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I am installing natural 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?

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2 Answers

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.

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The second layer of plywood is not to be considered underlayment as I understand it. Is this incorrect? "underlayment-- a layer between a subfloor and a finished floor that facilitates leveling and adhesion." –  Joe Philllips Feb 14 at 20:13
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In all my years of carpentry, the first layer over the joists has been subfloor. The second layer COULD be a hardwood finished floor, OR underlayment to better prepare the base for a stone or ceramic tile or linoleum or other finish similar to that. –  Jack Feb 15 at 6:10
    
Alrighty, I am Ok with calling it underlayment then. I assumed in a tile situation it is whatever would be allowing the subtle movements between the layers –  Joe Philllips Feb 15 at 7:13
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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.

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In my case the 2nd layer of ply should still be considered the subfloor. This is where I'm getting thrown off. –  Joe Philllips Feb 14 at 22:52
    
Doesn't matter what you call it. Bottom line, Don't attach the 1/2 inch to the joists. –  Edwin Feb 15 at 1:17
    
Ok sounds good. I read somewhere that I should not glue the two layers of ply together. Any comment on that? Also, how flat does my subfloor/underlayment need to be? The farthest range (using a laser) was 1/4" from lowest point to highest point on the joists –  Joe Philllips Feb 15 at 3:04
    
I think that some don't like to glue the underlayment, because they feel that they might want to take it up at a later date. You'll have to consult the manufacturer of the tile for the specs on flatness. –  Edwin Feb 15 at 3:22
    
The larger the tile the flatter the floor has to be. Not necessarily level, although I don't like saying it, but it has to be on the same plane. The smaller the tile, the less critical the flatness of the floor needs to be. Underlayment for 18X18 tile need to have less than an 1/8" up and down under a 3' straight edge, preferably a longer one. 12X12 not much break on the flatness, but when you get down to 6X6 and smaller, the tile is very forgiving over a floor that has more deflection in it. –  Jack Feb 15 at 6:43
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