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

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”.

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.

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.

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.