I have seen many tips and advice on the internet and rarely do I find the most important thing you will need to know when building a shower tray. Every once in a while I buy a book on home repair or bathroom remodeling that mentions the most important thing to know when building a walk-in shower, but I rarely see it anywhere else.
Most people waterproof their shower trays and never install a sloped surface underneath. There are a couple of ways to waterproof your shower tray. The first and most commonly used today is called hot mopping or hot tarring, a thick felt roofing paper. These shower pans seemed to work pretty well, but they can crack and start to leak.
The second most common way is to use a plastic liner that looks like an above ground pool liner. This material works well, but can often rip or tear if not assembled correctly. Each of these methods will be more effective if you install a sloped surface under them.
This surface will rise around the entire perimeter of the shower tray and will slope towards the drain. I rarely see anyone do this, but it used to be common practice. It can be made with concrete or plywood.
Any of these work well, you can even use concrete over a wooden floor to produce a sloped surface. However, I would recommend using concrete on concrete and wood on wood. The reason for sloping the shower pan area under the waterproofing membrane is so that water that gets under the shower floor tile can safely work its way down the drain, instead of remaining on the flat surface. , for long periods of time.
When you do not install a sloped surface under your shower tile waterproofing membrane, you can expect water or moisture to collect on the flat areas around the shower tray, under the tiles. Water will also soak into the concrete or cement used to slope the shower floor, directly over the waterproofing membrane.