Ours needs replacing and i can't seem to find an unbiased opinion on the web.

Main factors are longevity and price

ta

All depends on quality of work done. I would opt for fibreglass myself, but get a company that comes recommended. Seen some proper bodged roofs

Does it need to be flat? you can convert to pitched and would help water runoff rather than pond.

Is the whole roof flat or is this a section? ie how are you intending to manage the verticle/sloping to horizontal joints etc.

I would lay a heavy tpo or pvc membrane and then balast or green roof it to make it look nicer and protect from UV.

It's the whole roof, kitchen extension and no room for a pitched roof.

Hadn't thought of a green roof, will look into that

My father just had a fibreglass replacement done on two of his flat roof sections. He wasn't impressed at having to get someone in to do it (he'd always have done it himself with me helping in the past but I dont live there anymore) so he watched them like a hawk and got a decent deal on what seems to be a very good job. One thing it does do though, is creak and crunch loudly with temp changes. Was done with a light upper colour and dark sides so it reflects heat while not looking stupid.

We had a 'liquid plastic' job done…

was not these guys that fitted it (contractors name escapes me, was 4 years ago….) but this site may give you some pointers?

Ours was to seal a 'rotten' concrete roof – they fitted a framework of treated timbers which allowed them to put a very slight pitch on, which is all you need, insulated, boarded, fabric membrane, liquid plastic rollered on…. cost about 2.5k i think… not the cheapest but i was looking for a 'fit it once… ' option…

I'm just doing the same and have opted for felt with white chippings on top, the chippings are meant to prolong the felt life by about 10-15 years. Something to do with reflecting heat and light stops the felt degrading,
Or go for a green roof 40 years + lifetime I've heard

We have just had our entire roof re done by "the flat roof company" , very expensive £7 k but very good , they basically stripped off all the old roofing rebuilt with a very slight pitch and installed a shed load of high spec insulation .The main surface is fiberglass. we had one problem in exceptional weather which they fixed no fuss imediately. the roof comes with a twenty five year guarentee.

Go for the Fibre Glass. We do flat roofs and usually felt them,but it`s crap at the end of the day. All dependa if the customer can afford other options.

Sarnafil