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01-18-2017, 08:37 PM
Hey everybody,

This is my first post but I've been browsing the forum for a while now. I've searched the boards and found posts that have been similar to my situation but I wanted to provide my specific info and see what you guys think.

I want to lay 1/4 inch thick 12x24 porcelain tile in my kitchen and bathroom area that is 160 sq feet. I ripped out the old linoleum and particle board to expose the 1/2 inch plywood subfloor which is nailed down to 2x8 joists spaced 16" oc. The longest unsupported span is 11 feet with some 2x8 blocking in some areas. My house in located in NY with a poured concrete foundation. When i put my info into the deflectolator it says my joists can support ceramic tile. The joists are in great condition with no sagging, knots or cracks. The 1/2 plywood on top is in average condition but it sags and creaks in a 2-3 areas where the seams are not supported by the joists.

I'm trying to match the tile floor to a 3/4 inch high hardwood floor in the dining room as closely as I can. Can I screw my current 1/2 subfloor down more securely and glue and screw another 1/4 or 1/2 inch exterior grade plywood on top of it and then lay 1/4 Hardie Backer board and then the 1/4 tile? I know Hardie Backer wants a minimum of 5/8 subfloor so if I lay 1/4 or 1/2 onto my current 1/2 I would meet those requirements?

Thanks in advance,

Tom :cheers:

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01-18-2017, 09:44 PM
Welcome, Tom. :)

If you'll add that geographic location to your User Profile it'll remain permanently in view to aid in answering some types of questions.

Your joist structure is adequate, barely, only if the joists are of good species and grade. Only you can determine that for us.

Your subfloor is not. At all. I'm aware of no structural plywood available in nominal half-inch thickness with T&G edges. You could only produce a usable subfloor only if the existing surface is in such condition that you could glue-laminate a second layer of half-inch plywood onto it with a full spread of wood glue and lots of mechanical fasteners. Otherwise your best bet is to consider the existing layer nothing more than an unwanted spacer and install a proper subfloor of nominal 3/4" exterior glue plywood with no face of grade lower than C over what you've got as though it wasn't even there. If height is a serious concern, removing the existing half-inch material and starting over would be my recommendation.

My opinion; worth price charged.

01-18-2017, 10:27 PM
Hi CX, Thanks for the reply,

The joists are solid. The house was built in 1964 and has 1/2 inch subfloor throughout the first floor. 3/4 inch thick hardwood is laid on top of the 1/2 ply subfloor throughout the 1st floor except the kitchen. I guess my only option if I don't want to rip out the 1/2 subfloor and replace with 5/8 or 3/4 T&G and have a big transition between tile and hardwood is lay 3/4 hardwood in the kitchen. I'm pretty sure hardwood does not need the same structural stiffness that tile, thinset and grout do.

01-18-2017, 10:32 PM
Or could I screw and glue a layer of 1/2 exterior grade plywood onto the existing 1/2 and follow it with 1/4 Hardie and 1/4 tile and make up the 1/4 difference with a transition piece?

01-18-2017, 10:35 PM
The best option would be to tear out the 1/2" and replace with the 3/4" t&g. And in regards to your first post, 1/4" plywood should be used anywhere in a tile installation, and just an FYI hardi backer doesn't add any strength to a floor structure no matter its thickness.

01-18-2017, 10:38 PM
That is an option as long As you understand what CX was saying, you'll need to cover the entire subfloor you have now with a full spread glue, and screw the living daylights out of the next layer to laminate it together. Don't forget the thickness of the thinset that will be under the hardi, and under the tile.

01-19-2017, 04:55 AM
Since the cement board doesn't add any structural support could I tile directly over the second layer of 1/2 inch ply if I used a flexbond or polymer based thinset?

01-19-2017, 06:59 AM
I wouldnt. Your best options to install over would be...
Uncoupling membrane
Peel and stick like greenskin

01-19-2017, 10:41 AM
What Ryan said.

Well, except for the Greenskin.

My opinion; worth price charged.

01-19-2017, 11:14 AM
Thanks guys. What do you suggest to use between the two pieces of ply? Construction adhesive, Titebond? I know the second layer of ply should be laid perpendicular to the joists and the seams should be staggered with the layer underneath but should it be screwed into the joists as well?

01-19-2017, 11:33 AM
Tom, here's a good article from our Liberry ( on what I think is the best method of installing that second layer of subflooring with the exception of the glue. For glue I'd recommend Titebond II. Haven't done it for a while, but I think you can get maybe 2 1/2 sheets per gallon of glue. Don't skimp.

You probably don't want to intentionally fasten into the joists with the second layer, but I'm not sure it matters a whole lot with the situation you have. Make sure the first layer is well fastened before you install the second, though, and I'd strongly recommend you pre-drill the screw holes in the second layer

My opinion; worth price charged.