Slate on Deck -Advantech, Durarock, Ditra (Is Durock Needed)

[hevnbnd]

Building a deck that is NOT over an occupied space. Tiling two landings on it that are NOT covered. I am in the South. I was planning on possibly 2 layers of Advantech, a layer of Durock, then Ditra. After tons of research I have about decided on just using ONE layer of Advantech and then Ditra directly onto it. I have planned for a 2% slope.

According to http://www.schluter.com/media/DITRAH...v=201405080617 I can not see anything that says if the Durarock is needed.

Any advice would be great.

[Tool Guy - Kg]

Hi Brian,

Yes, you need cement board. Page 16 of the Ditra Installation Handbook you linked to shows just that. And the USG brand of "Durock" would work perfectly well as it's exterior rated.

Are you going to flash the Ditra into the siding using KerdiBAND or Kerdi?

[jadnashua]

Natural stone typically calls for two layers of ply (or in this case Advantech). Not sure why they'd changer that over exterior wood structure...might want to double-check.

[hevnbnd]

Deck is not going up against siding. Deck is free standing and will be held 1" off Brick wall.

According to page 16 on the document previously linked it states to use a supporting bed of unmodified thin-set
mortar over the substrate. However I have also ready that I must use a modified thinset mortar meeting ANSI A118.11 requirements to bond anything, including Durock to a wood subfloor. Any input on this?

"Not sure why they'd changer that over exterior wood structure...might want to double-check. "

Lost me on that sentence...

[jadnashua]

Natural stone tile everywhere inside, requires TWO layers of ply before you install any tiling substrate to it (cbu, Ditra, etc.). Spell checker isn't as good at catching some errors verses others!? Not sure why they'd change that for an exterior install which is why I suggested you check with them for clarification.

Most cbu's don't really care what thinset you use under them. If the one you choose requires a modified, use that. The thinset's primary purpose is to ensure there are no voids and the panel is fully supported...the screws or nails hold it in place. But, manufacturer's instructions trump generic info.

Depending on the size and shape of the area, if it is big enough, it will require expansion joints.

[cx]

Brian, if you'll look carefully at the Schluter detail for the exterior installation over wood substrate, you'll see that the instructions for item #1 in the drawing, the wood structure, says the "subfloor/underlayment configuration" shall be per one of the interior details listed as appropriate. In your case that would be D-W-S, which requires a properly installed double layer of plywood or OSB.

[hevnbnd]

Thanks for the help! Got my 2 sheets of Advantech screwed down today! Going to start on 1/2" Durock tomorrow. I have some Fortified VersaBond thinset. Should I use it under my Durarock? What is the difference from Fortified and Modified thinset? Thanks

[jadnashua]

They are the same thing...just to confuse people. Modified, fortified mortars are the same thing. The important thing is to verify that the ANSI spec listed on the bag corresponds to what the instructions for the products you want to install call for. If they want a modified, it is likely A118.11, but check.