This guy is one of the top tile inspectors and consultants in North America.
Check out his blog site. http://www.uofcts.org/2012/08/to-slope-or-not-to-slope/
This is a common installation for a shower floor that is not utilizing a shower ‘pan’, which has a built-in dam on all four sides. But, the floor should be thicker than ¾”. It sounds like they may be making a simple ¾” slab of marble and warping it so the water flows towards the drain. When installing a warped piece of marble for a shower floor, it leaves it vulnerable to cracking if the mortar doesn’t fill in all the voids. There is a shower base mold made for this and is commonly referred to as a Universal Shower Pan Mold. The proper slope is built in to the mold and the bottom lays flat on the floor. No warping and no chance of cracking. Another option is to use a shower ‘pan’ which would not require the vinyl but these only come in standard fixed sizes. If the area you’re installing a shower is not an exact size of a standard size shower ‘pan’, you can always have them install a bench if there is enough room or a shelf if it’s only a little bigger than a standard size shower pan size. Summing up; don’t install a ‘warped’ piece of marble for a shower floor. It’s a disaster (crack) waiting to happen. Find someone who can produce a shower pan from a mold made for this purpose.Attachment 23682Attachment 23683Attachment 23684
Originally Posted by kfp