I am having the same problem as this question: How do I fill a hole surrounding a boiler flue?

I notice the answer doesn't mention cement but other products which may have been due to fire safety issues. I spoke to a gas safety engineer just now and he said he would use cement. So I'm wondering if cement can and should be used and maybe why it wasn't mentioned in answer to linked question. I am in the UK.


  • That question may be in a different country, therefore different fire regs compared to what the gas engineer works with... you don’t state your location but you should - guess it is the UK...
    – Solar Mike
    Jun 21, 2020 at 18:17
  • To be pedantic, cement is a fine, dry powder which is combined with water and aggregate to make concrete. I'd guess you mean concrete, as cement would just blow out at the next light breeze.
    – FreeMan
    Jun 21, 2020 at 19:05

1 Answer 1


To use cement only would be difficult, since it will simply start to spill over into the flue. It may not matter, but it will only allow you to fill up the flue with so thick of a material, since it has to "mound up", no way to keep it flush with the inside of the flue. I typically use masonry cement, it is available in just add water format. That with some brick or broken block, lay cement and brick in alternating courses, this will allow the material to stand up more vertical to more effectively fill the hole. Do not try to fill the whole thing at once, do it in 2 stages, letting the first application set up overnight about 2/3rd to 3/4 filled then detail the rest in the next day.

You could fill it all they way at one time, it may just be a bit more tedious.

Do be certain the surfaces you apply the cement to is free of soot and other contaminates.

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