Best route for a new sewer pipe

[DIYWell]

I'm thinking of having an en suite built upstairs. It's probably going to need a second soil stack installed which will have to link into the existing sewer. The current one runs from a soil stack in the centre of the house and reappears about 250mm from the side of the porch. There there's a vertical pipe with a bend at the bottom about 1200mm deep. The main soil pipe joins into this horisontally abour 950mm down. This pipe runs off to the inspection chamber approx 1400 deep.

Option A: Through a cavity wall and trench about 4m across a garage. The floor is concrete of unknown thickness and hope to intercept the current sewer at some point. I don't think the current pipe is overly deep.

Option B: Trench down the side of the house and round the corner for about 7m to join in the existing drop pipe. The surface here is just earth with paving slabs on top. Also down the side runs the existing rainwater runoff piping from the rear of the house and the electricity supply. At some point the new sewer pipe would have to cross the rainwater runoff pipe from the front which may be an issue.

Option C: Through a cavity wall and trench about 6m diagonally across the garage and out the door to the existing drop pipe. This would bypass all the existing pipes and cables running down the side.

What are the pros and cons of each? What are the potential gotchas that would rack up the cost?

 

[Hugh Jaleak]

Externally (option B) would be the cheapest solution IMO, it'll be a lot easier to reinstate outside, existing services and drain can be worked around, and no guesswork where existing sewer is. You'll need to provide a chamber on the corner where the drain changes direction, but this run could be kept fairly shallow, then enter the existing drain at the manhole shown using a backdrop outside the chamber.

Digging up internal floors is going to cost as the concrete needs to be broken out and removed off site, then made good afterwards. DPM reinstated etc. Externally once pipe is bedded and surrounded with peagravel, most of what came out can probably be used as backfill.

 

[noseall]

It may be a faff (or not) but can you access the drainage in the WC below on the ground floor?

It may be possible to then extend the soil pipe vertically and connect the en-suite onto this.

 

[DIYWell]

It may be a faff (or not) but can you access the drainage in the WC below on the ground floor?

It may be possible to then extend the soil pipe vertically and connect the en-suite onto this.

Not possible - joists run the wrong way and there's a door in the way too

 

[DIYWell]

Externally (option B) would be the cheapest solution IMO, it'll be a lot easier to reinstate outside, existing services and drain can be worked around, and no guesswork where existing sewer is. You'll need to provide a chamber on the corner where the drain changes direction, but this run could be kept fairly shallow, then enter the existing drain at the manhole shown using a backdrop outside the chamber.

Digging up internal floors is going to cost as the concrete needs to be broken out and removed off site, then made good afterwards. DPM reinstated etc. Externally once pipe is bedded and surrounded with peagravel, most of what came out can probably be used as backfill.

I was hoping that 'B' wouldn't involve yet another manhole purely for visual appearances and the bend would be achievable by, well, and underground bend. I also thought the easiest entry point was the bottom right drop pipe rather than the big chamber since the direction of flow might cause problems.

Off the top of your head, what sort of effort would 'B' incur? Digging down the side would be a tangle of roots in most places. I'm guessing mechanical digging is out because of the unknown route of the other pipe, so man-with-a-spade. 2 days to dig, 1 day to lay, 1 day to break in to existing pipe, 1 day to backfill, so 4 mandays plus parts. No idea what that might cost in £s though.

 

[Hugh Jaleak]

If the new drain can be kept fairly shallow, then a shallow inspection chamber 225-300mm dia can be used on the change in direction. Underground bends in horizontal runs are only permitted immediately outside a chamber, in the middle of a run it's asking for a blockage to occur and you may find yourself with constant problems then.

Really needs someone on site to look and quote, impossible to estimate from afar, sorry.

 

[DIYWell]

Go on, stick your neck out Over or under £750? I honestly haven't a clue how many zeros.

Failing that, what trade would I approach to give me a quote for running a sewer pipe? Plumber? Builder?

 

[Hugh Jaleak]

I doubt you'd get any change from a least £1000. There's the labour involved and then plastic drainage isn't especially cheap, you'll need peagravel to bed pipes, hire/fuel costs of any plant used, your chosen contractor will have insurance and vehicle running costs to pay, apart from he may want to make himself something out of it to earn a living.....

Ask friends/relatives for recommendations, but a good Builder or small groundworks company will be your best bet. Plumbers dont usually take on drainage jobs of any size due to kit required to do this type of work. Whoever you choose, make sure work is done in accordance with regs and signed off. Otherwise it may well come back and bite you if you try to sell the property at a later date!