I have a long deck that must do sister joists on top of a beam. I was reading this thread Is deck joist sistering a sloppy repair? and look like it's better to use spacer in between.

Is it OK to use another treated lumber as spacer? I could just coat the lumber protection around that square 1x2 (?) piece. Or is there a designated metal spacer for such thing?

Update 1

This is what I am trying to do:

enter image description here

  • 1
    My understanding of the term "sistering" when applied to joists is that you would be adding a new joist in parallel along the length of the old one in order to strnegthen/replace the old one, not as in your picture where there is only a small overlap and your are extending the overall length.
    – brhans
    Commented Jun 2, 2016 at 13:00
  • 2
    If you're concerned about a short overlap, logically you must also be concerned about all the other places where one board contacts another. None of these are a real problem.
    – isherwood
    Commented Jun 2, 2016 at 16:05

2 Answers 2


Indeed, sistered boards may provide an area that is vulerable to moisture buildup and decay. However, in practice/experience the effect is not very pronounced. If I was concerned about it because of general moisture in the area, then I would use zinc paint between the joists to help reduce any mold issues.

If spacing is preferred, then I'd recommend a 1/8" space (similar to the space between roof sheathing/plywood). There might be some kind of special spacer, but chances are, it's a rare and unpopular product. You could potentially use washers, but that could be tricky unless you use bolts (I would want the nail to go through the washer to keep the washers from falling out).

Another way that this could be accomplished would be by using steel or aluminum shims. I would just cut my own from a piece (or three) of 1/2" x 1/8" steel or aluminum stock... maybe 6 foot long, and use 6" strips as spacers.

  • If the OP is that worried about moisture being trapped between the two touching joists, then he will also be worried about moisture being trapped where the joist touches the metal spacer. I think isherwood nailed the answer in his comment.
    – FreeMan
    Commented Jun 2, 2016 at 18:48
  • Doesn't aluminum react with the chemicals in pressure treated wood? I thought old galvanized or stainless steel were recommended for PT contact. Commented Mar 3, 2019 at 18:34

In case you're dealing with decay .... Sistering two joists is usually done to support a heavier load than the one for which the structure was originally built. On the other hand, if an existing joist is decaying, then there is no need to sister a second joist of the same dimensions because the new joist by itself will be sufficient. Just replace the existing decaying joist.

  • No, this is new deck construction. The sister joist is because the 2x8 I have is only 12 feet long and I need to run them across 3 beams with 9 feet apart.
    – HP.
    Commented Jun 1, 2016 at 5:49
  • ? 12' is longer than 9'
    – Merkle
    Commented Jun 1, 2016 at 18:10
  • Total of 9'x2 = 18 feet though
    – HP.
    Commented Jun 1, 2016 at 20:33
  • so your deck will be 18' wide/long?
    – Merkle
    Commented Jun 1, 2016 at 22:06
  • Yes, like |===|===| where are 3 beams total.
    – HP.
    Commented Jun 2, 2016 at 4:01

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