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Weathermaker01 01-07-2009 09:03 AM

Which Plywood?
What is the best plywood to use under a ceramic floor. I'm adding a 1/2" of plywood over the existing T&G plywood subfloor. I was thinking of using fir because I believe it has waterproof glue.

Bob Mariani 01-07-2009 09:26 AM

Exterior grade plywood must be used. Not luan, not OSB not CDX

bradnailer 01-07-2009 10:32 AM


Originally Posted by Bob Mariani (Post 208581)
Exterior grade plywood must be used. Not luan, not OSB not CDX

Why wouldn't you use cement backer board instead of plywood? Reason for the question, I'll probably be going through the same situation in a few months. House is a pier and beam.

Weathermaker01 01-07-2009 10:49 AM

It's my understanding that backer board doesn't provide as much strength as plywood does. I've read that the best method is approximately 1 1/4" total plywood with ditra on top. I'm sure that others can answer your question better than I can.

angus242 01-07-2009 11:25 AM

Yes, Bob is correct, exterior grade plywood. The glue is waterproof and that's what makes it exterior grade.
While it is an acceptable procedure to lay tile directly over (properly installed) plywood, I recommend against it. It's not a DIY friendly way to install.
So, for the other methods, you would install either 1/4" CBU or use a membrane (Ditra).
However, you cannot assume that by using 1/2" ply, your installation is correct. You need to consider what the substructure is. There is a deflection ratio that you need to be within in order to successfully tile. It does also make a difference on what kind of tile you plan on using, ceramic/porcelain or stone. The way to determine if the joist deflection will be within range is tell us the size of the joists (2x10?), their spacing (how many inches to center from joist to joist?) and what their longest unsupported span length is (how far between each end that rests on a support?). The species of wood the joists are made of is important too.
I know you said the subfloor is T&G (which is good) but can you verify that it's at least 3/4"?
Assuming everything works out and you have the correct specs, I'd still like to see 5/8" ply put down and then the Ditra. Not only do you have deflection (bounce) from the joists, you also get it from the plywood between the joists. The more ply above the joists, the stronger the floor will be overall....and the more success you'll have with your tile installation!

Bob Mariani 01-07-2009 11:26 AM

no you are right. 1 1/2" is ideal. He was planing to use 1/2 ply than tile. No good. 3/4 exterior of what he has now. 1/4 or 1/2" CBU over that applied with thinset and nails or screws (1 1/4" no longer) What I was saying is that the CBU needs exterior grade plywood. Ditra would indeed work (better IMO) but still should be applied over exterior grade not 1/2 AC or CDX plywood.

angus242 01-07-2009 11:28 AM


Originally Posted by Weathermaker01 (Post 208645)
It's my understanding that backer board doesn't provide as much strength as plywood does. I've read that the best method is approximately 1 1/4" total plywood with ditra on top. I'm sure that others can answer your question better than I can.

And just to clarify, cement backer board is not a replacement for plywood. They serve 2 different purposes. Plywood is for strength to the subfloor, CBU is a backer material for the tile. CBU offers no structural properties to the subfloor whatsoever.

bradnailer 01-07-2009 02:41 PM

Thanks for the comments. Makes sense now.

Weathermaker01 01-07-2009 03:22 PM

Just to clarify. The plywood at Home Depot (Canada) is either Spruce or Fir. Am I correct in assuming that the FIR is exterior grade plywood? There are no markings like CDX, or AC. I know that I can ask but I don't always trust the answer that I get from the staff. So from what I am reading should I be going with 3/4" plywood, I just don't want my floor getting too high if I can help it.

My current floor is 2X12's SPF, on 16" centers, 5/8" T&G Plywood, 16' span, 12" ceramic tiles. I did post my existing floor specs some time ago but I've been waiting since summer for my cabinets to come in. I'm expanding my existing cabinets and adding a breakfast bar. The cabinets were ordered to match the existing cabinets and then Canac decided to close up shop. They did honour the order but it wasn't to quick getting here.

Thanks for the info!!

Bud Cline 01-07-2009 06:16 PM

Ask for "underlayment grade" plywood, one side will be an "A" side. Underlayment plywoods are generally exterior grade pywoods.:)

Not to be confused with "treated" plywood - stay away from treated plywood for this purpose.:)

Speedball 01-08-2009 08:01 PM

Ah, just the answer to a question I have about some work at our place. Thanks Bud.

MrRyan001 06-13-2009 07:17 AM

non-exterior grade plywood subfloor
I recently installed a plywood subfloor in my new bathroom that I thought would be fine for tile. I asked the home depot worker if this is ok for a subfloor and he said yes. Turns out that it is not exterior grade plywood (I think its called sheething) What are my options? Ive already built the 4 walls on top of the floor and the tub is already installed so I can't completely remove the floor. I know that a certain thickness of exterior grade subfloor is needed. Can I install exterior grade plywood over this other plywood? Can I use a thinner exterior grade plywood because I already have the other plywood down? I am also going to use cement board so I don't want the floor too thick. Would it be better for me to cut out the old subfloor? Can I cut around the tub and walls and only replace the subfloor for the parts that tile are going to be on. I'd rather not cut out the floor.

Daniel Holzman 06-13-2009 09:53 AM

To clarify a few points. The only difference between interior and exterior plywood is the type of glue used to adhere the plies together. At one time, perhaps 20 years ago, interior plywood used non-water resistant glues, and would come apart quickly if it got wet. Currently sold "interior" plywood typically uses "water resistant" glue, not as water resistant as exterior plywood, but way ahead of the glues used 20 years ago, so there is not nearly as much difference between currently sold interior and exterior plywood. The most water resistant plywood is marine grade plywood, very expensive, limited availability, not generally used for construction.

There is no significant strength difference between interior and exterior plywood, the strength is a function of the type of wood used, the grade of wood, and the number of layers. You will not notice a strength difference between interior and exterior plywood from a structural standpoint.

Therefore, the only significance to a tiling job between interior and exterior plywood is the performance of the plywood if it gets wet. Interior plywood is going to give you trouble if it gets wet, exterior plywood is less likely to give you trouble, but you still should design your job so the subfloor DOES NOT GET WET even if you spill five gallons of water on your tile.

The simplest way to minimize the potential for wetting the subfloor is to use ditra under your thinset. Ditra is 100 percent waterproof, so if you get your ditra down correctly, the only place water can get through is at the joints between the sheets. So you plan to minimize the number of joints, and you can apply waterproofing caulk between the joints if you are really paranoid. The combination of ditra, thinset and tile is pretty close to impervious, so as long as you clean up the water within a reasonable period of time, you should get close to zero penetration of water into the subfloor.

This is a long winded answer to the question about use of interior plywood in the bathroom. It is not ideal for all the reasons previously noted, but I would use ditra, caulk the joints with an approved compound (the ditra install guide should tell you what to use), and don't sweat it. Just make sure you clean up any standing water on the floor.

Bud Cline 06-13-2009 12:00 PM


This is a long winded answer to the question about use.....
And never once was it mentioned that all tile installations start with thinset. Whether you use cement board or DITRA, you still use thinset to install the cement board and the DITRA.

THEREIN lies the reason for a plywood underlayment that is assembled with exterior glue and can resist moisture penetration issues.


MrRyan001 06-13-2009 06:15 PM

non-exterior grade plywood subfloor
I understand that thinset is needed to lay the cement board. While I have everything torn apart now, I want to do it right. I am not going to remove the tub so should I cut around it or can I lay another layer of exterior plywood on top of it? If I do lay another layer of plywood, can i make it thinner because I already have a thick layer of non-exterior plywood under it?

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