Kitchen Waste - under floor?

4 Jan 2012
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United Kingdom
Hi Guys,

We are moving our kitchen (having created a new kitchen-diner) and are intending to install a wet underfloor heating system (we have suspended timber floors). However, the sink is currently ~5m away from the drain.

I've looked at the regs, and we seem to have room for the pipe size and gradient required - assuming we put it under the floor. Given the insulation + board + tiles - I'm worried about any leaks.

How likely are sink pipes to leak? Is there anything I can do to prevent it? Is putting the waste pipe underfloor normal?


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Not sure about the permitted length of waste pipe from a sink, I know basins are only 3m from branch pipe.
Anyway someone will hopefully relay that info back.
I would suggest solvent welding the plastic waste pipes for under floor routing. If the pipes are insulted and protected from frost, it's the way to go.
You will need to purchase the pipes and fittings that can be solvent welded.
Make sure the pipe is well supported to avoid any strain on the joints.
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In addition to the sensible recommendations to use solvent waste pipes, well supported (but not tight) and insulation. Due to the presumably long and straight run it is best to allow for some thermal movement at each end. i.e. a change of direction without blocking the bends movement. Additionally use either an antivac trap (Anti-siphon) at the sink or an air admittance valve if you have like a washing machine trap plumbed in as well.

If you plan of connecting more than just the sink then I agree with Hugh below - Go to 50mm
Think you'll need to upsize from 40 to 50mm after 3m, avoid any bends under the floor and provide cleaning eye(s) in case they're ever needed! Provided joints are made properly they'll never leak!
There is no limit of length for a 40mm sink waste but after 3m an anti-syphon trap must be used.

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