When sistering a joist or rafter, must the ends of the sister be supported?

Question

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?

Answer 1

When beams and joists are sized, they are checked for 1) bending, 2) horizontal shear, 3) vertical shear, 4) deflection. Knowing the cause of the failure will help in knowing how to repair.

1. Bending will occur near the middle of the span and show as a crack or split at the bottom edge. (When we say “bending” we really mean the stress in the extreme fiber in bending.) When the wood fiber bends so much that it tears that extreme fiber then it starts to fail. This has nothing to do with “end bearing”.

2. Horizontal shear has to do with wood fibers sliding past each other due to deflection. An example of this is when you stack some 2x4’s on each other over a span and then stand on it. The 2x4’s will slide past each other at the ends of the span, but not so much in the middle. Again, this has nothing to do with end bearing...just the fibers in the middle of the joists at the ends.

3. Vertical shear has to do with crushing the fibers at the bearing point. Obviously if the joist is being crushed then the repair will need to be at its bearing.

4. Deflection is caused by overloading the joist to the point it bends (but does not fail) so that it’s springy or so it flex’s and cracks tile, etc. This repair is primarily in the middle one-third of the span and has nothing to do with bearing.

So, when you repair a problem, it doesn’t matter how FAR you go past the repair on each side...it matters HOW the load is transferred around the damaged area. (Generally it’s customary to go past the repair area by 3’-4’ or so. The reason for this is so nailing can be staggered across the boards without splitting the wood.) However, it really depends on the actual load on the joist. A small roof load could be repaired with a 24” lap, but a heavy floor load could require a 60” lap.

However, seldom does the repair need to extend to the bearing point, unless there’s dryrot, crushing from a post, etc.

Answer 2

Sistering is tying the two together to repair the original. If properly nailed or bolted no additional support should be needed. When I sister I look at the span if I have a top to bottom split over a foot 3 feet on either side is plenty of support but if I have a split over 6’ I would want 4 or more feet on either side. I have seen this with unusual snow loads splitting old growth . Not a long enough tie area can cause a new split.