Remove the old covering or if it is asphalt then re-lay a new material over the top.
If it is possible then build in a slope to 1:80.
Then cover with a material called Sarnafil which can either come in a PVC or a TPO membrane.
You would have to get a registered contractor to carry out the works.
This material is guaranteed whether there is standing water on it or not.
As the material is fusion welded at the joints this makes the roof one continious sheet, without the problems of other "one sheet roofing".
This type of roofing has 200% elongation which basically means that it will expand/contract and move with the building.
It also has it's own coated metal for making trims and flashings.
Don't believe all this flat roofing = problems nonsense.
It's is badly done flat roofs which equal problems.
Check out www.sarnafil.com
We have carried out roofs of 4500m2 in Moscow, Russia on the new British Embassy, so even temperature does not affect this type of roofing.