How should I install a plywood subfloor that will be covered by granite tiles?

Question

I am replacing my entryway floor and I have removed the old mud bed. I'm left with planks perpendicular to the joists. However the room is defined at a 45 degrees angle from the joists, so the planks and the joists underneath them appear diagonal (it looks like an X).

I plan to install one layer of 3/4 inch plywood over the planks, and a layer of 1/2 inch plywood over the 3/4 inch plywood.

Along which direction should I install the plywood? If I install them along the directions of the room then they will be at a 45 degrees angle with the planks and the joist. I know that usually plywood are installed perpendicular to the joists but what about my case? Would this be ok?

When I screw the 1/2 inch plywood, should the deck screws only penetrate the planks or should they go through the planks and penetrate the joists as well?

Answer 1

Would like to put in a suggestion for either Schluter Ditra or Laticrete Strata-mat.

As uncoupling membranes, they effectively stiffen floors by unitizing the tile layer. For stone installation, Schluter recommends double layer of underlayment. The second layer should be offset from the first layer, not following seams. The second layer should only be attached to the first, not through to the framing. The total thickness only has to be 3/4, but can be thicker as necessary for level matching. The first layer should be at right angle to the joists, along the the long direction of the ply.

Ditra stone tile install

Ditra plywood install

excerpted from Ditra installation handbook

Answer 2

In your question you reference installing granite tiles. You will want to put these on 1/2" concrete board not plywood. They sell concrete board screws and these work just fine.

Answer 3

1. Laying plywood on diagonal joists is ok if the spans aren't too far from each other AND from the edges. Fastening on diagonal joists requires more precise work because you have to "aim" at joists of a different direction than the edges. Drawing some guidelines might help this.

2. The longer screws the better fastening and tying all the deck together (not indefinitely :). It's unclear whether your entryway is heated or not. But if the temperatures differ more than in other rooms or if it even freezes sometimes there, strong fastening is a must.