Overlaying a grp dormer

[Wales]

As part of a roof replacement, about 3 years ago, we had a flat roofed dormer done with fibreglass. I knew nothing about flat roofs at the time, but now know that it was a poor job.

There's a slight rise around the 3 edges, and a 'belly' in the middle. So the water pools just in from the edges, and at the back by the roof tiles. It wasn't installed with a fall towards the front, and there are no side trims to guide the water that way; and no trough at the front. Also, the flashing at the back corners were poor, and have allowed water ingress.

Would I be better trying to upgrade the rear corner flashing and retrofitting side edging trims - if possible. Or should (and could) I overlay this roof with OSB board and EDPM, with a forwards fall and proper edgings?

[SteamyTea]

Apart from where the water is coming in at the back, which is poor design, rather than material failure, is there a problem?

Can you sort the roof to flat roof interface and get that section relaminated? 

Edited May 9, 2021 by SteamyTea

[Wales]

Hello SteamyTea,

Thanks for your response.

The patches you can see on the back near corner are Bostik Flashband that I've put on, and it did seem to work.

Over the last few days we've had high winds, hailstones and a lot of heavy rain. This is Wales!

Water pools just in front of that Flashband, and runs over the edge just where you see that rope coming off the roof.

So it could be that I haven't sorted the corners with the Flashbend, although there is no longer a leak inside at that top corner; or it could be the running water is going under the tiles and running along a joist. The current leak, inside, is equivalent to about halfway down the slope of the dormer, and about 200mm out from the dormer wall, towards the camera.

Hopefully not, but it could even be a problem caused by ineffectively fitted flashing of the dormer wall to the roof tiles. That's why I thought that a slight slope and proper side trims might take the water away from that area. 

[joe90]

Nothing wrong with water pooling on a GRP roof, it’s what boats do all the time. Yes the GRP should lap up the roof and your felt cover it by at least 150mm. I think the sides are a little high because preformed edges are usually used here. I would have built a slight slope to the front if it were me but I am a bit OCD. you can add to GRP by sanding back and patching with more GRP.

Edited May 9, 2021 by joe90

[SteamyTea]

51 minutes ago, Wales said:

The current leak, inside, is equivalent to about halfway down the slope of the dormer, and about 200mm out from the dormer wall, towards the camera.

Water will wiggle about till it finds and easy route, why it is really hard to find a leak.  Wind direction and rainfall can confuse even more.  I know, I live in even wetter and windier Cornwall.

I suspect that the problem is where it joins the roof, the GRP work looks alright, if a little rough.  As @joe90 says, we would have put some fall on, but then we know what we are doing when it comes to GRP, unlike most builders.

 

Either way, it can be sorted.

 

The Problem and then the solution.

 

[canalsiderenovation]

On don't jinx everything @SteamyTea

 

it's not leaking - yet!

[joe90]

57 minutes ago, canalsiderenovation said:

it's not leaking - yet!

And you won’t because you had (second time around)a roofer that knew what he was doing, I have done quite a few GRP roofs and never had one leak ?

[SteamyTea]

5 minutes ago, joe90 said:

have done quite a few GRP roofs and never had one leak

Same here.

Only viable long term covering.

[canalsiderenovation]

13 minutes ago, joe90 said:

And you won’t because you had (second time around)a roofer that knew what he was doing, I have done quite a few GRP roofs and never had one leak ?

 

Very true! ???