I have a garage that is being converted into an apt. and the floor will be 12" Mexican tile . The details are as follows: existing concrete slab, 2" x6" pt sleeper/ joists on 16" centers resting on top of slab, 3/4" t & g plywood glued and screwed to sleepers. What would be the under-layment recommended , with cost a big consideration?

I am not an expert but I think you are ready to go. Maybe add a few screws if you detect any movement of the plywood. I would read the manufacturer's recommendation on thickness of underlayment but the ones I have seen specify a minimum of 3/4" subfloor which you already have. It will be a cold floor (assuming you are in that type climate) but I think it will be structurally sound.

Hi all...

Old school will agree with smearcase. I was raised using 3/4" T&G plywood for subfloors with an 5/8" exterior grade plywood for tile installations. All tile jobs that we did were fine and lasted long time.

New school would have you glue and screw a 1/4" (minimum) or 1/2" cement board to the plywood. All seams get taped with an alkali-resistant tape and the joints get a layer of thinset mortar embedded into them. All screws are alkali-resistant as well.

The glue and screw is recommended to be a modified thinset mortar in which the cement board is embedded and screwed into the mortar which creates superb bond to the plywood subfloor (see instructions below).

I believe new school wins here as I have never seen a tile loosen from a cement board if all installation requirements are fulfilled... ;).

Check out these websites for more info...

James Hardie: HardieBacker 500 backerboard

HardiBacker cement board

Let us know if you have questions...

And Smearcase... you're an expert in my eyes. I always value your opinions!

Mark

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I agree with Mass' evaluation. I checked some sites and a min. of 1 1/8" subfloor is talked about nowadays.
The backer board is definitely the best surface for tile and goes for about 70 cents per sf for 1/2".