Oh Oh Doug. Now you have done it.
The story goes like this.
In the TCNA and TTMAC Specifications it mentions you need to follow the installation instructions of the product supplier. In Schluter's instructions it allows drywall. So, if you follow the TCNA and TTMAC recommendations about following product manufactures recommendations you get around this - so they say.
I went to my building department in North Vancouver and asked. I was told no.
I called Schluter and asked and they said Yes.
There is no inspection for this so really you will not get caught.
Other than Ardex I have found no other company recommending drywall. I asked my Mapei rep what thinset he recommends for installing Kerdi over drywall in a shower and he said none.
Ardex hooks you up if you want to go this route - but why would you?
There is more to the puzzle since if the drywall has been tapped with drywall compound then you need to prime. But they won't tell you which primer. Just it should be primed.
It's so easy using drywall instead of switching from drywall to backer board. No extra stud(s). pennies a sheet to install for most builders. Fast. Quick. Cheap. Easy. Tiny #6 x 1 1/4" screws.
Lots of drywall crews bang in Den Shield. The prep for the tile included in the Drywall crews scope of work. A good boarder making about $5.00 board he installs. Average sheet of concrete board requires about $6.00 in fasteners. Not much sense as a drywall contractor to use concrete board. If the city does not check and no one cares. Cheap and Easy wins.
Most times these shower codes (mentioned above) references refer to a typical shower pan with a 40mil liner(or hot mop). Concrete board or Den Shield approved for use over this liner as a backer board (if seams are tapped). These backer Boards approved just like that - nothing else. That is with no added topical waterproofing. Lapping some building paper or vapour barrier over the liner before boarding a wise idea if you go this route.
Using Kerdi is an upgrade and drywall used in this typical scenerio (installed into shower over liner) a recipe for disaster if it starts wicking water up the back. Kerdi used over drywall with a Kerdi drain and Kerdi shower pan much safer - but again Why take any chance.
When selling Topical membrane you are selling an improved shower design. How cheaply you get the benefits depends on how much effort you take and what measure you enlist.