My 65mm shower drain is connected to the 100mm toilet out-flow below the toilet u-bend below the first floor bathroom.
When the toilet flushes the fouls tend to cause a vacuum at the shower u-bend which allows the foul smells from the public drain to percolate through the bathroom.
The toilet drian is cased and therefore inaccessible. I had thought of making an extra u-bend in the toilet drain to slow down the "slug" effect.
Any ideas as to how to maintain a water barrier to the shower drain?

Brian Hedley, January 2005

I answered the same question some time back [Holland ring a bell?]
Follow Johnny's advice and re-route the waste.
Or an alternative method insert an Air admittance valve in the shower waste pipe on the run between the shower and toilet waste pipe, then the water should not be sucked out of the shower seal trap when the wc is flushed.

Go to www.wavin.co.uk this is the Osma pipe & fittings website. Select 'Technical tool box' on the top status line
then select the following in the order you see them 'Installation guide', 'Above ground drainage'
'Waste & Traps' 'Osma push fit waste Installation guide'
' Osma Vent 40 Air admittance valve'. Read the top of the page then scan down to see the drawings. the part number in my book is 4S.303 pge 30 of the Wavin/ Osma trade booklet
price £26.69. but expect more as that's trade.
you can also e-mail them info@wavin.co.uk they may well come up with another cheaper idea.
bob

bob, February 2005

try taking the shower waste pipe outside and into the main soil stack-toilet is causing vacuum because theres no air ingress at the joint between toilet/shower so it sucks water out of trap-be sure to blank off old shower joint to toilet

johny, January 2005