My son recently bought a 100 year old house. We have been working on some framing repairs in the basement, and have some questions about the best way to sister some joists.
This is a balloon framed house. The wall studs rest on the sill plate, and extend all the way up to the 2nd story. The first floor joist rest on the sill plate and were at one time nailed to the studs. The flooring is laid on the joists.
The joists in question are true 2”x10” lumber approximately 16 feet long (for the longest run). They bear on a 10”x10” wood beam that runs down the center of the basement, with a ~1” deep notch cut into the bottom of the joist, and extend to the outside wall. On the outside wall, they were also notched to sit on the sill plate.
Sometime in the past, the sill plate in this part of the basement was replaced. My son discovered, after removing some insulation, that the joists were no longer resting on the new sill plate. In some places the joists seemed to be short of the sill plate. These joists are also supported by a 4”x4” post (used as a beam) at their mid point, which means they are basically a long cantilever. We think this beam was added to support the joists which no longer rest on the sill plate. Oh, and some of the joist are split at the sill plate end. The sketch below tries to show this. The blue beam is being used to raise the joists to their proper height. New sketch showing another beam, in blue
We want to sister the joists with new lumber, 2x10 or 2x8. Our desire is to use 2x10’s, 16’ long that would sister a joist along its entire length, from the main beam to the sill plate. But, we don’t have room to slide a 16’ 2x10 in place – too many pipes. HVAC ductwork, and other things in the way. We think we can fit a 16’ 2x8 in place. A shorter piece of 2x10, 8’ long would also work (we have two of these dry fitted in place). See the second sketch.
The question is would we better to use a full length (16’) 2x8 as a sister, or an 8’ 2x10 that only sisters the bad end of the joist, the end that would rest on the sill plate? In either case, we want to remove the "added later" beam (shown in the light purple), so that the run from the main beam to the wall is uninterrupted.
Here's what we ended up doing. We sistered ~8' of the damage end of the existing joists so that the sistered joist (2x10), with the notch, would rest at the proper height on the sill place.