I put in a PVC Liner, then my final slope.
Can I Redgard the final slope all the way to the drain or will having 2 waterproofing layers on the shower pan trap moisture or something?
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That is how I do it. I make absolutely sure that the drain sides are covered properly with the Redgard. The PVC liner is a redundancy for me. I coat everything twice with a heavy coat, and the corners get 3 coats. Checking for pinholes on each coat.
If by some chance there is a pinhole, there is only going to be a very minor amount of water that will penetrate, and that is if there is a pinhole. Tile is impervious, I typically seal my grout, making it more water resistant. I treat the dry pack base as if it was not getting any Redgard and preslope the floor under the PVC liner. That way any water that may get through somehow will still get to the weep holes in the base of the drain in the cement base layer on top of the PVC liner.
If you pre-sloped it then you're directing the water to the weepholes. You don't want to trap water with capillary action or water tension like when you block the top end of a straw full of liquid. Same reason why drain pipes need to be vented. You want the water to travel through the mortar bed, hit the PVC liner then work its way to the weepholes using that system. I would say you are good as long as you have it pre-sloped and you aren't creating a sealed compartment to trap the water inadvertently with that capillary action or water tension. It's going to travel towards the drain. Another thing to consider is, if there are any dips in your final slope and you have that waterproofing, then you may have a shallow film of water that sits on top of your tile instead of traveling through the mortar bed like it is designed to do with that system. With that said, I would argue that the pre-slope is more important that the final slope if you are using the PVC liner method. Think about it, it makes sense, 9th grade physics class.