A lady grinds
Corner Blocking diagram, referenced on site
— Kerdi vs. PVC/CPE liner
Throw out the poly sheeting. You need rubberized shower membrane material. Poly won’t cut the mustard.
On the basement slab, you need a bond break. I’d use either 15# building felt of even Ditra. You don’t want the shower’s base bonded to the slab. Trust me, it isn’t going anywhere.
Then, you need a pre-pan that is sloped to the drain. Your water membrane will sit on this.
Let’s talk “mud.” You DO NOT USE CONCRETE in a conventional sense. You must use a mix of three parts clean dry sand to one part portland. This mix is available at tile suppliers. Per 50# sack, you mix in 1/2 gallon of water…No more. The lack of water makes it super strong. It will be the consistency of wet sand, but not “plastic” like concrete. The mud is packed into place using a trowel or float. Pack hard. It should be no less than an inch thick at the thinnest part, give or take a little. Sprinkle a little water on the surface once you’ve got it all shaped and looking good. I’d also recommend covering it with something (like the poly) to help it cure without too much evaporation. Once dry, remove the covering.
Once the pre-pan is cured for a day or more, apply the membrane. The membrane goes at least several inches up the surrounding walls and also gets incorporated into the curb. Use no fasteners except at the very top…Roofing nails work well, nailed into the studs. You’ll have to fold and overlap at corners, and they also make a solvent weld cement to make it easier. The membrane must be incorporated into the drain. They make special drains that clamp the membrane, available at tile suppliers. Also, your membrane should go under your cement backer board on the walls. Do not fasten the cement backerboard to the studs through the membrane. You need a few inches to hang down with no nails or screws. If you penetrate the membrane too low, it’ll leak.
Now set your drain.
Now you’re ready to place your final mud base. It is done the same way as the pre-pan, with the same mix, packed in place. I did mine about 3” thick, tapered to about 1-1/2” at the drain. You must have at least 1/4” per foot of slope. In the same “pour” as the final base, you should form your curb and place it as well. Same material. I always incorporate some strips of metal lath for reinforcement. That’s overkill, but it makes me happy. Be sure to slope the curb toward the drain if you’re using a swinging shower door. If glass/track are being sealed to the curb, the slope is less critical.
You’ll need some basic concrete tools to do the pre-pan and base. A 12” wood or magnesium float is perfect. I’d also suggest a small trowel, like a margin trowel. These are helpful for packing and smoothing the base as well as the curb.
I’d recommend going to a good tile supplier to get your membrane, base mud, and thinset. Their quality is typically better than what you can get at box stores. DO NOT USE PRE-MIXED THINSET. And for God’s sake do not use mastic. Use dry mix modified thinset.
Just two more suggestions…
Be sure that you waterproof your walls’ backerboard. Cement board, grout, and tile are not waterproof. RedGuard is a paint-on product that works great. It goes on the board, and once dry, you can tile. This is an incredibly important step.
Also, don’t forget to mud and tape the cement backerboard’s seams and corners. Use mesh fiberglass tape made specifically for backerboard. Use thinset to fill the joint, bed the tape, and cover the tape. Just like drywalling, only looks don’t matter.
Hope all this helps.”