When shower enclosures and trays are displayed in showrooms there is never a problem with waste pipework – they simply don’t have it. In the real world in your bathroom there is obviously a need for waste water to run out from your shower tray quicker than it comes in! If a waste pipe has a fall on it (as per the water regs) then this obviously helps matters considerably along with a fast flow waste trap.
However, many existing shower tray installations have the waste pipe running above floor level (boxed in) with the tray itself sitting on a riser kit/leg kit or wooden base. If the tray is then lowered to floor level the fall on the waste pipe then needs to be under the floor as opposed to boxed in above it. The floor would then need to be taken up to redo the waste pipe back to the stack ensuring an adequate fall. Even then this is not always possible or practical due to floor joist positions, distance back to the stack or the junction where the waste enters the stack being too high or badly positioned and not being able to get an adequate fall on the waste pipe itself.
If the waste pipe just pops up through a hole in the floor under a tray it is often then thought that a new tray can just be laid on the floor and connected up to the existing pipework under the floor. However, this could result in reducing the fall in the waste pipe back to the stack. I have been to many an installation where the floor has obviously not been taken up to check the waste gradient and just had a small access hole large enough to make the final connection from the shower trap to the waste. The waste pipe itself was flat/ bordering on going uphill and would not drain away quick enough for the power shower. The result is the tray fills up with water and the shower has to be turned off to give it time to drain away! -Not a clever situation at all and with no quick solution!
Obviously for those who are unable to step up into a tray then the floor should be taken up to check the waste pipe gradient and if still not possible/practical then maybe a wet room should be investigated?
Surely it is far more practical to have a tray on a riser kit with a removable plinth so the trap is easily accessible for removing and cleaning out, rather than a tray with the trap under the floor with no access and without adequate drainage and a shower that has to be turned off to allow the tray to empty out while the customer waits with the shampoo in one hand and a plunger in the other!