When sistering a floor joist, must the sister be full length to the original joist and must the ends of the sister be supported on a sill or beam for the sister to be truly effective? Assume the sister is attached to the original joist with adhesive and proper sized fasteners at an appropriate schedule. Is the same true for a rafter?
Is the answer different depending on the problem being corrected?
- Fixing a cracked or rotted joist
- Reducing deflection in the floor (e.g. for a tile installation or just less bounce)
- Strengthening the structure in anticipation of a greater load in the future
- Correcting sag
I see two schools of thought:
- Use a full length sister and support the ends. 3-4 inches at either end must rest on something. Otherwise it does nothing but add weight.
- Sistering over the middle 2/3 of the span (or 3 feet on either side of a break) adds the strength needed and the ends of the original joist will have no problem taking the negligible additional weight of the sister.
Seems you can't go wrong with duplicating the original joist when it's possible but are there cases where one is more appropriate than the other?