Please help - Installing soil pipe over flat roof

9 Nov 2015
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United Kingdom
Hi there,
let me just start by saying I am no plumber or expert and am looking for some advice on doing my first 'big' DIY project of turning a small bedroom into a bathroom, specifically with the toilet.

I want to put the toilet below the window on an external wall which is upstairs and overlooks a flat roof, the extension which is the current bathroom.
I think that the soil pipe will therefore need to go out the wall, drop down, then somehow run over the flat roof about 2.5m length to meet the stack for the existing soil (currently fitted with a vent).

Attached are some pics of the flat roof and wall that I want to put the toilet on.

I am aware that this is not ideal and should not lie flat (like next door!) and 1 plumber has already said he won’t do this part of the job.

I am guessing I can fit a T-shaped connector where the current vent is on the existing soil and then run a single piece of pipe up to wall. What would be the correct/best way to do this to ensure the pipe has the necessary drop (min 6 inches?) and is sufficiently supported?

I’ve thought of constructing a triangle ‘wedge’ shape frame from stud wall timber and thick MDF, then coat it in something weatherproof and sit this on the flat roof fixed in place with some kind of adhesive. The pipe can then sit on this all the way to the wall and be fixed in place with regular pipe brackets.

I’ve also thought of using something as ‘feet’ to support the pipe at intervals of half a meter or a meter (someone mentioned DuraFoot, maybe something like that).

Are either of these viable options?

Are there better ways I’m missing?

My ability levels are amateur for sure, but I can enlist several family members who have fitted bathrooms before to help.
Sorry for the long post but if anyone can suggest solutions and working order to do things in I’d really appreciate it!


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Thought that was my old house for a minute with the CR Smith brown windows :)

It'll be a nasty lashup having that pipe sitting on the flat roof- fixing it down carries the risk of causing leaks in the roof, using timber spacers (which would be simplest) means they'll be sat in the wet quite a lot and thus prone to rotting.

Given that your house looks about the same age as my old place, have you investigated whether the downpipe from the gutter goes into the foul drainage or some separate system (highly unlikely). Best way to check- there'll be a manhole at the end of the back garden, lift it, get an accomplice to flush the loo (to make sure it is the foul drain) then provided it is the foul drain chuck a bucket of water into the gulley that (presumably) that gutter pipe goes into.

If it is the foul drain then I'd suggest digging out around that gulley and put yourself a new stack pipe in- you could then put your loo either under the window as plan & pipe round the corner or put it on the other wall & have a straight through. You'd need to alter the guttering so the pipe drop is a metre or so nearer the house, not entirely sure about joining gutter pipe straight to the stack (entirely feasable technically but just feels a bit wrong & won't be in the ADs cos they all assume surface water will go to a soakaway). Probably better to have your new stack where the gulley is & have the gulley a bit nearer the main house for the gutter to drain into.
It looks like next door's soil stack runs parallel to their gutter down pipe on the main house so perhaps they share the same drain. Couldn’t you do the same with yours?
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Assuming that you don't take the better solutions above, may I suggest some tips for your original proposal:

1. I am not a favour of soil pipe ring seals on the lower part of a joint, whilst the chances of a leak are low the consequences can be severe (with smeg from the upstairs toilet leaking into your downstairs toilet (behind a boxed-in soil stack?). To avoid this you'll need to access the existing stack pipe lower down and solvent weld a join, so that the socket that the new tee fits into is facing upwards (and the new horizontal pipe just clears the roof).
2. Form the supports for the new pipe from treated timber (NOT MDF!), fixing it down with bitumen roofing adhesive, then felt over this new support to weatherproof it, using plenty of roofing adhesive under the felt where the bracket screws will penetrate. You should locate the supports so that the brackets are about 400mm from the new tee, and 300mm from the new bend up to the new toilet outlet.
3. I suggest fitting a tee instead of a bend at the point where the pipe will penetrate the upstairs wall, blanking its top outlet with a push-in blank. This allows access for rodding the horizontal section, and a convenient connection point should you decide to (or be required to) fit a basin near the new toilet.

I hope this helps.

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