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5/8+ 3/8 Subflooring

mc720's picture

I am working on bathroom renovation. The existing 5/8 subfloor has been mostlly ripped up, but there are some spots in decent shape that run under the wall and would be very challenging to block if it was pulled up. (return air duct in between joists)

The plan is to replace what I can with 5/8 plywood and then cover the entire thing with a second layer of 3/8 (11/32) plywood to increase thickness in prep for schulter ditra and tile.

A few questions:

First Is this acceptable? I hear mixed things about any subfloor layer thinner than 1/2 in. However floor height is an issue.

If ok, how should the second layer of plywood be attached? I was planning to treat like a wood underlayment and use 1/4x1in staples without adhesive/glue.

Finally, I need to build a wall to seperate the tub and vanity. Would you build the wall on top of the entire floor, or on top of the 5/8 (like the perimiter walls) and then put the 3/8 around that?

Appreciate all your help



I'll just throw in a curve (post #215113, reply #1 of 3)

I'll just throw in a curve here:  When we did our bathroom the shop where we got the tile recommended that (over thin plywood underlayment) we put down "expanded metal" mesh and cover that with thinset.  Worked out pretty well, and it's been completely solid for about 30 years now.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Yes, it's fine. I've used (post #215113, reply #2 of 3)

Yes, it's fine. I've used even thinner than that before. If you're going to tile you could also use cement board rather than plywood. Just make sure you bed it in thinset.

Florida Licensed Building Contractor, 50 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

Hi there,       Your (post #215113, reply #3 of 3)

Hi there,

      Your plan is fine.  Although I would recommend screwing the 3/8" plywood down to the 5/8" plywood intentionally missing the framing and straggering the seams between layers.  That is what the Tile Council recommends too. The idea is to provide a stiff subfloor for the tile.  The second layer being 3/8" on top of the 5/8" doesn't quite get you to the 1 1/8" recommendation for subfloor thickness under tile, but using Ditra you should be fine.  Keep in mind subfloor and underlayment are different things.  Cement board can not be considered subfloor.  You need 1 1/8' of SUBFLOOR before underlayment, whether it is Ditra, Hardie Backer, or cement board.  So 5/8" plywood and 1/4" cement board is not enough.  Good luck