Subfloor thickness question for 19.2" I joists using 24x48 porcelain tile tile

[cx]

There is no requirement for your subfloor to be level, David, only that it be flat. I understand the problem with getting your nominal 3/8ths" plywood flat, which is why I recommended the nominal 1/2" plywood instead. You'll find the problem with getting the 3/8ths stuff flat to exist whether your existing subfloor is flat or not. Still, you'll need to do any flatening on top of the top layer of subflooring before installing your sheet-type tiling substrate.

Yes, your second layer of subflooring will try to conform to the shape of the existing subfloor, and that is necessary for a proper structural installation of that second layer. The two layers must be in "intimate contact." Then you'll flatten as necessary before installing your Ditra.

My opinion; worth price charged.

[truthisone]

Hi CX, thank you for your reply. Yes, I meant to say flat I have spots with 3/8” deflection between joists. I chose 3/8 one because it was believe it or not Flatter and better quality than 1/2 one at the building supply when I looked. Plus it helps with my total height. I hope I don’t regret not going with 1/2” one

[truthisone]

On another note , is it beneficial to add subfloor glue to the top of 3/4” own prior to placing 3//8 sheets? I am thinking to use zigzags lines every 12” with Great stuff subfloor foam adhesive ?

[cx]

Don't do that, David, you're likely to do more harm than good, ending up with large voids under that very thin plywood.

If you'll install that second layer of subflooring (it's not underlayment) as recommended in this good article from our Liberry, you'll have all you need. But if you still want to glue it, use a full spread of a wood glue such as Titebond II. Not as easy as it sounds, but that's the only way to glue such sheet goods.

My opinion; worth price charged.

[jadnashua]

Construction adhesive works on a joist since it's a small area, and the adhesive will get smushed into a nearly full coverage. It's unreasonable to expect the adhesive to spread to fill the voids when used underneath a sheet as the pressure you can apply with fasteners will end up with a VERY small PSI, so there will be voids where the two sheets do not make 100% contact. A void implies potential deflection and tile doesn't like that!

Laminating the sheets requires full coverage with a more liquid glue, but is still hard to accomplish before the glue starts to set, and you must work from the middle out to ensure you squeeze out the excess without leaving thick or bridged areas...IOW, it's tough. Just installing the top layer as described in the article mentioned works reliably. While laminating, if done to perfection, will make a stronger assembly, it's often not done that well, and can end up worse.

[Tiger Mountain Tile Inc]

Personally, I wouldn't add the underlayment unless I needed to. Most people don't want the higher floor in their home.

With Ditra-Heat and 9mm LFT tiles you'll probably be around 5/8-3/4 finished height when it's all said and done. That's perfect to match up with most carpet and hardwood.

Schluter All-Set is really good mortar and if you use it to install Ditra-Heat and the tile then you get their warranty as well.