I have been roofing for 38 years and in my opinion fibre glass (GRP) is the best of the cheaper flat roofing systems. 

GRP will, if laid correctly,  last more than 20 years. It is laid on new decking and this creates an inseparable bond. 

if you find the roof has started to deteriorate in 20 to 30 years time, you simply sand down the roof and lay another coat of GRP, thus removing the necessity to rip up the old roof. This saves a lot of time, work, mess and, of course, expense. 

A GRP roof is immensely strong and durable,, this is the 21st century breakthrough in roofing technology.