Flat roof ponding

[Alfish]

I have a side extension that is 19ft by 5ft covered with a silver grey type thick sheet substance ( apologies no idea what it is) it is not leaking but ponding in the middle after rain. It is only a few MM deep but need quotes for repairing as selling house and buyers want to know how it can be fixed so it doesn't leak as home buyers report flagged it up. Has anyone got any ideas what can be done and how much it should cost ?. I am looking at a repair ideally not a new roof .
I have attached a photo of my roof if you know what material it is that would be great to.

 

[bernardgreen]

Ponding on a flat roof is not a problem provided the roof does not leak. Just the opposite in fact. The water slows down the rate of change of temperature of the roof when the sun is strong and this reduction in thermal changes prolongs the life of most roof coverings.

Some flat roofs have been designed to have water permanently on them to achieve this effect. But not many roofers ( or house owners ) are happy with the concept.

 

[Alfish]

Thanks for the reply. The only issue is anyone coming to quote will just say a new roof rather than the substance you have should be fine for a few years.