Flat roof, water pooling

[pauldm]

I have pooling at the rear of my flat felted dormer which causes all the rainfall to stream over the edge while nothing comes out of the gutter.
The pool is about 5 foot long. 2 foot wide. 3/8 inch deep. My plan was to bitumize paint a OSB3 sheet adhere it to the setion then refelt to heighten & slope this area. Also run a 1" inch high felt & bitumized guard rail made of say 1"x2" timber along the dormer edge (which couldn't be seen from street level) I realize that my dormer roof must not have the correct slope but without major works I can see this as the only fix unless anyone else has any ideas.
Two sketches included, thats not my actual dormer.

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[Xenon]

If its pounding, make sure the deck hasn't slumped first. If its all good, just put a check kerb in.

 

[pauldm]

I'm sure its not slumped. But a check kerb only will just cause an even bigger pool & won't push the water out to the front of the dormer which I think is at least flat surely? Like this>