Hi,
Twinkle's DH here.
I work for a flat roofing company. So declaring an interest from the off - however I don't know where you are based so not touting for business.
Firstly there is nothing wrong inherently with a flat roof per-se - it's what we have traditionally covered them with that is the problem.
You do need some sort of pitch even on a "flat" roof - a minimum of 1 in 80 but I would recommend 1 in 60 as better. Also make sure the architect takes the water from the property away to the edge of the roof, and not as I've seen before, from the edge of the roof back to the main building and then out through internal drains and all sorts - just bad practice.
Felt, has typically a lifespan of 7-10 years, slightly longer if covered in chippings. People ignore their roofs until they leak - which is about 2 years too late! Also, walking on it causes it to crack quicker.
There are however a range of products on the market now that have significantly longer lifespans. The product we use is called "Rubberbond" - it is an EPDM elastomeric single ply membrane with a fleece reinforcement layer. In a nutshell, fit this roof and you will NEVER have to re-cover it. The first roofs were done in 1962 in the US and no roof has ever had to be recovered (ok, sounding like a sales pitch over).
We give 25 year guarantee on it - most other installers will give 20-25 on it as well.
Do not confuse it with Firestone - which is a simple EPDM - it's cheaper, but no-where near as strong and won't last as long.
Insurance companies know about it, and whilst you still have to declare if you have an area of flat roof, once you tell them it's Rubberbond you'll be fine.
Do NOT fit fibreglass whatever you do. It's brittle, and whilst very strong, it has a tendency to crack.
There are also liquid coatings available - some better than others so do your research carefully on these.
Needless to say, Rubberbond isn't cheap - typically on a new-build it will cost 10-20% more than felt or fibreglass, but given the lifespan benefit it's well worth the extra.
Hope that helps, but for more info please PM DW.