Can I use Thin Set Mortar over Interior Grade Plywood?

JFERTICH

I am planning to add 1/4" cement board and tile to my subfloor. My subfloor consists of two layers of 1/2" to 5/8" plywood. The plywood was manufactured in 1983 and is marked C-D exposure 1, which I think means it is an interior grade plywood.

All the literature I read on applying thin-set is that it is only recommended for exterior plywood. I heard that applying wet thin set mortar over interior grade plywood could cause plywood delamination.

Should I paint or possibly seal the plywood surface with something first or just apply the thin set mortar down directly over the plywood and don't worry about it?

Also the plywood is very tight fitting with no gaps between sheets. Is that ok?

Thanks

oh'mike

Not a worry.

The thinset will not delaminate the ply----Go for it.

I've done this countless times without a problem.----Mike---

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