I have a ranch house on a slab that sits above grade and has no apparent moisture problems. I want to install hardwood floors (t&g cherry) in the entry, dining and living rooms and tile in the kitchen and bath. What is the best subfloor to install? Professionals have suggested (1) paint-on water sealant then floating 3/4" plywood then hardwood flooring; and (2) sealant then felt then two layers of 1/2" plywood cut into 2x2' squares overlapping so no seams connect the slab and flooring, then felt, then the hardwood. The kitchen/bath tile is 12"x12"x1/2" squares. Should I use the same subfloor (perhaps a little thicker so there is no significant transition?

To do a perfect job seal the floor and use dricore sub floor and tap con ply over that. In the long run dricore is better and quicker to install and lets the air breath under the floor.
Dricore Sub floor Systems Detailed Installation

To build up the tile floors use durock cement board instead plywood Especially if you get a bathroom leak and it gets past the new grout into the sub floor plywood and now its shot.

http://www.usg.com/USG_Marketing_Con...tion_Guide.pdf

Check with your floor manufacturer to make sure the warranty is kept on that floor and is compatible with any system


Signed 21 Boat

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I looked at Dricore, but would need to put another 1/2" of plywood over it to keep flooring nails from puncturing the plastic layers. Also, do I need to seal the floor if I use Dricore? I thought it served as both a seal and underlayment.

Andy,

First of all, is your slab in excellent condition with no cracks, or do you know yet? Is the hardwood you're considering 3/4" solid wood or is it a thinner engineered wood? Maybe 1/2" engineered hardwood? Hopefully. :)

If it is a solid hardwood 3/4" thick, although there are methods to install over a slab, it can get complicated and risky and the floor will end up at least 1 1/2" higher than the other subfloor. If you go this way, you must follow the wood manufacturer's directions to a Tee. What brand wood is it?

But why not use a 1/2" engineered hardwood instead? Engineered woods are glued down and can go direct over a good slab. The total thickness wood be just over 1/2" or so and you'll have a nice transition to the ceramic floors.

Most ceramic tiles are about 3/8" thick installed. I recommend first installing an uncoupling membrane such as Ditra, then your tiles. Ditra adds 1/8" thickness installed, and so the transition will be about perfect.

BTW, there is no method or need to ever install a cement backer on a slab. In addition to thinset under the CBU, it must also be fastened. Hard to do to a slab.

Jaz

Jaz and 21boat,

Thanks for your answers, they really help.

The slab has no cracks with 80% of the flooring up--no hints of moisture problems. Flooring is 25/32" Green River clear cherry solid T&G (Traditional Collection - American Cherry) that was given to me. I've planned for the finished floor to be 1 5/8" above the slab. Using the Dricore would be ok, but the FF would be a little higher than ideal, but still ok.

Given that thick subfloor, I will need to put subfloor under tiles in the kitchen and bathroom. In the kitchen I plan to use 12x12x1/2" stone (Adoquin Blanco 1/2" stone honed finish 12" x 12") with Raychem heater mats, so that will need ~3/4" of subfloor as well.

I guess I'm floating the idea of sealing the floor then using 3/4" plywood everywhere.

Thanks, Andy

Green River is an Asian company and I was not able to find any installation recommendations. Hopefully there will be some in the cartons concerning over slab installation. But then it may be too late. Try to get a set of instructions before you decide. Going over a slab is not a standard method and you will be on your own if you do.

I can tell you however that your plan to install plywood and then tile over that is not a method anyone will recommend. The wood will shrink and expand and will cause the tile grout to crack, and in a worse case the plywood will expand and buckle. We call it "tenting".

The only proven method the raise the floor that much is to it it with a "mud-job" over the concrete.

Jaz

Jaz,

What do you mean by a "mud job?" Is it a thin concrete slab bonded to the top of the existing slab?

Thanks, Andy

It's a very dry mixture of sand and cement to make a base for tile. I bet you have mud work in your house somewhere. It can be bonded to the slab, or if the areas is large or if there are cracks, it can be placed over a "slip-sheet" so the new mud doesn't crack too.

Some DIY'rs can do this method, but most shouldn't try it.

Jaz

OK, I should get my tile guy to build up the bathroom floor, but what should I use as underlayment under the tub? Maybe I should use a 3/4" or 1" pressure-treated plywood or if I want 1 1/2" higher I should use a continuous layer of 2x pressure-treated lumber?

Andy,

You should never use pressure treated lumber indoors. Again, any plywood over a slab isn't kosher either. Figure at different way the shim the tub.

Jaz