Waste from new shower

27 Nov 2007
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United Kingdom
I am installing a shower in an en-suite and I am wondering on the way to proceed with the waste in view of the possibility of back-flow. The shower is being installed next to the bathroom and I wish to run the waste pipe through the wall and join it up with the waste pipe from the bath, approx 1 metre from the bath u-bend.

At this point the plastic bath waste pipe is connected to a 40mm copper waste pipe which disappears under the flooring and runs approx another metre to a boxed-in internal stack pipe joining it below a corner branch to two wc's. Running the new shower waste pipe direct to the stack would be quite a task

All being well can I assume that all the bath water will run away to the stack pipe without backing off to the shower. If this could create a problem should I consider reducing the bath waste to 32mm for a short length before the tee where the shower waste joins it or put in a non-return valve into the shower waste if there is such a thing or should I be considering some other way.

I would greatly appreciate any guidance on this. Thank you
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min for a bath is 40mm waste
32mm is for basins
make sure you have the proper falls on the wastes and it shouldn't go backwards.

have you not a drawing or pic of how you want it ?
Thank you for reply however the bath and shower are at the same level and I am concerned about the flow when I release a bathful of water and its height and consequent pressure.
Will it fully drain away down the 40mm waste pipe or is there the possibility that it will flow back up the shower waste pipe. This is my worry and I wish to take any steps to prevent it at this stage.

Please can you clarify this for me. Thanks
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This work is governed by Building Regulations; in addition to the minimum diameter of 40mm pointed out by Seco, you must have a minimum fall of 18mm per metre & the maximum length of an unvented branch is 3m for 40mm & 4m for 50mm dia. waste. If you connect the shower into the bath waste run, you will almost certainly need an air admittance valve in any case or you will continually draw the water from the waste traps. In all cases the best method is a direct connection to the stack. Download & read Part H of the Building Regs. here:


Also be aware that if you’ve formed a new en-suite in the bedroom next door, it is “notifyable work” & you should have contacted Building Control; they will want to inspect the work to ensure Building Regs. compliance; no completion certificate could give you problems in the future.

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