Slate Deflection in Condo and 1/2" vs 1/4" CBU?

[risotttto]

Ok, Maybe I'm going crazy, but of course I now am just reading the Ditra Installation Handbook, and on Page 6 for Natural Stone it states you just need a minimum of 3/8" underlayment on top of 3/4" subfloor, and maximum joist spacing of 24"

Is this legit? Some sort of Ditra magic that invalidates the 16" joist requirement of F250 Stone and makes it 24", and one sheet 3/8" underlayment (OSB even) on 24" joists with Ditra, then you can lay 24"x24" slate tiles all day and invite all your friends over for a dance party? No mention of joist height, span or L/720?

[cx]

Originally Posted by S

Some sort of Ditra magic that invalidates the 16" joist requirement of F250 Stone and makes it 24"...

Nope, doesn't invalidate anything in the ANSI Standards nor the TCNA Handbook. It's Schluter's product and they can advertise its use in any manner they see fit.

Entirely up to you if you wanna follow their recommendation.

My opinion; worth price charged.

[risotttto]

Too good to be true as I suspected! So I'm not crazy because I just bought new 12x13 hex porcelain tiles which should work nicely with my deflection and look great.

I also just found the legacy TJI silent floor joist specifications from the late 80's, and calculated my defection ratio to L/458. Give or take 20% for some sloppy construction, and I'm right around L/360 plus the extra underlayment should be safe for 13" Porcelain for ages to come (thanks everyone again).

The bookshelf I built in needed to be scribed over an inch over 8 feet to flush up to the ceiling, so I'm guessing there's more sag than calculations predict!

Also for future visitors to this thread my quick calculations spreadsheet is attached.

Was considering trying out Ditra for the first time, maybe will do it on a smaller project that does not involve countertops, cabinets, and appliances!

[Tool Guy - Kg]

Any chance you’re using heavy stone countertops? If so, are you installing them before or after the floor tile?

[risotttto]

Tiling wall to wall first then countertops after the cabinets go in, the kitchen is also touching the exterior wall, so not placing all the weight in the middle of the span.

Really love Soapstone, but considering Corian, found a color that looks nice, and 27 square feet of soapstone is $4000! Would 20 PSF of soapstone really mess with the floor?