Replacing cast iron soil stack incl connection to foul sewer

10 Jul 2009
Reaction score
United Kingdom
Hi, I'm a new user and hope someone can give me some advice please. I posted in plumbing previously but was advised to try here in building instead. :)

We have an old cast iron soil pipe which we want to replace and hopefully move to a new location (only a couple of feet along same exterior wall). Job needs doing anyway as stack is fractured in top section and may soon fall down, but we also want to build a conservatory adjoining the wall where the stack is now (don't want it boxed in so would prefer to move it along a bit).

I am OK with most of the above ground work but unsure how to proceed with the underground section. I have part excavated it and found the old conversion piece/collar where the cast iron pipe joins the clay(?) foul sewer. The collar, short vertical section and bend (to run out to connect to main sewer line)are badly broken - they were only held together by the soil . I haven't yet excavated as far as the connection with the main run of the sewer - but it looks like it will be either a 45 or 90 degree tee joint.

Can I simply replace the soil stack with plastic and tee into the main sewer run in new position (replacing section where existing tee joint is) with plastic? (hope this is all clear enough).

Also, to save taking new stack through roof line can I fit an external AAV at top (I read something about build up of positive pressure possibly being a problem where there was no other vent to outside).

Many thanks - sorry for long post. Will attempt to attach pics below!
Sponsored Links

I have now excavated a bit more and found connection to main sewer run which enters inspection chamber visible at top of pic 4.

We were hoping to move the soil stack 2-3 feet to the right as you look at it (closer to the french doors) to allow a conservatory to be placed outside the patio doors (to the left of the stack in pic 2).

I think the rainwater downpipe visible the other side of the patio doors in pic 2 also connects to the foul sewer, but will have to excavate to confirm. I am looking to replace this as well with plastic (doing all soffits and fascias and gutters). I certainly can't see any other drainage pipes along back of house to carry this water away.

So can you see if it is possible to move the stack and if so, how and where do I join to the stoneware foul sewer please.

many thanks again

Moving the stack is easy peasy. It can not be replaced with an aav.

You can graft in a new 45 degree junction to the main run wherever you need to by using either fernco type connectors or proprietary plastic connectors. Once this is in place the rest of the assembly will be done in plastic. You may be wise to chop back all the way to the pan and fit a new pan connector while your at it.

The fernco (rubber jubilee clip type) make for an easier fit as they can slip up the pipe so grafting-in is relatively straightforward.

If this was done under reg's you may be asked to fit a small access chamber instead of a junction.

Make sure you terminate the stack 900mm above the nearest openable window and fit a balloon grate.

Looking at the images it seems as though the pipe goes vertically through the soffit as opposed to a 45 degree swan neck. You may be able to move and re-use the lead slate, but it may be a little bit tight around the plastic as c.i. is slightly smaller.
Thank you noseall, just a couple of follow up questions.

If moving the connection further downstream, what do I do about the old junction? The old clay pipe is about 130mm outside diameter. I can't seem to find anywhere on line that would sell the parts I think I need. The fernco connectors would be great but if I cut out the old junction I will need some new pipe to bridge the gap. Also if I'm using the fernco stuff do the two ends being joined need to be in contact, do they need to be sealed or can the connector just bridge the gap. I'm thinking that if there is a gap between the two ends then it will interrupt flow and potentially cause a blockage to build up over time.... :confused:

Also to minimise interference with the new conservatory foundation (likely to be v close to where we resite the new soil pipes) I am thinking of running the new line at a 45 degree angle away from the house wall to join the main sewer. I know we'll have to work round the sewer at some point, but dont want to have to bridge over part of the junction (not sure that would be allowed anyway)

If I went for the inspection chamber option - again what parts would I need and where could I get them from - all chambers I can see advertised on internet seem to be for 110mm pipe.

Sorry to bombard you with questions. :)

thanks again for your help
Sponsored Links
If you carefully cut the existing clay just above the collar on the junction, then further downstream on existing line where you wish the new line to connect in it would make life easier IMHO. Remove existing clayware between the two points, then using the Fernco couplings, graft in 110mm plastic, the correct couplings will take up the difference in the outside diameters of the clay and plastic. Wickes do one that will do the job I think:

Id advise fitting a small chamber in lieu of a junction where you're new 45deg leg from the stack joins. Make sure its all on the correct fall, bed all pipework with peagravel, and use a rest bend at the base of the stack.
Right, first you will remove the stack and the underground stuff, including the junction at the main run.

You will then cut into the main run further down stream and then renew all the way back to where you removed the original junction. Don't mess about with bits of old clay between.

The bore of the old and new pipes will be exactly the same. The thick wall clay turns out, as you say, at about 130mm and the new plastic is 110mm. When buying adaptors ask for thick wall clay (or old salt glaze) to plastic connectors. As i said earlier you can either buy an osma push-fit fitting or a fernco rubber type. I would use the fernco as you will need to slip at least one of the fittings back up the plastic in order to fit the pipes snug.

I would advise using a mini access chamber as most of the ones i have seen have a pair of 45 degree inlets which would be ideal for your scenario.
Something along the lines of this should do. Apologies to Hugh as he seems to have answered the Q. :rolleyes:

One more tip, go easy when fitting new to old as to not disturb the existing mortared joints. You will need to remove soil around the existing pipe to facilitate cutting but don't go mad. Only remove enough to work as the remaining soil will keep the pipes stable.
No need to apologise to me Noseall, seem to have far too much time on my hands at moment! Just like to try and help where I can.
Just so you understand the full picture ;) ; you should be aware that the work you’re doing is notifyable, your LABC should inspect the new stack/foul drain/chamber & in some cases may require to witness a pressure test; they will then issue a compliance certificate once they are satisfied the work you’ve done complies with Building Regs. You could argue that it’s maintenance but as your moving the stack & constructing a new access chamber, I think the chances are it would be regarded as more than that. The work should at least be done in accordance with Building Regulations (Part H).

There could be future consequences if you cannot produce compliance paperwork when asked, it will be regarded as unauthorised building work & could halt a potential sale. :cry:
thank you - I have now spoken to building control and they were pretty helpful but said I do need to complete a building notice application. They also confirmed I don't need an inspection chamber at the new junction if I include a rodding window at the base of the stack. I may go for this option as I would like to lay patio on top of the path area (visible in the pictures) once the conservatory is up and dont really want another inspection chamber within the patio area.

By the way does anyone have any ideas on how best to cover the other 2 inspection covers - can you cover them with patio slabs/stones but still have them accessible? Or will I have to have two dirty great inscpection covers in my patio? :(

Grateful thanks again to all . :D
If I'm having a soil stack replaced (the old is in disrepair) is this notifiable work? It's not being moved or anything, just a straightforward replacement.

The plumber hasn't mentioned this, but I thought I'd check. I found a Q&A from the ODPM site, it's a bit unclear but I THINK it's saying it's not notifiable. But I'm not 100% sure.

(r)(i) If I want to install fittings such as a WC, shower, washbasin, or
kitchen sink within my home, or any other type of building,
will the Building Regulations apply?

YES – if the installation of the fitting or fittings will involve
alterations to, or new connections to, a drainage stack or an
underground drain. You should also check with your Building
Control Service to find out how much information is required
about the fittings you intend to use.

NO – provided the work only involves installing an additional
fitting or fittings in the same or a new location which does not
involve extensions to the drainage pipework.
If I'm having a soil stack replaced (the old is in disrepair) is this notifiable work? It's not being moved or anything, just a straightforward replacement.

I personally believe this to be maintenance work so would not go rushing to b.c.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links