Help needed on flat roof low spot

[deanogtv]

We moved into our house around 2 years ago now in which time I have managed to refub every room in the house. Being a 1970s build house the exterior is pretty bland so this is my next long term project. The house has a small hall way entrance with a cloak room which leads into a converted garage all of which is contained under a front roof. The garage was converted around 4 years ago and i’m pretty sure the flat roof was replaced then with an roofing felt system, excuse my non builder terms here. Its water tight and functions fine. However there is a large low spot which does not drain, its about 5-7 mm lower than the drainage hole. In the winter water will sit there for weeks on end. I’m guessing in the long term this isn’t good. To solve this I need to ideally raise this level to stop water ponding here, but what can I do to do this cheaply.
Would a shingle help displace the water?? I’m guessing I would want a lite covering which will not add weight and depress the low spot further.
Next door have shingle and a moss covering, I’m guessing the moss has grown naturally over time.

Here is a street view before the garage conversation, I'm the house on the right.

[deanogtv]

Its not rubber, its definitely felt.
I have read that the shingle protects the felt which I may cover just to help anyway and I'm guess the shingle will help to displace the ponding water anyway.

Should I use normal pea shingle, 5mm stone for a builders yard??