Fibreglass or rubber to cover dormer's gently sloping roof?

[David001]

We've bought a house that had a "flat" dormer-roof fitted into a sloping ceiling to create an emergency exit from a first floor room.  This was done 12 years ago and the roof of the dormer was covered with fibreglass.  Since last autumn water's been coming in through the ceiling, directly under a corner of the dormer-roof.  I got a ladder out and climbed up to see what's going on.  The surface of the fibreglass in the area above the leak looks almost glass-like, i.e. somewhat milky and transparent, seemingly made of ice-like brittle plastic, and in its surface are numerous cracks and crevices.  The dormer roof slants slightly toward the centre of the house, so that rainwater runs towards the centre of the house but exits right and left of the dormer, then off down the slate tiles on both sides of the dormer to the gutters.  I think the leak has been caused by autumn leaf-fall, which is huge, when lots of dead leaves get caught in the exit area, so the rain-water banks up a bit, and then on very cold days and nights the mess of leaves and water freezes, probably causing the cracking in the fibreglass directly beneath.   The rest of the fibreglass surface of the dormer-roof looks as if it is in good condition.

 

A builder has recommended laying a new timber framework over the dormer so that the new roof-surface will slope away from the centre of the house, directing rain-water to a new gutter on the edge of the dormer, which would then empty into the already existing gutters at the edge of the main roof.  As for the surface, the builder said fibreglass or rubber would be suitable.  I've since done a bit of research and read that fibreglass is poured onto a dormer in liquid form, so if that's the case I imagine it would flow over the edge of the dormer into the gutter and cause mayhem. 

 

The house is 350 metres above sea-level, so it's pretty cold here most of the year, as well as wet.  The dormer is on the north-facing side of the house, so it doesn't get much radiation in winter.

 

The photo below shows the state of the fibreglass in the area where the leak has existed (and which I have temporarily covered with a roof sealant).

 

 

 

The photo below shows the dormer as it currently is.

 

 

What should I go for?  Fibreglass or rubber? 

 

Thank you!

 

 

Edited May 10 by David001
To add photos.

[joe90]

@SteamyTea is your man on fibreglass, he will be along shortly (it looks like damp got in during the cure when it was laid).)

[nod]

I used fibreglass on ours I had no experience but found it pretty easy with some good advise from a boat builder friend of ours 

[Libertinedreamer]

Hi,

Flat roofer here…the system is almost secondary to the installer. 
 

The arguments for fibreglass are always things like ‘the coat boats with it’ but houses and boats function very differently.

 

I dont like any of the systems proposed but I think you will have less issues long term with rubber. 

[SteamyTea]

8 hours ago, David001 said:

I think the leak has been caused by autumn leaf-fall, which is huge, when lots of dead leaves get caught in the exit area, so the rain-water banks up a bit, and then on very cold days and nights the mess of leaves and water freezes, probably causing the cracking in the fibreglass directly beneath. 

Hard to tell from the pictures, but you may be right about the drainage.

 

It also looks like someone skimped on the materials.  It could be a mismatch of resin types, but that is an unknown now.

But most likely, as @joe90 says, it was initially laid up onto something that was damp.

 

You could try recoating it, once cleaned up, with one of the polyurethane systems.  They have good reports. @Onoff did his roof with it and it seems to have worked.

[David001]

@SteamyTea, your suggestion of re-coating is very interesting.  Thank you for your help.

 

As I mentioned, I thankfully stopped the leak by spreading a layer of a roofing "gunge" over the cracked area.  I did it crudely (the finish is not at all smooth, but is full of ridges from the spatula I used).  So I can see sense in the idea of having the whole dormer-roof "simply" coated with a thick layer of a liquid fibreglass.  On the negative side this would mean sticking with the slant of the roof going towards the centre of the house, but on the positive side I imagine it would save a contractor having to pull up the existing roof (and thereby possibly causing all sorts of knock-on problems).

 

But if using pour-on (liquid) fibreglass, how would the vertical edges of the dormer be treated?  Is there a method of pouring the fibreglass over the edges so that it runs uniformly down the sides but catching it before it falls elsewhere?

 

When I first got up onto the dormer to try and find the source of the leak I immediately noticed a bit of stick-on, aluminum-like, flashing close to the area where the fibreglass was cracked, evidence that the previous owner had also tried to deal with a leak in the same area.  I looked all around the edges of this (cheap) adhesive-flashing but it seemed to be well-adhered, without any openings through which rain-water might be passing.  If a thick coat of liquid fibreglass were applied to the whole dormer roof could it also be applied over that small area of cheap adhesive-flashing?

 

Thank you very much for your help.