Proper way to install double-layer of subfloor?

Question

I am installing natural stone tile, so I've had to reinforce the joists a bit and I'm currently working on installing a new subfloor.

The subfloor is 3/4" plywood with 1/2" plywood on top of it. The underlayment will be Ditra.

My main question here is about how to secure to the joists. Normal application with just 3/4" I would squirt down some construction adhesive to the joists, lay the 3/4" layer and screw 8" on edges and 12" in the middle. This becomes slightly more complicated because I have two layers now and I need to know whether I should still do the 8" and 12" method on the first layer as well as on the second layer, or if there's a different way I should be approaching the screwing and gluing.

As a side question, should I glue the first and second layer of plywood together with construction adhesive?

Answer 1

Fasten your first layer plywood subfloor as you normally should.

The second (top) layer of plywood joints should not coincide with the joists and the second layer should not be fastened to the joists.

Article quote from - Position of Underlayment to Prevent Cracked Tile and Grout By Frank Woeste and Peter A. Nielsen For publication by the TILE LETTER Magazine (read whole article):

The 2003-2004 Tile Council of America’s (TCA) Handbook for Ceramic Tile Installation contains numerous details for a double layer wood floor system supporting ceramic tile. The thicknesses of the subfloor and underlayment are given in each case. Specific guidance on where to butt the underlayment end joints is not given for any detail. For example, for F142-03, the TCA Handbook states, “offset end and edge joints of the underlayment panels by at least two inches from the joints of subfloor panels; they should not coincide with framing below." It further states, “underlayment fasteners should not penetrate joists below." In the case of F150-03, the offsetting is not mentioned, but it does state, “underlayment fasteners should not penetrate joists below." The same holds true for F155; however, it also states, “face grain of plywood should run perpendicular to trusses, I-joists, or sawn lumber for maximum stiffness." The purpose of this article is to propose specific guidelines for the orientation and placement of underlayment, including end and edge joints, beyond the rules given in the TCA Handbook, to improve the performance of double layer wood systems. These guidelines are based on engineering science and field observations.

Also, since you will be using Ditra, make sure to stick with their guidelines for warranty purposes. Specifically see page 8 and 23 from The Ditra Handbook for your application.

Answer 2

On the 3/4" subfloor, you have the right idea. Use subfloor adhesive, and screw 8" at edges, 12" in field. For the 1/2" underlayment, ideally, you want to screw every 6" all across the panel. If you use adhesive for the underlayment, you can get away with fasteners every 8" using ring shanked nails. Using my experience, I would fasten the underlayment with adhesive and 7/16-inch crown staples every 6 inches driven with a pneumatic stapler. The stapler saves a lot of time. You want to choose fasteners that will not penetrate the bottom of the subfloor. This means that for 7/16" underlayment and 23/32" subfloor, the longest fastener you can use is 1-1/8". Adhesive will help you here, as the subfloor plywood will have voids in spots that will not hold a fastener. I use the term "underlayment" to mean a layer above the subfloor, but not the finished floor. In your case, you need the underlayment to prevent deflection along the edges of the subfloor between the joists.

Answer 3

1. Make sure first layer is fastened to national code.

2. Install second layer with all edges off set, leave 1/8-3/16 gape and fill with silicone.

3. Nail or screw 6 inch on edge and 8 inch in the field.

Side note: Follow MFG specs.