This effect is known as ponding. Flat roofs are not technically flat and should have a minimum gradient of 1:40. However if this is not done correctly or the structure is is altered over time (usually due to foot traffic) then ‘ponding’ can occur. If the water remains this creates a perfect environment for moss growth and over time the water can freeze and thaw causing further problems. In the short term there is no cause for alarm, however in the long term the weight of the water can cause sagging of the rafters, leading to cracks in the membrane and eventual leaking.
No. Any part of a roof can be repaired but consideration should be given to the following:
Fibreglass is a fantastic alternative. Using fibreglass can create an aesthetically pleasing and useable roof space. This strong and durable material has little intrinsic value and therefore is not attactive to thieves. More properties and benefits of this material can be read here.
Yes. You will commonly find GRP on: