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Re: Tiling bathroom floor with DITRA Underlayment
maplemale wrote:

This is the first I've heard of this stuff...

So, how strong does my sub-floor need to be? My test which has served me well over the years: If a 200lb guy can jump up and down on the hardiebacker, and it doesn't give / move at all, I'm good for tiling it. Usually I end up with 3/4 inch subfloor and hardiebacker. If my subfloor is something in the 5/8- range, I end up having to supplement it with more sub-floor and possibly blocking between joists to get the zero movement.

I guess my question is, how does a DITRA underlayment help with any of that? Or does it? For instance, I can a get away with a 3/8 subfloor on 16 inch center joists or even TJs if I'm using this stuff? Or, is just a little bit of extra protection on top of my stable subfloor in place of hardiebacker? If that's the case, then it seems like it's more expensive and doesn't provide a whole lot of extra benefit?

It is for decoupling the plywood (or concrete slab) movement from the tile job. Lateral (shrinkage/expansion) movement, not flex. You need to shore up/confirm floor system stiffness before proceeding with any large-format tile or stone job.
No, it won't magically turn thin plywood into a stiff subfloor. It's better than hardibacker, because it's thinner, and it's waterproof.
BTW a 3/8" subfloor is never structurally sufficient under any circumstance. 5/8" fir ply on 16" center framing is OK for carpet. It's a rule of thumb that you need 1 1/8" of plywood on 16" OC framing for tile jobs. The framing is ideally at least L= 1/480 (deflection factor 1/480 of the span length.) L720 is a factor safer but of course is more costly because of more material.


Re: Tiling bathroom floor with DITRA Underlayment
HoustonRemodeler wrote:

TCNA is the Tile Council of North America which sets the industry accepted standards for setting tile, making tiles, along with installation methods and materials. They've been printing these guidelines for 50 years. Recently they have incorporated natural stone and glass tiles. The TCNA works side by side with ANSI, the UPC, and international bodies of related industries.

Ditra isn't waterproof on its own. To achieve waterproofing, the sheets must be seamed with kerdi band as prescribed by Schluter. Ditra does offer some measure of in-plane sheer movement protection, which is its main claim to fame for the last 27 or 28 years. There is no membrane which offers protection from vertical displacement. Ditra is used to replace CBU for several reasons; moisture management, thickness (DitraXL for example) labor savings, weight savings, speed of installations, and integration into an overall waterproofing system such as balconies or decks.

All of the manufacturers of CBU's agree that ceement backer boards add nothing to the reduction in deflection. Visit their websites to double check if you'd like. You will find no such claims of reducing deflection even when properly installed.

If you want to discuss the measurable, scientific standards for the "Jump Test" I'd be happy to do so. But please, for the sake of the DIY folk reading along, do not try to pass the Jump Test off as some sort of industry standard which they should find acceptable, even if you do.


I work with Paul and, if you want to trust someone that relies on the "Jump Test," God bless you.....but keep Paul's number. You might very well need it in the NEAR future. You wanna "Jump Up & Down" on the floor and try to convince any reasonable homeowner that you know what you're doing? Good luck with that......


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