Flooring Tile - Fastening backerboard: adhesive or not?

Question: I plan to use 1/4" Wonderboard under slate on our bathroom floor. Should I embed the board in thinset and screw it down, or just screw?

Question 2: Do I need some kind of waterproof membrane, and if so what should I use and how/where?

Other details: Joists are 16" OC, 11" TJI 350 series, span is 10'. Subfloor is 5/8" Advantec (a high performance OSB) over which I plan to install 3/8 exterior ply as an underlay. Backerboard will go on top of the underlay, and then a Warm tiles floor heating system embedded in scratch coat, and then the slate. Moisture on the floor will mostly come from stepping out of the shower or tub.

Thanks in advance for sharing your expertise!

Should I embed the board in thinset and screw it down, or just screw?

Thinset is a must when using backerboard, but read on....

Do I need some kind of waterproof membrane,

Why would you?

Other details: Joists are 16" OC, 11" TJI 350 series, span is 10'. Subfloor is 5/8" Advantec (;) a high performance OSB:D ) over which I plan to install 3/8 exterior ply as an underlay.

Good so far.

Backerboard will go on top of the underlay, and then a Warm tiles floor heating system embedded in scratch coat, and then the slate.

The scratch coat is bad idea. If you would embed the Warm Tiles in Self Levelling Compound you can eliminate the backerboard and level the floor beyond the heat mat at the same time. This would eliminate the need for the backerboard.

Moisture on the floor will mostly come from stepping out of the shower or tub.

Ah-ha!!! So no need to waterproof the floor then, huh?:)

And since the slc will give you such a nice flat surface, ditra over the slc will give you the waterproofing that you are looking for.:)

Terrific Bud...thanks so much for the fast response. I love your suggestion because it reduces floor height, labour, and likely cost too! If you'd humor me, I'd love to increase my understanding further:

I've gleaned that backer board is used because it adheres well to thinset, is dimensionally stable, and acts as an uncoupler. It is not a moisture barrier. Correct?

I know very little about self leveling compounds. Do they serve the above roles that are important in my case? In other words, can you expand on your suggestion that by using it I can eliminate use of backerboard?

Thanks again.

HJ...I'm trying to avoid Ditra because of cost. I get the sense that moisture shouldn't be a huge concern given the warm floor and limited dampness to start with. Can you put Ditra over top of the heating system???
Brian

I don't like the proposed subfloor thickness at all, especially for slate. I would want a second layer of 5/8" or thicker plywood underlayment. Over that I would recommend Ditra, although 1/4" CBU is fine.

As for your I joists. I don't believe Weyerhaeuser makes TJI 350 11", are you sure they're not TJI 360-11 7/8"? Either way they are designed to meet L480 at max span. No way to know the rating at your short span, but I think you're ok? You could check with the manufacturer if you'd like. The recommended max deflection for natural stone tiles is L720.

Yes, Ditra goes over heating mats.

Jaz

JazMan...yes, they're 11-7/8 joists. I have porcelain tile over a comparable area on the main floor with no problems, but these were laid by a contractor with no heat system underneath. The Advantec on the main floor is 3/4, not 5/8 like upstairs, and they used just 3/16 underlay like you'd use under vinyl flooring.

Somehow I'd like to keep the floor height reasonable. What about going with the 5/8 ply over the 5/8 advantec, and then the heat system embedded in SLC, and then the tile (without CBU)? Or...if I bite the bullet and go with the extra cost for Ditra, can I reduce the overall thickness somewhere?

Thanks.

Martin,

Ceramic and porcelain have different deflection requirements from natural stone tiles. Natural stone requires a stiffer subfloor system.

I like the plan for a double 5/8" plus Ditra. You might even go with 1/2" over the 5/8" subfloor. We're talking about differences of 1/8" now. It's not a good idea to install tile direct over plywood, high risk of at least a few grout cracks. Not very professional, but it can work in dry areas at least.

It's never a good idea to change methods to arrive at a certain height. You do the job right, then use an appropriate remedy at the threshold. In extreme cases you can even add plywood to the other room too.

Jaz