Shower heads, wastes and pipe flow rates

7 Jan 2010
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United Kingdom

As part of my bathroom refurbishment, I'm looking to install a new shower and shower tray, which will also require new waste piping to the outside stack.

Ideally I want a shower tray that is 900mm wide x 1300mm long with the waste at the narrow end but this seems to be an obscure requirement. I also want the shower tray to be flush fitted to the tiles as it is a walk in shower.

The best fit I have seen is the Merlyn Truestone rectangular shower tray 900mm x 1200mm, however, this is only 30mm deep and appears to have very little fall. It comes with a 90mm "fast flow" waste, which achieves a 28 litres per minute flow. This doesn't seem particularly fast to me given that McAlpine make one capable of 42 litres per minute!

I plan to fit a ceiling mounted "rainfall" type shower head, probably the 30cm round diameter size. The current waste pipe for the shower is 40mm, but I believe upto 50mm pipe can be used for waste. The shower head will end up being fitted directly above the waste, which according to the Merlyn website, is something that should be avoided. I assume this is because the fall of the water interferes with its ability to smoothly flow out through the waste. Unfortunately, the only way to avoid this would be to turn the rectangular tray around so the waste is at the "entrance" and therefore the furthest point from the outside wall, meaning 3, rather than 1 joists would need to be drilled/strengthened to accommodate the waste pipes.

Given this is a very shallow tray, I want to ensure the tray doesn't fill up with water as there will be nowhere for it to go apart from over the bathroom floor.

What do I need to consider to ensure this doesn't happen?
-- Shower head flow rate
-- Waste pipe size
-- Waste flow rate - is 28l/min good?
-- Size of grate in the shower tray. I assume this is the first point that would limit the flow, so there's no point in using a 42l/min waste if water can't get through the shower grate faster than 28l/min?

Thanks in advance for your advice.
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