I understand that the toilet flange should sit on the finished floor. I've got porcelain tile on top of a Ditra uncoupling membrane for waterproofing. To attach the flange securely to the sub-floor, I would have to screw through both the tile and the waterproofing membrane, which would seem to defeat the purpose of the membrane. Is there another method for attaching the toilet flange when using an uncoupling/waterproofing membrane?
Attaching toilet flange through tile and uncoupling membrane
flangetoilet
Related Solutions
You should screw the flange through the floor, through the subfloor, and ideally into some kind of framing. Use all holes. I don't fully understand the reason for this, but all flange manufacturers are very clear that you need to screw the flange down.
What you are looking for is a riser also called extension flange. But considering you are able to get to the pipe below - you could just extend it to what you need, but cramped space and a crappy job - and possibly not being able to seal it properly well - could be really ...
Humor aside take a look at the links here I think you will find it to be easier.
I do believe they make spacer flanges in @ 1/2 and 3/4 inch increment.
See this link Extension Flange
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Best Answer
You should be fine going through the Ditra and tile into the wood. You don't really need to be concerned about the waterproofing under the toilet as it's open around the closet flange anyway. The uncoupling aspect will not be affected. The process of uncoupling is to allow the subfloor to shift without cracking the tile. A few points fastened down won't hurt. If you are still concerned, use a high grade construction adhesive and hope it's not you who works on it next.