roof levelling ?

[Megnom]

We have a conservatory with a flat roof that dips in middle and retains rainwater to a depth of approx 1cm, this stagnates.
Would like to use some form of leveiling compound such as sold by Wickes for floor levelling but am advised it is not suitable for extrnal use.
Can you suggest what would do instead please?

 

[Alastairreid]

build it up with torch on felt.

 

[Nige F]

Or you could use Wicks cold roof felt adhesive and cold mastic repair compound -your choice Tho` the torch on would probably be a better repair