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I am replacing about ten floor joists that are almost completely rotted through, one’s even been cut in half for a four inch drain. The joist size is exactly 2x7.5" with about a 16 foot span.

My questions are:

  • What type of wood should I replace it with?
    • I'm looking at SYP 2400 their thickness is 1.5" with a correct height give or take a couple mm/8th of an inch.
  • Should I replace them at all or sister them next to each other?
    • The middle is more or less solid for a 100yr old house and having water damage, it’s where they rest on cinder blocks that the wood has rotted from bad siding and a leaky window both on opposite sides of the joists in question.
  • If I sister them then would the rot degrade the pieces I put in?
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  • At 16 foot span, I would not be jumping up and down with 2x8s. There are chemical treatments for rot stop, but probably better to remove the rotted wood.
    – crip659
    Aug 15 at 22:46
  • Agree that 2x8 with 16’ span sounds like a lot of span.
    – Aloysius Defenestrate
    Aug 15 at 23:46
  • That’s what I’ve been seeing a 2x8 wouldn’t be the proper size and the reason it worked before it was damaged is because it’s actually 2” thick if I go with a 2x12 would I just notch out the cinderblocks to fit or is that where the blocking comes into play don’t know what that is thought it was small pieces fit between the joists
    – Jake
    Aug 16 at 2:34
  • The small pieces between the joists (blocking or bridging) help reduce joist wobbling. Some people prefer one or the other, YMMV. As suggested, 16' is too long for 2x8s now; the recommended size depends on your spacing, which I may have missed.
    – Huesmann
    Aug 16 at 12:20

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It depends on the wood species and grades you have available at your local lumber yard, but whatever it is, your best bet is to look up the maximum spans here https://awc.org/calculators/span-options-calculator-for-wood-joists-and-rafters/

I suspect that 8" is too little. You might have to go 10 or even 12". If you do 12", add blocking.

If the current location is wet or you have insect damage, you either have to fix the conditions or use pressure-treated wood. Rot will not spread from one board to the other, but wet conditions will duplicate the problems.

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