You seem swayed towards this fibreglass thing ... about 6 months ago I did a bit of fibreglassing and made this ...
http://i39.photobucke...xitagain/DSC03758.jpg
I'm no expert ... but fibreglass is good stuff ... I have made gutters from it and done roof repairs with it. But when It comes to covering your roof you will never get the smooth finish you may wish for..( do you overlook the roof ?)
As chas has already said .. it is not always 100% waterproof ...
When I made the boat I know it is 100% because I am applying the gel coat to a mould ... but when the gel coat is the last thing to be applied on a roof, usually with a brush. I personally would'nt be so confident.
Personally I could easily do my roof in fibreglass ...
But sorry it's Butyl every time for me ... less than a mornings work to do my whole house, no waste and it will be wrinkle free and 100% waterproof.