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Help me fix this leak!


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Can anyone advise on the following please?...


My home is a dormer bungalow with the dormer flat roof on our side being two year old fibreglass and next door’s is slightly older EPDM rubber. We have a slow leak, leading to slight mould inside our bedroom on our side only (neighbour’s is fine). This would seem to be coming from the join between the fibreglass to EPDM, which looks like it’s been overlaid with some felt at some point (the GRP on our side was done by a builder who 'flipped' the house before we bought it, I'm guessing did the roof himself and didn't do a great job of the join).


I did a repair in September 2022 on the join area with 10 Year Roof Seal, which held for about a year, then started leaking again. Next door were having their garage re-roofed with EPDM last year, and their builder did another repair in 2023. I don’t know what he did, but it looks like grey chewing gum. I’ve been up on the roof myself yesterday and took some more photos and a video you can view here:




First off, do you think this is repairable with a proper join? If so, what would you recommend. I’ve seen videos and forums on websites talking about ‘upstands’, the importance of using the right sealants, but it’s all different, so hard to know.

The diagram is one method I was considering (I drew the diagram based on a discussion with a GRP supplier). Again it uses an upstand, but what is the purpose of the upstand? Is it a barrier, or does it act as a kind of expansion joint? 

Another roofer recommended simply putting some reinforcement matting across the messy join area and coating it with 'Britannia Durashield'.

Another supplier recommended another product, something called 'Overcoat' which seems like an industrial product similar to the Durashield, he said it gets used used on the big metal Amazon warehouses for example. Just a heavy duty coating with high stretch capabilities.

So there seems more than one way to tackle this. I could of course rip up the whole roof and lay down a new one, but I can't afford that at the moment, plus the two roofs aren't that old, so in theory a proper joint would be ok I'd have thought.

Any tips and advice would be greatly appreciated!

roof situation 3.jpg


roof situation 2.jpg

roof situation.jpg

Screenshot 2024-04-11 at 07.43.23.png

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*on the diagram for the upstand and termination bar method, I'd like to know what kind of sealant to use for  'Sealant A' and 'Sealant B'. And also where I've put 'GRP sheet screwed down' I meant the timber batten upstand is screwed down, not screwed through the GRP sheet. I'd like to know what the addition of the upstand gives? Is it worth the extra work? Also do I coat the whole thing over the join area with something like 'Durashield' once done?

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There are a few good experienced roofing people on here who I hope will be along to help you. I am not one of them but hope the following will help kickstart the conversation for you.


My observations:

1. You will require the roof to be thoroughly clean and dry in the area to be repaired. This is the KEY AND MOST IMPORTANT part otherwise any repair will not adhere properly

2. I would ensure the repair covers the whole length of the join between the current GRP and EPDM layers including, very importantly, up under the tiles. You could start the cleaning in the wet, but be careful up there!

3. I would avoid lifting any of the existing GRP or EPDM if it is clearly well adhered to the subbase - in your diagram it appears the proposed fix is to insert the GRP repair piece under the EPDM. In fact I would be looking to remove anything that is not clearly stuck down, then you know the extent of what is to be repaired. You may need to temporarily cover with a good thick sheet of plastic after cleaning and before repairing - have this to hand!!

4. Whatever you use is going to need to be flexible to cope with expansion/contraction. This is where you should rely on other knowledgeable folks on this forum.

5. The repair piece will need to overlap "good existing roofing" by 100mm+ I would think (maybe more) and have flexible sealant between the existing roof and the new repair piece and some way to ensure that rainwater falls onto the existing roofing a good distance away from the sealant to avoid it collecting. I show a double run of sealant (orange blobs) in the diagram below to act as an insurance policy (FYI blue line is the repair piece (see 4. above), black lines are the existing GRP or EPDM). Others may be along to disagree with me 🙂 





Edited by BotusBuild
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Hi BotusBuild. Thanks for the feedback. Good point about the tiles. I really should have checked out what's going on under those. It could be that there's where the water's getting in at the moment - I'd assumed it would be where I've taken photos, but maybe not. I'm also wondering if it's worth laying some OSB board over the 'mess' of a join at the moment, to cover the two repairs, a bit like you've shown on your drawing. Like you say, being higher than the roof either side then water should run off the join area. Hopefully I'll get some more replies, and get to an achievable process. Thanks again.

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