New Construction Tile Subfloor

[Craigsproject]

The project here is a remodel addition of about 1000 square ft that includes a 6x8 half bath with ceramic tile. After days of reading online, I have determined (I think) that for the bath I will use a 2 layer plywood subfloor followed by Ditra and then 12x12 ceramic tile. The first layer of plywood will be 3/4" and perpendicular to the floor joists.The second layer of plywood will be 1/2" and parallel to the floor joists. This will result in only 1 seam in the center of the floor. Since this is new construction, both layers of plywood will be installed before the walls are framed. The big question for me is related to the top layer of 1/2' plywood. How should it be fastened to the structure? Is it ok for the top layer to go under the wall framing and be part of the subflooring in the adjoining rooms or should it be separated somehow? Should I create any expansion joints or perimeter joints to allow for expansion/contraction? Please keep in mind that this room is only 6x8 and I am planning on using Ditra. Kind of a long post but thanks for reading. Any advice would be appreciated.

[Levi the Tile Guy]

Craig welcome,

First off I would wait to put your layer of 1/2" in until the walls are up, then leave 1/4" or so around the perimeter of the room. To Fasten it us screws, but make sure you don't hit the floor joists, also you do no want to glue this layer down. You don't need any other expansion joint just the small gap around the perimeter.

[bbcamp]

Graig, the second layer of plywood should be perpendicular to the joists, just like the first. You need to offset the joints from the first layer and about 4 inches from the joists. Leave 1/8" gap between sheets. Use deck screws long enough to completely penetrate the first layer and extend 1/4" beyond the bottom. Avoid the joists, but don't sweat it if you hit one. The plywood can go under the walls. Stop the Ditra and tile about 1/4" away from the walls.

[Edthedawg]

This is more for Bob...

One thing that caught my eye is the plywood orientation. I know the traditional response is that plywood should always be set perpendicular to the joists. But if he passes the Deflecto for joist structure, and has the 3/4" subfloor layer, then wouldn't it not matter which way the 2nd layer of plywood was oriented in this case?

And for that matter, assuming 16" OC joist spacing, does he even need that 2nd layer? More is always better, yes. But just OOC...

[bbcamp]

You orient the plywood to gain the most stiffness. To do otherwise is to waste money. Granted, if the floor didn't need the second layer of plywood, it shouldn't really matter, but then, why install it? If it covers the entire floor, it is for stiffness, not height.

[Levi the Tile Guy]

Craig, do you need the layer of 1/2" through the rest of your house? If not I sure wouldn't waste the money it. just put up the walls then address the second layer where you are going to tile.

[Houston Remodeler]

The reason you don't need the second layer: ceramic tile can go happily over 3/4" alone. If you elect to use natural stone however, the added 1/2" is needed.

What is the condition of the 3/4"?

[Levi the Tile Guy]

Originally Posted by Paul

What is the condition of the 3/4"

I would hope it's in good shape it is new construction. I know just as well as anyone that doesn't always mean anything though...

[Houston Remodeler]

I visited an upcoming project yesterday. There were a number of places the 3/4" ply was unsupported around the edges. No way I'm pulling a mud floor over that.

[Craigsproject]

Thanks everybody for the responses. My proposed method was based on intuition and I am not an engineer or a tile expert so I will follow your advice. Based on your responses I will proceed as follows:
1) First layer of 3/4" plywood perpendicular to the floor joists screwed and glued.
2) Install the walls
3) 2nd layer of 1/2" plywood also perpendicular to the floor joists but with staggered seams and joints. Seams will be offset by 4" from seams of the first layer. I will leave a 1/4" perimeter gap secured to the first layer (but not the floor joists) with deck screws but no glue.
4) Instead of running the 2nd layer of plywood under the walls to gain floor height in adjoining rooms, I will install the 2nd layer after the walls are up. I will then install nailing blocks on top of the wall sole plates to provide an adequate nailing surface for the drywall and baseboards.
5) Install Ditra with a 1/4" perimeter gap and finally the ceramic tile.
This is for my own house and I really want to get it right (the first time)
Thanks again for your help.

[The Kid]

sounds pretty good! Do make sure the ply wood is the proper kind suited for a tile install and to please schluter anything BC or better exposure 1 is what you want.