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Cut a hole in a large concrete roof...

Discussion in 'Building' started by David Costelloe, 18 Aug 2021.

  1. David Costelloe

    David Costelloe

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    Roof.JPG House is three storeys, 100 sqm flat roof, no access to the roof without tower, cherry picker or long ladder and good ladder skills. The picture below is when the 30 year old felt covering blew off.

    Roof get a lot of leaves on it. There are four drainage outlets, draining to four downpipes inside the house. The gulleys go unswept for years due to lack of roof access.

    The roof structure, from inside up, is joists, chunky battens, c75mm of wood wool, concrete, OSB-PIR board and GRP.

    House was built 1963. Would be grateful for views on:
    • What are the chances that the roof is reinforced with rebar?
    • Is it possible to confirm that with a detection tool, along with centres etc
    • Is cutting a hole even a starting proposition?
    • Assuming a structural engineer and BCO approve, how difficult would it be to cut an access hole if it is reinforced?
    • Whether or not it is reinforced, would you need kango/percussive tools to do the job?
    What would you recommend and why?
     
    Last edited by a moderator: 18 Aug 2021
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  3. Tigercubrider

    Tigercubrider

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    I can't see an image

    Wouldn't it be easier to bore a hole? I guess it depends on how big a hole is possible and tool access.
    But I would have assumed that a circular hole is easier to achieve and seal.
    Certainly wouldn't think a breaker- a disc based cutter would make more sense
     
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  4. Mottie

    Mottie

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    What about a permanently fitted access ladder starting 10 foot up so that you need another ladder to get to it? Might be better and cheaper than cutting a hole in someone’s roof for access? Something like this :

    upload_2021-8-18_14-2-15.jpeg
     
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  5. David Costelloe

    David Costelloe

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    Hopefully I've fixed the image.

    For an access hatch, I'd need have a hole 800mm x 400mm I think, cutting and trimming one joint - and yes, using disc cutter to form the hole in the roof in manageable sections makes sense, whether it is reinforced or not. And simple.

    In the picture, you can just make outlines between the cast concrete. I presume in 1963, it was unlikely that they would have been pre-cast reinforced slaps.
     
  6. David Costelloe

    David Costelloe

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    Very practical to have a ladder like that. Aesthetically a bit ugly though. Wonder if you can get one that "hinges" away onto the roof.
     
  7. bernardgreen

    bernardgreen

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    You should remove all personal information ( your address etc ) from the images
     
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  8. David Costelloe

    David Costelloe

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    Good grief, god know what I did there!!
     
  9. bernardgreen

    bernardgreen

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    This may be a mad idea

    Will the chimney ( the one that is laying on its side ) be re-built and used ?

    If not then maybe the hole in the roof for the chimney could be enlarged to provide access. It would need some alteration to the chimney stack below the roof
     
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  11. David Costelloe

    David Costelloe

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    Bernard, you're a genius!! We just capped it off and I'm pretty sure we would not even have to trim a joist, just remove some non structural walling.
     
  12. Harry Bloomfield

    Harry Bloomfield

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    A metal detector might help to confirm whether there is rebar in the concrete, I would suggest that there must be for such an area - though your mention of 'joists' confuses the issue, unless you mean for the support of the ceiling?

    Any hole you make, or if you extend the chimney hole, will need an up-stand/ lip around it and some sort of over lapping lipped and lockable metal cover to prevent water ingress.
     
  13. David Costelloe

    David Costelloe

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    Thanks Harry. I believe the build up of the roof is wooden joists, battens, then woodwool, then the original concrete deck (which would have had asphalt on it in 1990), then we have the modern stuff - OSB with integrated PIR and GRP, which replaced the material that blew off.

    I've always assumed the wooden joists above the ceiling are the main support for the roof (ie transmit roof's load to earth via the side walls) including the concrete, though it is conceivable that the side walls and central spine wall of the building could be taking the roof load directly from a stiff reinforced slab. One for a structural engineer.

    If reuse of the chimney hole is conceivable, I'll put in a very low U-Value hatch with proper upstands.

    I find it a tough call on whether it is worth it, as the idea of puncturing the structure is pretty scary. But at the times when I need access, not having it without a tower or brave soul going up an 8.4m ladder, is a right pain.
     
  14. oldbutnotdead

    oldbutnotdead

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    Hopefully the chimney sketch will work, otherwise you're looking at cutting. Breaker would be very unwise, you could cause cracking all over the place.
     
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  15. Harry Bloomfield

    Harry Bloomfield

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    I would not expect so.
     
  16. David Costelloe

    David Costelloe

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    Thank Harry - which bit though - the joists supporting the concrete, or the wall taking most of the load directly?
     
  17. Harry Bloomfield

    Harry Bloomfield

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    I would not really expect timber to be supporting concrete, except as temporary shuttering.

    Concrete is a rigid material, with minimal flex before it breaks. Wood is a flexible material with considerable ability to bend before it breaks - therefore it is useless as a permanent support for a concrete roof.
     
    Last edited: 19 Aug 2021
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