Downstairs WC, suspended timber floor, existing soil stack

[immortalben]

We're about to buy a 3-bed semi in Cambridge and want to make a few modifications, starting with installing a downstairs WC. The only sensible position for this is under the stairs. I'm totally new to home renovation so I was hoping to get some advice on what I would like done and what I can reasonably expect to pay/where I can save money by doing work myself - we're on a typically tight budget.

Floor plan http://imgur.com/KnO5C5H - there will be two new soundproofed partition walls built immediately afterwards.

The property has a suspended timber floor with a 30cm void under the joists, which by my calculations provides plenty of vertical space for the required 1-in-40 slope. There's a (reasonably recent, plastic) soil stack outside the front door which drops down into the concrete drive - manhole is about 4m away. I'd like to connect to the existing stack if possible as my better half would not love another one being installed.

Plan is: run a 100mm pipe along the path shown, then dig up the relevant section of driveway, cut through the foundation blockwork and connect the pipe through to the existing stack approximately 30-40cm beneath ground level. (I'm assuming it goes at least that deep.)

Is there a better way of doing this/is it daft for some reason?

Based on the floorboards I think there's likely to be a sleeper wall between the WC location and the soil pipe (along the route of the new partition wall) - will check this when I next visit and can peer under the floorboards. Otherwise I'd be tempted to run the pipe up until it was level with the stack to eliminate a bend, although getting to the external wall under the kitchen floor (which is adequately tiled for now) would be a pain. I'm guessing that going under the staircase and digging up more driveway to eliminate the other bend isn't worthwhile either.

What sort of costs am I looking at if I hire a professional to do this? (And do I want a plumber or a builder? Not quite sure...) Not including actually installing the WC, supply pipes or any making good etc. - just the below-ground stuff.

 

[Hugh Jaleak]

Really need to ascertain depth of the drain first to ensure adequate fall would be achievable, however whether your proposed setup would be allowed is another matter. Bends in a 'horizontal' drain run are frowned upon, drain should have a manhole at every change of direction, and you've indicated 2 90deg turns in the proposed run.....

Difficult to advise further without seeing the site, but if the drain could run straight from the WC position to outside the property (under the stairs and hallway) then this would be far superior. A mini chamber may need to be provided outside, although this would probably involve more work, ultimately if the drain blocked, it's accessible to clear.

IIRC you would also need a handbasin in the room to comply with Building Regs. Some Plumbers will do underground works, if not then a Builder. Work is notifiable under Building Regs and will require inspection by Building Control. Oh, and avoid a Macerator like the plague.....

 

[immortalben]

Hi Hugh,

Thanks - sounds like I underestimated how bad bends are...! (Not macerators though. Hellspawned things.) Okay, so I'll look at going under the staircase.

I realised while re-reading Document H last night that I might be able to get away with feeding straight into the drain if it's within 1.3m of ground level, rather than going via the soil stack. I presume the existing drain will run straight from the soil stack to the manhole, i.e. diagonally across the driveway, so in principle I could connect to that with a straight run of drain from the WC. Can I really do that or is it a bad idea?

Er, last newbie question, how do I find out the depth of the drain? If I peer down the manhole and measure the depth of the pipes there, will that give me a reliable estimate of the stack depth - or do I need to pay for a flexi-camera inspection?

 

[Hugh Jaleak]

Existing run should be straight from base of stack to manhole, bends are frowned upon, unless immediately outside a manhole. You can connect to the drain at the most convenient position, but bear in mind every part of the drain run must be accessible for rodding. Rods wont go around sharp bends, so if the drain were to block beyond a bend then it is unlikely you would be able to unblock it using rods. You may therefore, need to install a manhole/chamber where the new drain connects to the existing to allow access along the proposed new run.

If you have a look down the existing manhole, then measure from ground level to the bottom edge of the outgoing pipe, this distance is known as the Invert. The 1.3m rule is measured vertically from the crown of the WC (top edge of the WC outlet on new pans) to the invert of the drain. Really no need for a CCTV survey unless there are other, major problems with the drains, e.g. constant blockages that need investigating.