GRP flat roof problem.

[Jonzjob]

We have just moved in to a 'new to us' bungalow. It had a kitchen extension built about 4 years back with a GRP flat roof and a lantern window in it. Since moving in we have noticed some evidence of at least 1 leak and were told of another that had been fixed. After a lot of tries on the phone I managed to get hold of the bloke who did the roof and a couple of weeks back he came and had a quick look. He then said he would be on the Friday, 3 days away, but neither sight nor sound of him. It then took me another week and a bit to get hold of him again and he said that he had been thinking about it and would probably complete the repair by fitting a 'membrane' over the roof. He didn't enlarge on the membrane bit and I was so surprised to get to talk to him I forgot to ask.

The only type of membrane I can think of is the EPDM rubber type? Has anyone any ideas as to whether this would be what he is on about or are there other types? Also, is it a good idea to put any kind of membrane on a GRP roof?

I doubt very much if I could get him on the phone again and would like to find out as much info as I can before hand anyway.

 

[MikeG.]

The GRP is a nuisance and in the way. Remove it and put an EPDM sheet down in its place*. GRP is rigid, and expands and contracts at a different rate from the stuff all around it, inevitably leading to cracking. This always happens in the end.

*Edit:
I've just spoken to a supplier and they reckon this seldom works, as the fibreglass won't come up cleanly. Their suggestion is to overboard the fibreglass (ie screw another layer of board down on the top), and glue the EPDM to that. DO NOT attempt to fix EPDM direct to fibreglass. The glues won't work.

 

[Jonzjob]

Ta for your reply Mike. It has been a long and stressful journey over the past almost 2 years. Getting into this place after a double move from France to rented to here.

I will be fascinated to see just what the idea is with this membrane and how it will be 'installed'? We are also getting the attached garage flattened and a bedroom built to replace it and Ifter reading about EPDM it's what I want on that flat roof.

 

[mindthatwhatouch]

If you can find out the system used for the grp roof. Some of the manufacturers offer a gaurantee.
Am I misunderstanding here. The guy that messed up a grp roof you are now using to put down epdm?

 

[Jonzjob]

"Am I misunderstanding here. The guy that messed up a grp roof you are now using to put down epdm?"

Not in my wildest dream MTWT ! We are getting the job done by a roofer recommended by the people who are designing the bedroom for us. The boss of that company has a superb reputation and he wants to keep it.

 

[mindthatwhatouch]

Phew, that’s alright then.

 

[Jonzjob]

Has anyone any idea as to the cost difference between a GRP and an EPDM roof ? A sort of ball park figure??

 

[MikeG.]

I think a budget figure of around £9 per square metre is a rough guide for EPDM, assuming DIY.

 

[worn thumbs]

Living in East Anglia I know a number of GRP laminators from the local boatyards.They don't have a high opinion of their counterparts who only work on roofs.In fact one of them told me about the roof on a extension to a bungalow his daughter bought and that he had been asked to go and remedy.He was shocked to find it was a 1 1/2 oz laminate and that the fellow that had done it had attended a one day course to learn the process.Andy cleaned it up and added 4 oz more and it hasn't been a problem since.It would seem that the quality of such jobs depends hugely on the people doing the work.

 

[Geoff_S]

Do you know where it is actually coming in?

 

[Jonzjob]

There is a parapet wall about 1 foot high around the 3 sides not attached and there seems to be several small fold/bends where the GRP finishes about 6" up the parapet wall about a foot out from one corner. There has been a leak before we bought the place and that was repaired. If what I think was the repair then it was done by spreading some kind of rubberised mastic over it and surprise surprise it has started to peel up around the edges. I haven't tried pulling it very much because it seems that it's still solid in the middle at the moment and therefore still sealing the leak?

I have worked with GRP for about 25 years now, but not on the roof size scale. I am into radio controlled model aircraft and there's a lot of GRP on a 4 metre model. Plus I made a set of steps for our swimming pool in France. 5' radius and 4' high with 5 steps. They lasted about 7 years before they were taken out and I replaced them with brick built in ones when the liner was changed.

These are they. The only change to them after all the time in the chlorinated water is they are nearly white.

 

[Geoff_S]

OK, the reason I ask is because I had exactly the same problem, flat roof with Victorian roof light and parapet wall. Two gullies through the parapet wall to let the water out.

This was a new extension. The roofer is highly recommended and has a superb reputation, but even he was foxed.

As you might be able to see, there was some patching work done, the brighter blue/grey around the roof light. This didn't fix the problem because that wasn't the problem, more a stab in the dark.

After nearly 18 months, I lost it and cut out a large area of the ceiling and actually found out where the leak was coming in. I had asked the builder and the roofer to do this many times, but they were for some reason reluctant.

What was happening was this. The GRP had been wrapped up the parapet wall to the bottom edge of the ridge tiles, not actually over the top of the parapet so that the ridge tile could sit on the wrapped over GRP. This is where the water was getting in, all along that top edge of GRP.

The solution as you might be able to see, was to torch on felt over the ridge and the parapet.

Problem solved!

Then I found out why the builder had been reluctant to cut out so much of the ceiling. He didn't want to cause me a lot of fuss! Bless :|

If I am properly understanding your setup, your GRP finishes half way up the parapet wall with an exposed edge. This is wrong under any circumstance and would be my first port of call.

 

[Jonzjob]

It has also been my 1st port Geoff. When the roofer did come and have a gander that was exactly what I pointed out to him. The top of the GRP is slightly wrinkled and he could see that. The top of the parapet is done with flat overhanging Bradstone slabs to match the rest of the bungalow and with the inner overhang of the slabs the edge of the GRP is somewhat sheltered.

I think if I were doing it and keeping the GRP roof then I would put a complete glass ribbon tape around the top of the edge and make sure it was in the correct place. I would also use epoxy resin as it is a lot more flexible than polyester resin.

You ain't arf got some posh ridge tiles there init

 

[RobinBHM]

Are you sure it is the GRP that is causing the leaks.

abutments, lanterns and parapets are all potential areas of leaks.

parapet walls are very prone to leaking unless properly built with proprietry cavity trays.

There is nothing inherently wrong with GRP. The biggest problem is the fact its often fitted or detailed wrongly. I also understand that the cheap GRP kits from roofing merchants use the cheapest base material, which has no flexibility.

Ive built orangeries, ie with a central lantern for years with GRP. The only problems Ive experienced are incorrect detailing at abutments. Its important that abutments to house etc are kept separate from the house. The best way is to form an upstand that is screwed to the new flat roof, GRP up it. Then dress lead down over. This allows movement.

 

[Jonzjob]

I am not trying to be alkward Robin, but could you put that in laymans language please as I honestly do not know a lot of the building terms? Or if possible could you point me at a good web page that explains them?