Problem With GRP Flat Roof

Decking Over GRP Roof Pimlico

Problem With GRP Flat Roof


Could you give me some advice?

7 years ago we bought a 3 storey house in Pimlico with the intention of retiring there sometime around 2014. One of the attractions of the property was that the entire roof was a roof garden of some 45.1 square metres.

We did the place up in order to rent it, which included installing a new GRP roof and new lead flashings. We then put timber decking over this as the walking surface of the roof garden.

Since then we have had nominal water penetration in two places. Firstly at the front which I suspect was a failed flashing, and now at the rear. The rear is so slight that it might be condensation.

I want to lift the decking and do something which is really foolproof to the roof, so that I can forget all about it until I am dead – in other words something with a truly certain failure-free life of 30 years. Once she has got all her plants growing up there the thought of tearing them all out and removing them through the house so that we can do some repairs to the roof is inconceivable. Not a great believer in guarantees, so it is the quality of the product that is what matters to me – not the paper guarantee.

I would also comment that the easiest way to do the job would be to lift the decking and stack on one half of the roof, so that whatever roofing works are needed can be done to half the roof. Then re-lay half the decking and stack the other half on top of it so that the second half of the roof can be done. Of course this may cost more than it saves. My guess though is that if all the decking was lifted, thrown away and new decking provided, then there would not be much change from £3k or £4k.

We could put a scaffold up but undoubtedly the cheapest way to access the roof is up the stairs. This may affect your thinking as to the appropriate product to use.

Finally I would be certain that the roof is not laid to a perfect fall (and probably ponds in places beneath the decking). Would this concern you with regard to point loadings from the timber decking support framework and do you have any thoughts as to how to overcome this. (I was thinking of TimberTech Easy Fit support pads but your experience may point us in a different direction.) We could of course change the timber decking to something else if there was a real advantage but the cost feels prohibitive.

I look forward to your advice.

Please send over the any photos. If you follow any of my blogs, you will know that I don’t believe in guarantees, but I do believe in doing a job correctly, then you don’t need a guarantee.

The best system on the market today is the 3M SD650 system. Try looking at my videos on YouTube. Also think about whether or not you want to insulate the roof; if you don’t insulate it, remember that when you do this roof, you won’t be doing it again for a very long time.

You indicate that the existing roof is GRP and is leaking, probably from the flashings. 95% of leaking roofs I get asked to look at, leak from flashings – more detail, more work, more to go wrong. So, whatever you do on your roof, pay attention to flashings. It sounds to me as if you have already lost confidence in the existing GRP roof. It’s probably only defective along the flashings. That said, I personally think they lay the GRP onto things and don’t build in expansion.

  • To move the decking from side to side and to re-fix on completion: £1,500-ish + VAT.
  • To lay 3M™ Scotchkote™ Urethane Roof Coating SD 650 over existing GRP (if possible): £2,600 + VAT.
  • If you want to insulate to the new Building Control standards: £1,800.00 + VAT.
  • You are going to need to do something with the flashing, what, I don’t know, so I suggest £800 + VAT.
  • All the work can be carried out without scaffolding and up through the stairs, if access is good.
  • There may be a cost for covering carpets etc.
  • Decking, supports and ponding water.
  • Ponding water is not a problem to GRP or 3M™ Scotchkote™ Urethane Roof Coating SD 650, so unless it’s a big problem, there’s no need to address it.

Decking has its problems but tends to have less than other coverings, so is still the main choice. If you have the room to raise, use the support pads so the water can run more freely under the decking.

This link takes you to the five videos I have on YouTube, showing the 3M™ Scotchkote™ Urethane Roof Coating SD 650. It’s definitely the best system on the market. All we need to do is work out what to do with the flashings and decking and the old GRP roof. If it’s not sound underfoot we may have to strip it.

Hope this all helps,


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Comments (1)

  • Paul Greenfield. Greenfield Architects Reply

    I would value your advice please.
    New flat roof on single storey extension with perimeter upstand parapet wall. 36 sq.m total area (well within recommended maximum overall area). Fibreglass roof finish on OSB t&g decking. Warm roof construction. Persistent loud knocking noises and tapping noises started soon after completion which seem to be temperature related – expansion and contraction related to heating up and cooling down. What the roofer undertook recently was introducing movement joints with proprietary expansion joint detail so that area is now in four smaller areas in the hope that noises would stop. Unfortunately the noises continue. Have you any advice as to possible cause/solution, please?

    12th June 2019 at 9:34 am

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