In a previous post, I talked about beams. One special kind of ad-hoc beam is constructed by “sistering” two boards together. In older homes you might notice sagging floors, or if you were to inspect the floor joists directly, you might see twisting, warping, cracking, or bending. Essentially what’s happened is over time, the stress of the load the existing joist was baring was greater than the material or construction could stand. This might be because of poor initial choices (undersized beams or spans that are too great for the material/construction), or mitigating factors over time (moisture or insect damage, for example). It’s pretty rare that a structure will reach immediate failure. You tend to read about those kinds of things in the news. A building collapses, a balcony falls, train tracks twist and derail, etc etc etc. The more common story is that things will fail slowly. Small cracks will get bigger over time. Floors will sag. Doors and windows will get sticky. The list goes on and on. Occasionally you will need to bring in a structural engineer to assess the problem and give you direction on what to do about it. However, that kind of thing can be expensive (a few hundred dollars at least), and the issue might no be large enough to warrant that kind of action.
Which gets us to this idea of sistering. When you attach a new piece of structural lumber in parallel to an existing piece in the same way as you would construct a beam, you can provide greater rigidity and potentially stem the slow failing of the wood. The process is very much the same. You’re going to glue and nail the new board to the old board. If there’s sagging or warping in the old board, it’s best to do what you can to straighten it out. This process might mean using a bottle jack to push on the center of the board, to nudge it back into alignment. Once you’re straight-ish, you can attach the new to old. If done properly, the new beam will be stronger than the old board. Not as strong as a beam of the same size, but a single piece, but certainly better than before. You should do your best to reenforce over the entire span of the old board. If that isn’t possible, a shorter span may help, but I’d recommend extending at least three feet on either side of the mid point of concern (i.e. the point of sag, crack, etc). Additionally, you should use the same or larger dimension of lumber. Adding a 2×4 to a failing 2×10 is about as helpful as spitting in the wind.
For our project, we’ve had to deal with extensive termite damage in the joists under the 1st and 2nd floors. In some cases, this meant replacing the joist all together. In others, it was easier to just sister new lumber over the span. Even damaged wood is still proving some support. Termites will weaken boards, but it’s rare that they devour it to the point where it completely falls apart. Often the harder, core wood is still intact. So it’s actually strong to add to what’s there rather than completely replacing it. This conclusion isn’t always the case and certainly isn’t a good idea if you have an active termite infestation. But if, as in our case, there are no bugs, and a lot of the wood is still good, sistering is the way to go. In all, we added 5 new joists made of laminated veneer lumber (an engineered lumber project stronger than sewn boards), 2 new josits made of regular old grade 2 (structural) lumber, and sistered about 7 more.