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In my bathroom there is vinyl on top of plywood, which is nailed to the sub floor. Should I pry up the plywood before putting down cement backer board for tile, or can I go over the plywood?

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Have you considered what adding backer-board will do to the height of the floor? – The Evil Greebo Dec 18 '12 at 17:59
I was mostly worried about the flatness. Where could the floor height present a problem? I can think of a few like toiletbowl seal, and under cutting the door /frame. Also, the plywood ends an inch short of the bath tub. Can I extend the plywood? – Ampersand Dec 18 '12 at 19:22
I'd screw down the plywood where the nails are. Then use the self leveling cement to start the surface. – Anagio Dec 19 '12 at 12:15
up vote 5 down vote accepted

The plywood is there to make the vinyl the same height as the rest of the flooring in the house. If you add backerboard + tile on top of it, you are going to add somewhere in the range of 5/8" to 3/4" of height. That is enough to be a tripping hazard, and it also wouldn't look very nice.

Pulling up the plywood and the putting down the backer board is the right thing to do.

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+1 - I fully agree with this answer. – Michael Karas Dec 19 '12 at 9:31

Tile should be sitting on a minimum floor thickness of 1-1/4". Assuming the plywood sitting on your floor joists is 5/8", you need another 5/8" plywood which must be screwed down with floor screws every 6". Drive longer screws into the floor joist to secure the original plywood and the new securely.

Another option is to use Schutler Ditra which is a 1/8" "membrane" that provides the same strength as 5/8" plywood but without the added thickness of the plywood. The product is fairly expensive however. The also have a thicker membrane as well.

Bottom line: insufficient thickness will result in cracked tile and grout over time.

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There is diagonally laid wood subfloor under the plywood. Thanks for the info about thickness. – Ampersand Dec 19 '12 at 15:46

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