There are different ways to water proof a shower. We use foam boards and membranes which are fool proof. Some will use what we call 'roll on' membranes or a 'liquid' membrane. One example is Red Guard by Custom. When you're getting bids and the competition tells you they're going to use a liquid membrane, ask them if they'll be using a wet film gauge when applying the membrane. I'll bet they aren't.  What we find in the field is most guys will paint one or two coats of liquid red guard on the walls and think that's good enough....Oh no, my friends. Manufacturer specifications insist you install liquid membranes to a certain thickness. The only way to know that thickness is with a wet film gauge. It measures the thickness of the waterproofing liquid. Just painting on two coats of red guard and calling it 'good enough' is not sufficient and it leads to the failure of many showers and the only way to fix a leaky shower....tear it out. Don't be the victim of a construction nightmare. The industry is moving towards foam and waterproof membranes for a reason...That reason being using cement board and liquid waterproofing is an antiquated installation technique.