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How to identify smell coming through gaps in floorboards

Hi, very recently we have started to become aware of a noticeable smell coming up through gaps in the floorboards in downstairs rooms of our 1930s house. If I had to try and describe it is almost like a sweetish, boiled cabbage smell...but not the obvious strong smell you would associate with a gas leak. I've managed to lift a couple of boards and there seems to be air circulating underneath, coming in via the house's external airbricks. The smell seems to get worse overnight.and receed to some extent through the day, possibly as we have the heating on.

We had work done early 2022 to our kitchen extension (knocking through some walls and reconfiguring the kitchen & utility space)...but I am convinced it is not something as simple as a leaking pipe from that as the smell is also coming from other rooms (and is stronger in those).  We also had a new patio laid in autumn 2022 where I can see they have raised the ground level and come up close to the airbricks,.leaving recessed gaps to allow airflow.

My head says it could be the patio work that has led to the issue as the timing is right...but I am convinced air is still making it into the house as there is a strong draught pushing the smell up through the floorboards.

I have a general distrust of trades and wouldn't know who to get round to investigate. It doesn't feel like a plumbing issue and I see lots of bad reviews of damp specialists. The general builder who did my work is going to pop around at the end of the week to see if they can offer any thoughts...even though I am fairly sure they haven't caused this

Any ideas from the forum would be well received

Thanks in advance


  • edited 21 February at 10:52AM
    SilvertabbySilvertabby Forumite
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    edited 21 February at 10:52AM
    Dead rat?  Check for access holes, perhaps created by the recent patio work.
  • Lions_johnLions_john Forumite
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    Dead rat?  Check for access holes, perhaps created by the recent patio work.
    Thanks. Not a bad shout as we do have a lot of wildlife round here. Will follow up on that and confirm...
  • edited 21 February at 12:20PM
    MicronMicron Forumite
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    edited 21 February at 12:20PM
    Hi, very recently we have started to become aware of a noticeable smell..... like a sweetish, boiled cabbage smell...

    My guess would be foul air from a damaged or ill fitting sewer, drain or vent pipe being sucked or blown in the air space under the floorboards.
    Dead rats and mice give off an unmistakable odour which I would never describe as 'sweetish'.
  • edited 21 February at 5:59PM
    Eldi_DosEldi_Dos Forumite
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    edited 21 February at 5:59PM
    If you still can lift the floorboards,get a small garden trowel and dig down into the solum and see if it is dry under the layer of dust you usually get under there.

    If you can see the rear of air bricks while you have floorboards up is there a build up of debris behind and below air bricks.With the age of house I would imagine there will be if it has never been cleared out. This material can sometimes smell like damp soot in certain weather conditions.

    I always think a experienced joiner is a good start for advice about smells coming from under floor, they can develop a nose for what smell indicates what problem.

    If you have a distrust of trades best not to show it when they are there to help you.

     And the most obvious one, have the pipes etc checked by a gas engineer or the gas board.
  • greyteam1959greyteam1959 Forumite
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    Even if you think there is the very slightest chance it is a gas leak report it & they will attend very quickly.
    There will be no charge even if it is not a gas leak.
    Better safe than sorry ?

  • twopennytwopenny Forumite
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    I had a 20/30s house and just occasionally one room would have a smell like this in late winter/spring.

    It did worry me but then it would vanish for the year.
    Never found out. Nothing untoward happened. Put it down to damp or some such.

    The only normal people you know are the ones you don’t know very well

  • mi-keymi-key Forumite
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    Have you checked the house didn't used to belong to Dennis Nilsen ? 
  • markinmarkin Forumite
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    The patio should never have been raised, What is the actual level below the joists?
  • ch3shirecat4ch3shirecat4 Forumite
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    I know several people who have had issues with a strange smell in their house and both times its been a dead rat, they really can make a horrible smell, I hope you get this sorted 
  • georgeheregeorgehere Forumite
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    If you have had rodent issues and put down poison then a dead creature is a possibility. If not, then either the kitchen or patio work has caused a drains problem (mind you, getting the gas checked is never a bad idea).

    Smells can unfortunately travel a long way under floors or behind wall dry lining. Also, they may develop over time eg. the issue could be the kitchen work, but then something done when the patio work was completed caused a change in airflow that then allowed the previously undetectable kitchen issue to 'appear'.

    If the patio work did not involve drains - then it's the kitchen work, made apparent by a later change elsewhere.
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