I've had a loft extension done on my house, all has gone pretty well. The last snagging point was that I spotted that the soil pipe hadn't been sealed in where it exits the wall. The guy on site put some expanding foam around on the internal wall so you can no longer see daylight from the inside, but they've not cemented in around the pipe on the exterior wall, and they are saying they don't need to.

The hole they have cut is a pretty good fit for the pipe, so it's not a huge gap (it's actually quite hard to see a gap from the outside) but it looks to me like cementing on the outside is a pretty standard thing to do.

I'd like your opinion on whether I need to push back on this. Is there any chance of water getting in to the brick work?

enter image description here enter image description here

2 Answers 2


First of all, did you have any agreements on how the pipe is to be fitted? If paperwork mentions sealing the pipe entry in any way, the final result should match the criteria laid out in the agreement you had with your contractor.

That aside, you said that it is hard to see any gap from the outside of the building so to me, the question on whether or not you need the soil pipe cemented in depends on where the soil pipe is entering/leaving the building.

If the pipe is entering the loft and you are not looking to inhabit the loft, the expanding foam will help with mitigating any water ingress if the expanding foam is protecting the wall cavity from rain water and so I would agree that you don't need to have the pipe cemented in. In fact not being cemented in will make things easier for you or anyone else if the section entering/leaving the building needs removing for repair or replacement. It all depends on how you wish the aesthetics to be.

If the pipe enters/leaves an inhabited space, the interior part may aesthetically be better if it had some cement around it, but that also depends on how the decoration is going to be sorted. If you are plastering the interior around the pipe, you dont need cementing done as the expanding foam used will protect the plasterwork from water ingress.

  • 2
    Expanding foam is going to disintegrate if it's left exposed to the elements for too long and will no longer provide any protection. It can also be chewed through by insects or rodents. I believe cement or some kind of sealant is required on the outside to protect the foam and properly close the gap.
    – Carl
    Mar 20, 2020 at 19:59

Standard construction methods would include a minimum of a silicone seal to prevent water entry at the pipe. Cement or mortar is not required but sealing to prevent water entry is.

  • 3
    Also a great entry point for little creatures that you may not want in your house. Mar 20, 2020 at 15:25
  • This turned out to be a good point. I did get them back to cement up in the end. We've had quite a few cluster cluster flies coming in from the hatches through the eves even with this sealed up, so I'm pleased we did.
    – gbro3n
    Mar 20, 2021 at 10:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.