Waterproofing a shower.....

When building a shower the old school way, you're supposed to do a pre-slope and then apply a vinyl sheet followed by a second coate of floor mud. This is because water will go right through the grout, thinset & floor mud and make it's way down to the vinyl sheet. The grout, thinset and floor mud are all porous.

But using the new school methods, you lay a waterproof membrane and seal the seams with thinset.

My question: if the thinset is so porous that you need the vinyl sheet under the floor mud, how is it able to act as a sealant for the membrane?

 

You have minimum 2" of overlap when taping the membrane fleece that is used on seams, corners and fasteners, and the very thin layer of thinset that the fleece gets bedded into gets flattened out and squeezed to the point that once the thinset dries in the fleece, the seam is for all intents and purposes waterproof. If you go less than the recommended overlap, the joint will not be warranted as waterproof.

 

I understood the need for the overlap. But if a porous material is used as the sealant, can't water migrant between the layers?

 

If the overlap is less than 2" it can. Over 2", they say it's good.

 

I converted a tub and shower to a shower only a few years back. I used a heavy duty membrane at the bottom wrapping around the sides 4-5 inches high up the walls and all the way to the top of the curb. The corners need to be flattened in such a way that they won't create lumps when placing the wall layers over top and secured to the studs, typically stapled. I used concrete sheet all around the shower because the old stuff the builder had put in was falling apart (water resistant sh*t, my neighbor had to have his redone on account of that). I then used a liquid rubber compound to paint on several coats of that all around the shower walls. After that I laid my tiles. Good luck.