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. Often times mold is 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.