replacement of a severely damaged beam are needed in this situation. Another codification approach involves the removal of the damaged part and the replacement of it with a new piece with the same dimensions as the old one. A matching dimensional board (such as a 2×6 or 2×8) must be placed between the new and old portions of the joist, with the new piece of the joist extending 2- or 4-feet beyond each end of the repaired section.

Avoid Errors Despite The Absence of A Building Code For Sistering Joists

Even without a building code for sistering joists, we can define some of the best practices by looking at common errors that we all have or had in the past.

I explain in detail these errors, here, in an article about the process for sistering floor joists.

Making repairs and improvements to your home may save you money while also instilling a feeling of pride and ownership in you. It is possible that you may be tempted to try your hand at floor joist repair sistering if you have the necessary equipment and time on your hands. The following are some of the difficulties you may encounter along the road.

• 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 in this article.
• 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.
• Joist becomes Useless As It Is Not Strenghtening Its Sister: 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.
• Joist Bays Must Be Unobstructed: The joist bays must be free of any obstructions, such as electrical cables, ductwork, and plumbing, while you are doing the work.
• Enough Nails To Transfer Structural Loads: 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 Required: 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.

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 exist and they have to be followed and are subject to inspection as well.

Assesment Of A Structural Engineer As There Is No Building Code For Sistering Joists

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 ones. 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, hardwood is difficult. Predrill and drive 3/8-in. x 3-in. lag screws if you are 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 an 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 In The Building Codes

There is codification about a joist lap at a load-bearing wall. 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, which 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 2020-12-08. Search was building code for sistering joists.
3.  Canada, Government of Canada. National Research Council. “Codes Canada – National Research Council Canada”. www.nrc-cnrc.gc.ca. Retrieved 11 October 2021. Building code for sistering joists
4. Northampton Borough Council, Building Control – regularisation charges, accessed 15 March 2021