Here is a typical flat roof issue.
A “flat” roof is meant to have a slight slope on it so rainwater can drain away. If it doesn’t standing water creating a permanently damp environment causes moss accumulation as we have here.
In turn, the moss means the felt remains almost permanently damp resulting in more moss growth which leads to a faster breakdown of the felt than should be the case….and so on.
So, every now and then check your flat roof and carefully remove any moss with a soft brush to ensure the felt stays a clean and dry as possible.