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GRP Roof Problems

Discussion in 'Roofing and Guttering' started by electrogear, 3 Feb 2020.

  1. electrogear

    electrogear

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    Hi,

    I laid my first GRP roof a couple of years ago and I have learnt a few lessons (including not to attempt it on the hottest day of the year in blistering heat ). The main problem I had was where the new GRP roof meets the felt garage roof. It delaminated after about 6 months and let water in (unfortunately my poor fall calculations also meant that water was standing along this join). I had a local roofer come and have a look. He fitted another piece of felt which lapped over the GRP. This lasted a few months and then let water in (again). I think when he was blowtorching on the felt he damaged the GRP. He came back In December and put some Flexaxryl paint on and said it would last until the weather warmed up to sort out a proper roll mop trim. After he’d gone I took another look and I realised there were a number of small cracks (about the size of a 1p piece) in an area near to the where the leak had been. I picked off the topcoat in these areas and saw the ringwire nails I’d used underneath. I think what’s happened is the board where the leak has occurred has got wet and maybe warped, causing the nails to come loose or move or react somehow with the GRP/topcoat. Does anyone have any experience with this? It’s only a small area (the roof is approx 6mx5m and the damage is confined to one corner approx 1.5sqM) so I’m hoping I can resolve this without replacing the whole roof. When I fitted the boards I left a 5mm gap between them and bandaged well, so I’m hoping this will prevent any moisture bridging to the other boards. I’ve used more flexacryl to patch up the damage for now until I can implement a more permanent fix. I don’t think the roofer will be coming back given his multiple botched attempts. Any advice would be appreciated.
     
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  3. datarebal

    datarebal

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    All sounds a bit of a botched attemp ..
     
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  4. Nige F

    Nige F

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    Maybe you could cover the lot with Decothane made by Sika ltd. Have a look on the web.
     
  5. oldbutnotdead

    oldbutnotdead

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    Sounds like your roof design is the main issue- felt won't bond to GRP and yes torching it won't have done the GRP any good at all. Probably better to form an upstand between the felt and GRP rooves and cap it with lead or flashband.
    Are the boards plywood or OSB? Whichever, if they've blown due to damp you'll not get a good bond to them with laminate, proper job would be to cut out and remove the damaged board, put new board down, relaminate (you'd have to grind the topcoat off at the cut, acetone the laminate, fair bit of messing about to do).
     
  6. noseall

    noseall

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    GRP = gash.
     
  7. electrogear

    electrogear

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    Thanks for your helpful response. I’m confident I can sort it with a bit of advice. I have already bought a GRP roll-mop style trim which a roofer had mentioned I should fit. I’m not sure how easy it would be to install lead capping over the top of a GRP trim. Would a timber upstand be a better idea?

    I’m yet to find out if the ply board has been compromised yet but it’s my gut feeling that it has. I’ll find out in a couple of months when the weather’s on my side! It was very well keyed when I fitted it, and it doesn’t seem to have delaminated - it’s not crunchy underfoot, and when I peeled off the top coat I couldn’t see that the fibreglass underneath was damaged or blown but the top of the nail did seem to be showing (difficult to tell without getting the belt sander out).

    quick question - when removing the topcoat, what’s the best tool to use? I have a hefty belt sander? I fear that an angle grinder with wire brush attachment might be a bit harsh?
     
  8. oldbutnotdead

    oldbutnotdead

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    The grp rollmop trim is really for expansion gaps, you may have problems bonding it to the felt side- PU or Acrypol or black jack might give you a 10 year repair but I doubt it. The upstand doesn't have to be structural, bonus is you aren't trying to bond different materials. Use a 90 degree trim on the grp side. Ideally you'd roll the felt back a bit, fix upstand to deck then let the felt relax into the upstand.
    Plan B- new strip of felt blackjacked or torched to upstand and felt (fix upstand on existing felt or where felt runs out). Then 9 inch lead (code 3 or 4), form it into a U and push it onto the upstand (EDIT that's assuming a 100mm upstand which is a bit big but is the standard height for the GRP trims. You'll probably get away with using 75mm or even 50mm upstand- def easier on the eye & you'd only need 6 inch lead for that)
    Wire brush on grinder will do a quick cheap job on the topcoat, belt sander or da works but is slow & paper clogs quickly.
     
    Last edited: 15 Feb 2020
  9. DIYnot Local

    DIYnot Local

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