Follow the Ditra manufacturer's installation instructions no matter what anyone else says. They've been doing this forever.

It's a mixed bag of advice that can be due to the differing requirements from the different manufacturers. Or some folks just may not know how to do things.

I highly encourage that you yourself check out the installation instructions for whichever material you are planning on using. They will often give you the A, B, and C steps regarding what other materials you'll need to install their product.

Then you have to read the installation instructions for "A", "B", and "C". That will allow you to see the Big Picture.

With that out of the way:

If you install a SLC over plywood, I highly recommend you use a primer. Always. The primer itself has instructions on how it gets installed with dilution rates and drying times (if needed).

Technique: Some folks prime the plywood, let the primer dry, then set the heating mat. I prefer to minimize walking all over the primer. What I'll do if installing SLC over plywood is I'll set the mat over the plywood, then spray the SLC with a garden sprayer. It coats the mat and wood. Wait the appropriate time for the primer to dry, then pour the SLC.

Some SLCs require some reinforcing material to go down over the plywood before the SLC gets poured; expanded metal lathe, for example. You may be able to omit the metal lathe if you use cement board. Others allow you to omit the metal lathe if you'll be using a mesh heating mat, as the mesh mat will provide some structural integrity to the SLC.

You can often get things settled with a call to the manufacturer's tech support 1-800 line.

You won't find that on the SunTouch pages. You'll find that on the SLC pages. That's where one layer of research leads to the next.

FWIW, If you use cement board over plywood, any time you put cement board on a floor, use thinset to bed the cement board on the plywood. Then use nails or screws to fasten the cement board to the plywood. The thinset acts as a filler between the cement board and the plywood, preventing vertical movement within the floor.

So, with Suntouch? Unless their installation instructions have changed (I'm going from memory here), over a ply base you can set the mat, apply primer, then SLC. Then thinset and tile.

But when you research the SLC, as I wrote earlier, most SLCs also want some sort or reinforcement. Metal lathe, etc.

You get different layers of independent instructions because you're using different layers of non-related materials. Manufacturer "A" may not know or care about Manufacturer "B", etc.

When you stay within a single manufacturer's product line for your entire installation, you can sometimes streamline the process with fewer products, because that manufacturer knows how one of their products will wok with another one of their products.

Example: If you used Laticrete SLC primer, Laticrete SLC, and Laticrete heating mat, Laticrete allows the mesh of the mat to act as the reinforcing material for the SLC. And they will warrant that installation. I found that out when talking to their tech support line about an installation I was working on.

That doesn't mean that the SunTouch mat won't provide the same benefit when used with Custom's SLC. It might. It might not. It just means that most manufacturers won't provide warranty protection, or they haven't cross-tested their products with another manufacturers products.

A lengthy reply to a short question. But hey, I was out splitting firewood and you gave me a reason for an extended lunch break. Back to work...