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Joists are an integral part of many homes, but over time they can bend, twist, and flex. When this happens, other components such as walls and ceilings usually do as well.
A joist is designed to carry a specific load and must be constructed in such a way that it can support it, or it will fail. If a joist falls victim to age, poor design or disrepair, the solution is often to “sister” the joist.
Sistering a joist means adding another identical joist, like a sandwich, thereby doubling the thickness of the joist. Most building codes, including the most widely used, the IRC, do not directly address sistering joists, but they do address girders, which are similar.
Today we’ll discuss how a joist should be sistered using specific code requirements and general engineering principles.
What does Sistering a joist mean?
For lack of a better analogy, you can think of a sister joist as an identical, combined sister. For example, let’s say your joist is in the floor, which makes it a floor joist. If your floor joist measures 2” x 8” (approximately), then you have a 2” x 8” floor joist. To install a sister joist, you would build another joist the same size as the joist in the floor and attach it with nails, screws, or bolts.
Why would I sister a joist?
When you add a joist, you are actually creating a double girder. Most people think of a girder as a long piece of steel used in the construction of skyscrapers. Girders are very strong, and are used to provide support on an unsupported span, such as a bridge. In the case of a floor joist, it acts as a single girder and supports the floor above it.
Over time, joists can bend under the weight and create a gap between the ceiling and adjoining walls. Sistering the joists adds the full strength of the new joists to whatever support is still being provided by the original joists . Professionals typically consider creating a double girder by nailing two boards together, while creating a sister by adding an identical board to the existing joists.
Will adding floor joists fix my sagging floor?
Adding a sister to a loose floor joist is the best way to fix a loose floor , however, there is a correct way to use it. Sistering a floor joist (or joists) usually requires some preparation work before installing the sister on the floor joist.
You don’t want to add another addition to a loose floor joist because that will make the looseness permanent.
First, you must raise the original floor joists to their original position. Most professionals and do-it-yourselfers use telescoping support jacks for this task.
Telescoping support jacks have a threaded rod at the end that increases the length of the jack with every turn. Telescoping jacks are useful because they function as both a jack and a support post simultaneously.
What are the rules when installing sister floor joists?
To add another joist to a floor joist, you must follow certain rules to ensure that your joist is doing its job properly. Professional carpenters follow these rules to ensure they are following building codes, and because it is the best way to do so. You can think of a sister joist as a splint on a broken finger. First, you must straighten the finger (the floor joist) before you can apply the splint (the sister).
Make sure the sister joist has bearing support
The first step in adding sister joists is to make sure there is bearing support wherever the sister joist will be installed. Bearing support refers to the posts, girders, concrete or other materials used to support the floor system from below. In most cases, a girder is installed beneath the floor system to support the floor joists.
Wherever your sister ends, you will need support from below in the form of a concrete post, telescoping support jack, or other support. In other words, your sister joist needs support every time the two ends join together. Generally, just make sure your sister joist is the same length as your original and you shouldn’t need an additional post.
Always drill and cut in the right place
A common amateur mistake is to drill a groove or hole in a floor joist or sister, but in the wrong place. Drilling these grooves or holes in the wrong place can destroy the strength of the joist. Always follow these rules when cutting holes or grooves in floor joists:
- Never remove more than 1/6th of the width of a joist .
You should never cut a notch or drill a hole that removes more than one-sixth of the joist material. For example, if you have a 2” x 8” floor joist and you cut a 4” notch in its side, from an engineering standpoint, you now have a 2” x 4” board supporting your floor where a 2” x 8” joist should be.
For example, if you have a 2” x 8” joist, any notch you cut should be no more than 1/6th of this width. So, you can safely make a notch of about 1” in a 2” x 8” floor joist or its partner, as long as it is in the middle third of the span. - Never cut a notch in the middle one-third of the span .
If you need to run a cable or pipe through a sister joist, never make a notch in the middle one-third of the span. For example, if your sister joist spans 60”, never make the notch further than 20” on either end. Always use the outer two-thirds of the joist and never extend more than ⅙ of the width of the board.
Floor joists and sister joists sometimes have notches and holes in them, although it is best to avoid them. The important thing to note is that if your plans call for 2” x 8” and you cut a 2” notch on the edge, you have violated code because you have effectively installed 2” x 6” where 2” x 8” is specified. - Never drill a hole within 2” of the edge of a sister joist.
You should never drill a hole close to the edge of a board because you effectively shorten the board. Like taking a notch that is too large, this weakens the sister joist. If you do need to drill a hole, always make sure it is at least 2” from the edge. Always drill a hole as close to the center of the sister joist as possible. - Never drill a hole larger than ⅓ of the width of the sister joist .
If you must drill a hole to accommodate a large pipe or vent, never cut a hole larger than ⅓ of the width of the sister joist. For example, if your sister joist is 2” x 8”, you can safely drill a 2 ½” hole in the center. However, a 3” hole, even directly in the center, will not work because it will be more than ⅓ of the width of the sister joist.
Install the correct fasteners for the sister
When installing sister joists, make sure your fasteners are appropriate for the project. When adding a sister (or sisters) to a joist, it is important to connect all the boards to each other, not just the board next to it. In some cases, you may need to add two or three sisters to a joist if the situation is critical.
Never use #12 or #16 nails if you are adding more than one sister to a joist. Although these are commonly used throughout a home, they are not long enough to penetrate a three sister joist. The best solution when using more than one sister joist is to bolt all sisters and joists together with appropriately sized galvanized carriage bolts.
You should not use lag bolts to sister joists if you can avoid it. This is because the sister is only as useful as it is tight. Lag bolts can easily break when trying to pull the sister and joist together. Carriage bolts require a washer and nut, so they can be tightened more effectively without damaging the wood.
To make your connection even stronger, don’t align the carriage bolts evenly. For example, mark the locations for the carriage bolts on one edge with odd numbers, such as one foot (12”), three foot (36”), five foot (60”), seven foot (84”), and so on. On the other side, start with even numbers, such as 2 foot (24”), four foot (48”), and so on.
From an engineering perspective, this pattern spreads the load more effectively. If you are only installing one sister joist, you can use #12 or #16 sinker nails to attach the sister using the same pattern. You can also use screws, provided they are 2 ½”-3” long. Most professionals use 3” galvanized wood screws with a square drive head, but Phillips or Torx screws are fine as well. Just make sure the sister joist is as tight as possible.
Sister crown oriented up
Professionals will typically select a sister joist for its size and crown. Ideally, they are looking for a board with a mild crown, meaning the edge of the board is slightly higher in the middle. An example of an exaggerated crown would be the rocker on a rocking chair.
Professionals look for a board with a light crown because it will be a little stronger in the downward direction. By keeping the crown higher, the sister is a little stronger from top to bottom, where it needs to be strongest. You only want a light crown, about ¼”, because a severe crown can cause a bulge in your finished floor.