Sistering Floor Joists: 6 Mistakes to Avoid

Wooden floor joists and rebar laid out for a wood foundation.

Properly sistering floor joists is very important and should not be rushed to save time. Since this is a repair of a faulty or damaged floor joist, it is imperative to do this project correctly, and there are a few big mistakes to avoid. Be prepared to get a little dirty, but rest in the fact that fixing this problem properly will help avoid major headaches later.

1. Not Treating the Initial Problem

Most often floor joists need to be sistered because of two problems, either water damage which leads to rot or termite damage that crumbled the joist. These problems must be addressed firsthand prior to implementing any restoration steps. If there was water damage, make sure that whatever leaky pipe or condensation issue caused the rot is fixed.

Mold is often associated with water damage and it is good to address this problem as well. If there was termite damage call a professional exterminator to treat the house if it hasn't already been done. If these problems remain unfixed, sistering the damaged floor joists is only a band aid and the damage will only happen again.

2. Over Notching or Not Jacking Enough

Generally, newer home floor joists are made from 2x8s or new engineered 'I-beams.' Older homes, however, may have 2x6s instead. While it is a good idea to replace rotted material with 2x8s, it may require some notching to fit between the sub floor and ledger board. It is very easy to over cut the notches and remove the waste in two swift cuts, but over cutting the notches in this manner weakens the structural integrity of the replacement lumber.

In situations when sistering damaged floor joists with lumber of similar dimensions, it can be easy to notch and slide the sister board into place. However, doing this does not fix the slant that the floor has taken since the old joists got damaged. Take some extra time and jack the damaged area up so that the new lumber doesn't need to be notched.

3. Not Level

Usually when a joist gets sistered the new lumber does not span the entire length of the old joist. Because of this, the tail end of the new lumber is not supported. It is important to jack this end level to keep the floor from sagging any more and to keep the joists from splintering later.

4. Not Enough Security

The amount of weight that each floor joist handles is pretty amazing. When sistering these pieces together it is easy to put just a few nails in the boards and move on. However, shifting and settling can work these loose and separate the boards. Ideally, nails should be used to temporarily set the piece and then either lag bolts or, more preferably, carriage bolts should be used to lock them together.

5. Not Enough Good Lumber Covered

It can be easy to overlap the new lumber with good lumber from the existing joist with just a few inches. By doing this you save lumber but do nothing to secure the pieces together. Overlapping the boards by two feet or better is best.

6. Not Supported

Rarely does rot occur only in the center of the joist, but on occasion, the most damage does occur there and it may seem easier to just span the rot. But sistering a joist where the new lumber is not supported by either the center beam or the ledger board often only makes the problem worse. Make sure to start a sistering joist where there is support.

Sistering Floor Joists FAQ

Is it OK to sister floor joists?

Adding a "sister" floor joist reinforces and strengthens floors and ceilings and it will not do any harm. If anything, it will improve the integrity of any building.

However, sistering floor joists will not solve all structural problems and may not be enough to resolve some issues.

How do you secure sister joists?

Sister joists are attached with standard construction glue and ordinary nails, which are placed every 16 inches along the joists.

What is the bolt spacing for sistering joists?

Bolts in floor joists are typically spaced 24 inches apart.

How much stronger is a sistered joist?

Adding a sister joist is adding a new jew joist, so that's the amount of strength you will get. You will increase the support of the structure by the power of one joist.

Do sistering joists need bolts or screws?

When securing sister joists, bolts are stronger and more secure than screws but nails are better than screws. Ideally, secure joists with carriage bolts or something similar but when you cannot do that, use standard nails.