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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Plumbing Questions >> Fall on waste pipe. https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1125937338 Message started by Timmo on Sep 5^th, 2005, 5:22pm

Title: Fall on waste pipe.
Post by Timmo on Sep 5^th, 2005, 5:22pm

According to document H1 of the approved documents, for an unventilated waste pipe from a WC a fall of between 18 and 90mm/m is required. However, reading further on to page 10 section 1.20 states the following: If the figures in Table 2 and Diagram 3 are exceeded the branch pipe should be ventilated by a branch ventilating pipe to external air, to a
ventilating stack (ventilated branch system) or internally by use of an air admittance valve. So is 18mm the absolute minimum or for example is 12mm acceptable provided the pipe is ventilated? It appears a bit grey as there are no details in H1 for ventilated pipes.

Title: Re: Fall on waste pipe.
Post by chrishutt on Sep 5^th, 2005, 9:37pm

My copy of H1 (1992 edition) gives the figure for a WC branch as a minimum of 9mm per metre. The maximum branch length (unventilated) is 6 metres. Perhaps you should have another look at your copy.

Title: Re: Fall on waste pipe.
Post by CWatters on Sep 6^th, 2005, 7:22am

I think the bit about "if the figures are exceeded" is referring to the maximium fall not the minimum.

My understanding is that if the fall is too steep the flow can suck the water out of the U-bend. Thats why the solution is to ventilate the pipe.

If you are under the minimum then the flow rate isn't fast enough to keep the pipe clean and it's prone to blockage.