Is Joist Sistering Effective?

Joist sistering is a very popular repair and bolstering method that you will find on every DIY enthusiast website and forum, but we recommend that you avoid it altogether. For a start, it will not address the cause of joist over spanning, and it is not a permanent solution. 

Sistering Over Spanned Joists 

Sistering is not an effective repair method for over spanned joists as it will not resolve the underlying issues. After all, in many cases, the main issue is not damage to the joists, but their placement, number, or the suitability of the wood. Sistering is just the process of adding extra material that can compensate for damage or weakness, and while that can be helpful in some cases, it will not help a fundamentally defective joist. 

The second main issue with sistering over spanned joists is that the process could make the damage worse quite quickly. This process requires that a joist be moved to make space for the extra material that is being added to it. If you move an already over spanned joist or weak joist, you could cause a lot of damage and make the sagging in your flooring even worse! 

Sistering Damaged Joists 

If you have a joist, or several joists, that have been damaged by rot, foundation movement, flooding, mold, or a sudden impact, it could be bolstered with sistering. This is not a permanent solution to any kind of damage, of course. Sistering a damaged joist can make the problem worse, especially if you are not skilled or experienced. Of course, even if you successfully sister the joist, this will not give a long-term resolution. 

You see, the materials used to bolster the damaged joist will become damaged over time. Why? Because they will be subject to the same pressures as the damaged joist, and this will cause them to deteriorate in tandem with the over spanned joist. As such, you should consider sistering a method of delaying floor collapse. It will buy you time, but it won’t bring the matter to a close.