I've done some research lately on joist sistering as a solution to bouncy floors. The logic seems to revolve around the following principles.
- The stress of carrying a load is at the center of the span
- 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.
- 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?