Flat Roof

In our last house we had a flat roof porch on the south side which was felted and got extremes of heat. It cracked and leaked. We had it replaced with fibreglass. It was flatter than it should have been so puddles did form on top but it never leaked in the next 20 years until we left. I notice the ‘new’ people are now having it rebuilt - but that could be because the wooden sides were starting to rot when we moved 14 years ago!

We put a properly draining fibreglass flat roofed utility room on the side of our new home and that has never been a problem.

You need expert advice on this, speak to your architect they usually are very good.
I personally wouldn't buy a property with a flat roof too many nightmare scenarios from people we know with small areas of flat roof.

Flat roofs are not a nightmare, trouble is caused by people doing nothing when they first notice a leak so it gets worse and they may dread it raining but that's because they chose to do nothing at the first sign of trouble. I have 2 areas of flat roof on my property, both are part of the original design but they weren't designed to last for ever and they haven't, both have been replaced and been problem free. I also have a conventional tile roof and two areas of that have needed attention, requiring the flashing to be replaced, tiles removed and replaced and the felt under the tiles being replaced. It was somewhat inconvenient but hardly a nightmare. I've lived here 20 years so expect to renew and replace parts of the property surely that's normal wear and tear. So as long as you are aware that certain parts of the building will need replacement over time I can't see a problem.

sarnafil,guaranteed for 20 years and will last far longer. Needs to be put on by a registered company. Was problem free for us