New construction advice

[microcosm]

I am interested in opinions on a tile job I plan on doing in my new home. Bathroom is 70sf with 3/4 osb decking sitting on engineered trusses 24" on center. I plan on installing Warm Tiles (coil) underfloor electric heat as well. Either 12x12 or 18x18 tiles.

What is the best subfloor and thickness to use (cbu, ditra, anyone heard of easy mat by custom building products)?

I thank you in advance for your advice

-Mike

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[Danny Ferguson]

I would add another layer of plywood 3/4" and use Schulter Ditra.

[cx]

Welcome, Microcosm. Please give us a first name to use. :)

You gotta add some more subflooring. Half-inch plywood would be my minimum, but Schluter's minimum is 3/8ths for Ditra over that joist spacing.

Using a membrane over the heating system is a good idea. Lotta guys won't do the heated floors without one.

We're familiar with the Easy Mat. I wouldn't wanna put it over a heating system, though. Too much insulation.

You've read all the instructions for your particular heating system?

My opinion; worth price charged.

[microcosm]

CX: Do you mean 1/2 plywood, heat coil, skim coat, membrane, thinset, tile?

I like 3/8 better only because I am concerned about the final height of the floor with carpet in the adjoining bedroom.

[cx]

Do you mean 1/2 plywood, heat coil, skim coat, membrane, thinset, tile?That would be the order using Ditra or similar membrane, Mike. You need to verify the installation of the heating part with the manufacturer of that product, of course.

If you'll go to the UserCP above, find Edit Signature, and enter your first name there it will appear in each post for folks to see. :)

[microcosm]

CX:

The Warm Tiles website explains the substrate install exactly as you have described. A couple of additional questions:

1. Is the type of plywood important?
2. Why not use cbu instead?

Thanks for your advice.

[cx]

1. Yes. It must be EGP (Exterior Glue Plywood) with no face of grade less than C.

2. CBU provides no structural strength, only a tile-friendly substrate. You must have enough plywood to achieve the necessary deflection standard.

And Because that's what the Tile Ranger says. :shades:

My opinion worth price charged.

[microcosm]

CX:

Been doing some reading on the forum, great site! I have a couple of more questions/concerns with the joists being 24" on center.

1. Would it help to glue the 1/2 plywood to the 3/4 osb subfloor?
2. Can I use osb for the second layer?
3. I thought I saw you write something about the plywood extending a quarter of the way passed the joist for maximum strength.
4. Does it matter with the Ditra if the subfloor is screwed directly to the joists or should the be avoided?
5. Any special length/type for the scews?
6. Lastly, would a floor leveler be better over the heat cables than a thinset skim coat ? Otherwise leveling out the areas that won't get the heat cable is a bit cumbersome.

Just want to make sure that I do this right. I will have quite a bit invested with the in floor heat and ditra (never used before) and want to make sure that I am confident with the particulars.

I appreciate all of your advice.

[cx]

1. While I'm a big fan of gluing, I don't recommend trying to glue to OSB. Doesn't seem to work so well. Can't prove it, just anecdotal evidence.

2. Yes. Here (http://www.tile-assn.com/tileletter/pdfs/Underlayment-Nielsen-Woeste-0604.pdf) is a good article on that theory. I recommend installing your second layer in accordance with their recommendations to the extent possible.

3. Avoid the joists with the second layer fasteners per the Ditra instructions. I think it shouldn't matter with the Ditra, Schluter does. It's their product.

4. Some kind of coated or plated screws long enough to penetrate the bottom layer. Decking screws work well.

5. Out of my league. Some folks think it's a lot easier with the SLC. There are other considerations when using that stuff, though. Like primer. And caulking alla holes and cracks and gaps and stuff. Wait for more knowledgeable respondents. :)

My opinion; worth price charged.

[microcosm]

One last question: How many screws would you put in a sheet of plywood/osb (4", 6" or 8" apart)?

Thanks again.