I have a twelve foot span. The floor joist are damaged. How far do I have to sister the joist to make them structurally sound?
I can't answer this without seeing your situation. There is no code written to deal with this type of problem. Each case is different. I can tell you this, when we have come across damaged joists, we typically have gotten a structural engineer to advise us on a solution. The city building official has required us to do this.
Solutions to damaged joists have included attaching a full joist alongside the damaged joist, from end to end. There are specific nailing patterns, such as 4 rows of 16# nails, evenly spaced, 16" on center. We have attached BE" plywood on each side of the joist, 4' on each side of the damage, with a similar nailing pattern. Both of these remedies were determined by a structural engineer.
Floor joists sizes and spacings are figured by floor loads and types of lumber. Damaged joists throw off the calculations and there is no simple way to correct this problem, short of replacing the joists.