Flat dormer roof - best covering material?

[Keith (Dorset)]

Hi, we are having a dormer loft extension built which needs to have a flat roof. My late father was in the building trade and then a surveyor and always swore to avoid them like the plague. Our prospective builder has however told me that things are 'much better these days' and they offer a 25 year guarantee on a GRP flat roof.

We live on a very exposed area of coastline where the roof would be subject to a great deal of UV and windy weather (although the prevailing wind comes from the opposite side of the house.

Does a GRP (fibreglass) roof sound like a good idea, or is there another material that might be more appropriate - i.e. do an equally good job at less cost and/or need 'even' less maintenance?

Thanks in advance for any help.

Keith

[Mike]

I'd rate a GRP roof as one of the best solutions to a flat roof. But having just had to put an incline on one due to rotted timbers below caused by leaks, I'd have another think about at least a slightly inclined solution. With GRP, even a few degrees is helpful whereas tiles need a lot steeper of course.

[Rick]

They build boats out of GRP which shows how waterproof it is. If done properly its virtually indestrutable, and will last till the end of time.

Rick

[dale hammond]

[Bob Mannix]

Lead will probably last the longest, then "zinc", then GRP then ashphalt, then felt.

It rather depends how long you plan staying there! Lead/Zinc are 80/40 years (impossible to say, really, but these are guidelines)

GRP (as your builder and the other poster says) 20-25

Ashphalt say 15-20 (not very common but I was offered this)

Felt 10-15

ALL THESE are subject to the job being done properly in the first place.

I had lead on mine and am very happy with it.

[Pete C]

Hi,

How thick was it and how did it fail?

cheers, Pete.

[The Natural Philosopher]

It is UV degradeable though. Slow, but it happens.

But you can always slap a new coat of resin on top itf it starts looking sick.

[dale hammond]

if memory serves me right it was 3 layers of 2 oz mat but on top of that i used gelcoat with wax additive (flowcoat) as for the fall what fall lol i just did the grp the base was plywood which some cowboys had put in

[dmc]

Any feel for the comparitive prices of these?

Darren

[Cynical Git]

So a felt roof (x around 20+ locally to my knowledge) in perfect condition at 40 years+ is unusual then? Then again the very well regarded local builder was *extremely* good.

[Bob Mannix]

Ah, maybe if you could go back in time and get a felt roof 40 years ago...

I am always happy if felt roofs last longer than people expect (who wouldn't be!). The figures quoted were what the builder(s) I have had said, are certainly true of ones I have known round here and was true of mine, which started to show problems at about 12 years and which was replaced at 15, by which time it needed it! I would be planning to have the money for a replacement in another 15 years.

It does also depend on how much sun it gets and if it is ever walked on.

[Bob Mannix]

Not as much difference as you might think but I can't give today's figures. I was going to have asphalt but in the end the loft company man said he had so much lead reflashing to do round the chimney he might as well get the lead man to do the roof at no extra charge. He made an excellent job of it. Lead roofs aren't "flat" as the lead comes in sheets which are formed over wooden formers where they overlap - I will send you a piccie if I can find it.

[Pete C]

Thanks, what was the thickness of the fibreglass itself, am not too good on oz-> thickness. Also did it split under the gelcoat or did the gelcoat come off first?

cheers, Pete.

[The Natural Philosopher]

My garden shed roof had the felt totally disintegrate after 8 yaers. Cheap s**te agreed, but don't expect felt to be the be all and end all of longevity.