I'd say that is more than a 1/4" Jim.
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So I checked in with Schluter today and ask some questions;
I asked about corner beads and was told you could or you could skip them. I asked about drywall compound over drywall and was told by the USA rep that dry set mortar will not stick to it. I was told to use a latex primer first. When I called Schluter Canada I was told to be careful of which latex primer to use - some can re-emuslify from the non-modified thin set and cause a bond breaker. I asked which latex primer Schluter endorses and was told to email Dale Kempster.
Dale was out today but did send an email off to me that he would get back to me tomorrow.
Jim do you know which latex primer's work with thin-set? Do you know which ones are endorsed by Schluter? It's odd that the US Schluter tech girl didn't mention this. Apparently on the Kerdi Website some where it makes mention of latex primer if drywall compound is used.
I also checked with one of the heads at NTCA about Jim's claim that the TCNA endorses a shower install with dry wall and Kerdi. This NTCA official was unaware of any system allowing drywall.
I also checked with my Laticrete and Mapei reps about their thoughts. Both reps felt this practice is wrong and that it falls outside of industry guidelines.
So you have reps from Laticrete and Mapei telling me it is bad procedure and not industry standard. You have a head official at the NTCA who tells me it is not recommended by TCNA or NTCA. The TTMAC and ANSI specification guides state drywall is for dry locations only.....
Jim and Schluter say it's OK.
If the Kerdi is installed perfectly.
If drywall compound is not used (if used - a non re-emusiflying latex primer is needed)