Long 32mm sink waste (and joining to shower waste) All aspects of plumbing questions and answers, help, tips and information

[allotmentboss]

Hi All,

Two questions, if I may:

(1) I want to change the location of the sink in my bathroom. In order to do so, it would require a 4 metre (or so) waste pipe to get to the soil stack. I can achieve the required 18mm/metre fall throughout and I have an abundance of pipe clips to stop sagging. I read somewhere that the maximum length for a 32mm waste is 1.7m. Is this true? If so, why? And, what happens if I exceed it? Does this length of waste pipe sound like a dumb idea?

(2) I don't really have any great new spots on my soil stack to add a new boss for my sink waste, so I'm thinking of joining it in to my existing 40mm shower tray waste outside of the property. The join will be ~3.5m away from the sink, and ~1m away from the shower. They'll be joining at a point where both will have been on a shallow fall (18mm/metre) for some distance (1m shower, 3m sink) - that is neither the shower nor the sink will be going through a vertical descent anywhere near to the tee point. The sink trap will be ~3-4 feet above the tee point. The shower trap will be 6-9 inches above it. Are there any rules of thumb as to whether this set up is likely to lead to syphoning or water gurgling back up into the shower when the sink plug is pulled?

I hope they make sense! Many thanks in advance for your help.

Best wishes

Phill

[plumbbob]

Well, its not ideal to have very long runs on small diameter pipes, but I am not sure about the 1.7 mtr length you mention. Very few installations have a run less than that.

The problem with long runs is the possibility of airlocks and syphoning. If you keep the fall, it should work all right. If you want, increase the bore to 40mm.
Also, use an anti syphon sink trap.

Do not use combined sink and shower waste. When the sink plug is pulled, the water will rise up into the shower tray.

[allotmentboss]

Hi Plumbbob,

Thanks for your reply. I'm going to upgrade my sink waste to 40mm after the first metre or so, so hopefully this will make life a little better.

While ideally I know I should run a separate waste for the shower and sink it's going to be dead tricky. There are already strap bosses on the soil pipe in all of the available spaces, so I'm going to have to do substantial work to it if I'm to keep within the rules of part H.

Would installing one of those Hepworth anti-siphon valves on the sink and shower do the trick?

Thanks again

Allotmentboss

[plumbbob]

Yes use an anti syphon trap on the sink

If you have to combine the waste do it as close to the stack as possible and try and make it as low as possible. Consider how the water will flow and make it as difficult as possible for the water to flow into the shower trap.

You could fit a one way valve in the shower waste, but in my experience, they don't work too well, and may be prone to blocking up.