My 5/8" plywood subfloor is screwed and glued to joists. It has 1/8" seams between individual plywood pieces. It also has 1/8" seams at the boundary, which is existing laminated hardwood floor. The seams are only 20 lineal feet. They are not going to be seen.

I've seen a variety of answers about which caulk to use in the seams (from places where I maybe shouldn't have asked the question in the first place). So I'm asking here.

I have 100% silicone caulk (tube is open but recently used) and latex caulk with silicone. Elastomeric caulk was one suggestion that seemed to have merit since the new floor is right by the deck door.

I'm prepared to visit a home improvement center to get some, if that's the recommendation. A brand would be good to consider.

{P.S. and FWIW: After sealing the seams, I will install Hardiebacker over the plywood using thin-set mortar between. I haven't used concrete board before, so I may be back!

{Finally, I will install 3/8" granite tile. I've done 4 tile floors in our old house, but not granite, and not 3/8".}

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    If you are going over the subfloor with cement board and tile there is no reason to "seal the seams" of the subfloor. If you insist, just use the siliconzed latex for ease of application.
    – Kyle
    Aug 1, 2022 at 0:03
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    Those are not "seams" they are "expansion gaps" and there's very good reason not to fill them...if they were supposed to be filled they wouldn't be there at all.
    – Ecnerwal
    Aug 1, 2022 at 2:27
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    @Ecnerwal, please make that an answer, there is absolutely no reason to fill the gaps between the plywood. If anything, it would do more harm than good, other than a waste of time and money.
    – Jack
    Aug 1, 2022 at 13:08

1 Answer 1


Those are not "seams" they are "expansion gaps" and there's very good reason not to fill them...

If they were supposed to be filled they wouldn't be there at all. They are to provide empty space for the plywood to expand without buckling. Filling them defeats that purpose

  • What am I missing (besides experience and knowledge)? Please explain why Custom Building Products says this about installing WonderBoard (WB): "For best results, be sure plywood subfloor panels are gapped 1/8'' at seams .... Keep joints free of mortar" and about using ProLite Tile Mortar for adhering WB to subfloor: "Expansion joints and perimeter joints ... should never be bridged with setting material. They must be brought through the tile work and filled with an appropriate elastomeric sealant." How else to "Keep joints free of mortar" than to fill them with elastomeric sealant?
    – DSlomer64
    Aug 2, 2022 at 20:19
  • Furthermore, I assume the perimeter joints between subfloor and external boundaries should be filled with elastomeric sealant to control air infiltration and prevent water leakage, especially since the (small expanse of involved) floor borders a deck door, so why not similarly fill the expansion joints/gaps?
    – DSlomer64
    Aug 2, 2022 at 20:45

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