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I've done some research lately on joist sistering as a solution to bouncy floors. The logic seems to revolve around the following principles.

  1. The stress of carrying a load is at the center of the span
  2. Taller is better than wider for a joist, i.e. a 2 by 12 will be a better joist than a double two by six.
  3. For joist sistering the sister can be less wide than the original joist, but it should always sister over the middle of the span.

Most of what I've read involves glueing and nailing an identical piece of lumber to the original framing member, i.e. sistering a two by eight onto the original two by eight.

One thing that occurred to me though: if it doesn't have to span the original distance, and the load is at the center of the span, then why not use a large piece of lumber for greater stability? If I sistered a 2 by 12 onto the original 2 by 8, wouldn't that be stronger than sistering with a 2 by 8?

I'm sure there is some logical problem that I'm not seeing. Are there any hidden dangers to the idea of sistering with an oversized piece of lumber?

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Other than loss of head room I cannot think of a single, plausible one. –  The Evil Greebo Sep 7 '12 at 14:30
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up vote 13 down vote accepted

The reason that joists are sistered using the same height of wood is simply space; generally there's no extra room. If you've got nothing below and there's space then there's no reason you can't use a taller piece of wood. Things to keep in mind:

  • Spanning as much distance as you can so you transfer the load as evenly across the original joist. If it's too short it's going to still bow, just in a different spot, and you'll add additional stress on the existing joist.
  • Don't weaken the original joist by drilling holes or cutting
  • You'll need to raise the joist into the correct position before you glue and nail, otherwise you'll just solidify it in a sagged position!
  • Prepare the surface of the old joist properly for gluing if you want it to take
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