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I am working on replacing decking and found a couple of rotted joist I decided to replace as well.

The joist size is 2x8-12. I purchased the replacement joists however when I took one rotten out and compared with the new one, it turned out the new one is 1/4" wider. The label on both old and new joists says 2x8-12, but the exact width of the old one is 7" and new one is 7 1/4". Did I get wrong product? The old deck may be 10 years old -- is it just result of the wood shrinking over years? If so does it mean I can't replace a few joists and should do it for all of them? Technically I could cut off the extra piece but if it is shrinking, then it will become uneven in a few years.

Any guidance would be much appreciated

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    Kiln dried after treatment wood is harder to find and more expensive but it should be of same dimension as 10 year old naturally dried lumber.. sometime you get lucky at smaller lumber yards and find boards that are totally dried from sitting under a shed for months.
    – Kris
    Aug 11, 2018 at 20:22
  • You could notch the bottom ends of the new joists so that the tops of the new joists would be at the same level as the old ones. (They would hang a little lower in the joist hangers.) Then if later you wanted to change all the joists you could flip the notched ones over.
    – Jim Stewart
    Aug 11, 2018 at 22:43
  • I've seen different producs with sizes that differ by about 5mm but I've not shopped for lumber in USA. and here the advertised dimensions are acurate,
    – Jasen
    Aug 12, 2018 at 3:21

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It's hard to know how much of the 1/4" difference you're seeing is going to remain after the wood dries out. I'd take just a 1/8" notch out of the ends of the joists so they set 1/8" high in the joist hangers now, with confidence they will shrink at least 1/8" as they dry. If they shrink a lot and make a noticeable low spot, you can shim between the bottom of the hangers and the joists. (As long as you have access to the joists when the decking is on.)

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