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Thread: Banging grp roof

  1. #1
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    Banging grp roof

    I have just had a new warm flat roof installed with a GRP finish, it is just over 36 square metres.
    I am suffering from banging noises which can be very loud,its worse when there is a large temperature variation between day and night, Spring and Autumn are the worse times.

    I also noticed that when I initially walk on the roof it emits a series of cracking sounds as though the GRP is moving. Once I have walked around the roof and exhausted all the cracking noises, it emits no further sounds, its as though the roof has settled down. However, if I walk around it an hour later, the same thing happens. This is very repeatable providing I leave approx an hour between walk arounds.
    The roof has been in place 9 months.

    Does anyone have any ideas as we have to move out of the room when the banging gets loud.

  2. #2
    Super Moderator Alex's Avatar
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    Hi Graham,

    Yes there a few threads about this same phenomenon with some warm roofs. The problem is either a board expanding against an immoveable surface (like a wall) or getting the upper deck clamped down well enough. The wood is heating up and cooling down causing a contraction and stressing the GRP enough to make a popping noise (which is rigid and fully bonded). It is important to note its the wood that is making the noise, not the GRP. In most cases it has been cured by either finding the expansion constriction or more probably applying more fixings to stabilise the upper deck to stop it moving around.

    Interestingly we have noticed that if the roof is very dark grey or black, the issue is certainly made worse, and a cure has been found by recoating with a pale grey or white. A direct treatment of the symptom, yes, but in most cases that's all that's needed.
    Last edited by Alex; 06-10-2016 at 08:48. Reason: The names have been changed to protect the innocent.

  3. #3
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    Alex,
    The decking is square edged OSB3 with no gap between adjoining boards. I thought this might be an issue as nearly everyone suggests a 3mm expansion gap is left between adjoining boards. What do you think, are gaps important?
    The top deck is secured to the joists though 150mm thick Celotex with long screws. Its difficult to tell if I have enough. I have tried to use a metal detector to locate the screws but it picks up the foil on the Celotex.
    I do not quite understand how painting the roof a lighter colour would prevent the initial creaking when I walk around the roof
    Regards,
    Graham

  4. #4
    Super Moderator Alex's Avatar
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    Sorry to say that gaps are important, the boards are likely pushing against each other as they heat up and they have no room to grow. In some cases it can cause ridges to appear in the roof at the boards edges as the pressure pushes the edges up, this can also cause the GRP to fail (crack) if severe. This could be the expansion problem regarding the noise. Its not uncommon for creaks as you walk on it, your weight on one specific spot is causing flexing. Painting a lighter colour is purely for heat reflection, the less temperature differential between cold and hot the less expansion movement you'll get. Like I said this isn't the cure, but it can alleviate the symptom.

  5. #5
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    Alex,
    Thanks for the quick reply.
    I already have a light coloured finish so based on your comments it looks as though the roof need relaying, this time with expansion gaps between the square edged OSB3 boards or use T&G instead. Any idea what the approx cost would be?
    Thanks

  6. #6
    Super Moderator Alex's Avatar
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    If you can see where the joints are you can saw through the fibreglass and decking then refibreglass the joints, tape first so the joints don't fill with resin. Hopefully the whole thing wont need ripping up.

  7. #7
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    Alex,
    Thanks for your help

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