I am building up the concrete slab floor in my mudroom. I plan to lay T& G
plywood over 1/2" x 4" sleeper joists 2' o.c. with 1/2" rigid foam between
them. I may put down tile and I understand tile will not tolerate floor
flexing before they crack. What's the minimum plywood thickness that I can get
away with? I am hoping the foam would provide some floor support. I only have
2" from the concrete slab to the bottom of the doors and I don't want to raise
You have two inches for sleepers, insulation foam, subfloor, wonder board to
support the tile, and tile flooring?
Two inches is not enough space.
I never heard of a 1/2" thick sleeper, but I can't say it's not OK. Nails
will barely bite it, and adhesive won't necessarily solve the problem
because foam will be between the sleepers and subfloor, so I would use lots
and lots of screws to grab the sleepers. I *think* you need pressure treated
sleepers. I can't seem to figure out if you need a 6 mil barrier on top of
the sleepers and under the foam, but maybe you have a barrier under the slab
that's sufficient, but do not take my word for it.
Insulation 1/2 inch, well, ok. Provides little or no strength to floor.
Subfloor. Here in NC, on 24" OC joists, it has to be 11/16 if perfectly
perpendicular to joists, or 3/4 if on an angle to joists. If you add more
sleepers to achieve 16" OC, the subfloor can be 5/8 thick.
We're up to 1-5/8 already, and you cannot get wonderboard and tile in 3/8
You have enough space for ... linoleum.
Did I understand your question?
Thank you for your comments. I think I will go carpet or hardwood. I
completely forgot about the wonder board under the tile.
I am also worried about only a half inch bite. I didn't clearly describe the
foam. I plan to place strips of foam in the "bays" between the sleepers, not
over them. I hope lots of srews and glue will help.
I bought pressure treated sleepers because I could not justify tripling the
cost for the synthetic deck boards.
I question the position of the plastic too, but a website (seems reputable)
that I visited recommended placing it over the sleepers/foam.
I think I will put 5/8 plywood over the sleepers situated 16" oc.
The book I have states that ANSI standards recommend that a plywood setting
bed for a floor or countertop be a minimum thickness of 1 1/8".
It also recommends laminating two sheets (1/2" & 5/8")together to achieve
this. This was from 1998 so things might have changed.
Of course this depends, to me anyway, on the joist spacing. In my bathroom
I used 5/8" plywood with 1/2" Durock. I should have gone with 3/4" ply and
1/4" Durock, but it's been fine for several years. I also added some
blocking between joists to eliminate deflection.
I've seen many recommendations of 3/4" plywood with 16"O.C. and that will
I'd double up your sleepers to a 12" o.c. just to give a bit more leeway.
Slap on some 3/4" T&G and you'd be good to go.
Thanks for your help. You and the other response that I received mentioned the
Durock that I completely forgot about. I can't squeeze everthing within the 2"
limit so I think carpet or hardwood over the 3/4" is the new plan.
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