In recent years EPDM/Rubber (giant sheets of rubbery plastic) has reappeared on the market mainly because 'felt' contractors can't get naked flame insurance to lay their felt. Apart from being cheap EPDM/Rubber has nothing to commend it.  

It can also wreck your roof since in high exposure areas (North of England & all of Scotland) it should have ~1 tonne of stones per 10 square metres to hold it down. A typical 25 sqm roof extension will have 2.5 tonnes added to it - your joists and Artex won't like that!  

We've lost count of the EPDM roofs we have replaced.  Using what is basically a pool liner for roofing work just doesn't work when things get difficult - like where the material meets edges, walls, tiles, slates, pipes, chimneys, gutters, drains, skylights, vents, parapets etc etc. 

Stretching, caused by shrinkage, is the primary failure indication for EPDM roofs. This is usually discovered when the building leaks and a roof inspection shows that the membrane is pulling away from inside corners and angle changes. Building owners are frequently reporting leaks after five to seven years, with replacement required in the first 10 or 12 years of service.

Related Links:

Roof System EPDM Shrinkage: Further information on EPDM Shrinkage

I will tell you why, Rubber is not seamless, the seams are held together with glue.
This will eventually come apart allowing water to seep in. The membrane is also very easily punctured and cannot be walked on.

Fibreglass is totally seam less, no joins means no wateR. It is 3 times thicker than rubber so also can be walked on. It also cannot be punctured with a knife and is the toughest.