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Warm roofing and GRP Fibreglassing Single Storey Flat Roof

Discussion in 'Roofing and Guttering' started by jtmk, 30 Jul 2018.

  1. jtmk

    jtmk

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    Looking for advice please. I've got a flat roof single storey bathroom that needs a new roof and I've looked at GRP Fibreglass as the route to go down. I also wanted to insulate the flat roof and have read all the advice stating that a warm roof is superior to a cold roof. If the Celotex for the warm roof sits on top of the OSB3 board then do I need another layer of OSB3 board above the Celotex for the fibreglass to go on?

    Fibreglass
    -----------------
    OSB3
    -----------------
    Celotex
    -----------------
    Vapour Barrier
    -----------------
    OSB3
    -----------------
    Rafters
    -----------------
    Celotex between
    -----------------
    Plasterboard
     
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  3. geraldthehamster

    geraldthehamster

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    Yes you do. Personally I'd use EPDM rather than fibreglass. Easy to DIY, less messy, quicker and more durable.
     
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  4. Notch7

    Notch7

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    Yes you do need the outer osb3

    silver tape on the celetex joints as an extra vapour barrier if you want

    I dont know if the vapour barrier is needed if you tape the joints, I never bother -if your celetex that is in between is set flush to the top of the joists then taping across each joist will act as a vapour barrier.

    If you have firrings make sure you dont have voids on the cold side anywhere.
     
  5. jtmk

    jtmk

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    Thanks for the responses. What thickness of OSB3 board would you suggest? I was going to use 18mm OSB3 tongue and groove directly above the rafters.
     
  6. jtmk

    jtmk

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    Would I use 18mm OSB3 tongue and groove above the rafters and above the celotex insulation? Also, as I have downlights in the bathroom, can I use 200mm knauf mineral wall insulation between the rafters instead of celotex and just use the 120mm celotex above the vapour barrier?
     
  7. Notch7

    Notch7

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    we use 11mm osb above rafters -its only to spread the load on the insulation.

    18mm T&G is ideal on the deck, although I never bother -how many joints would you need above a bathroom?
     
  8. jtmk

    jtmk

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    Thanks for the advice on using 11mm OSB3 on the Celotex. How do I fix the Celotex in position? Can I use a grab adhesive? Also, how is the 11mm OSB3 fixed in position on top of the Celotex.

    Is it OK to use Knauf loft insulation between the rafters because I've got LED downlights in the bathroom that I'll be putting insulation hoods over?
     
  9. Alastairreid

    Alastairreid

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    you dont need any insulation between the rafters if you are creating a warm roof.
     
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  11. geraldthehamster

    geraldthehamster

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    You don't need to fix the Celotex, just screw down through the top layer of OSB, through everything, into the joists. You can get proper fixings that supposedly avoid cold bridging. I've done two roofs like this and just used 6 inch screws.
     
  12. jtmk

    jtmk

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    The video I watched on Youtube states that you do need to insulated between the rafters otherwise you can get thermal bridging at the fascia:

     
  13. jtmk

    jtmk

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  14. datarebal

    datarebal

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    Wrong side of the insulation though.....
     
  15. bobasd

    bobasd

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    thats a good video but the part about cold bridgin is ref to projecting eaves.

    would fillin the joist bays with insulation and settin celotex aabove them cause any problems - i dont know?
     
  16. Alastairreid

    Alastairreid

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    The cavity insulation meets up with the roof insulation.
     
  17. bobasd

    bobasd

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    what do you mean?
     
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