Water test around the drain right at the pipe with a little water running just at the drain only, keep on moving the water source a little bit to the outside of the drain until you get it to leak again to pinpoint your hole, then you will have a better idea who you need to fix the problem. I would say a roofer if it is on the roof are decking.
If it is around the drain it will be caulking with a type of silicone if it is getting between the drain and the roof membranes. it has a lot to do with what type of roof systems is on there now if it is leaking out away from the drain. There are different types of flat roof systems if the roof is small enough under 20 ft wide there are one pc rubber roof made by Good year and last a lifetime that will have no seems and will never leak again. This same type of membrane I have used over 30 years and never a problem, and I have also applied for pond liners and lakes.
When you add layers over and make several seems there will always be that chance of a seem leaking if the seems fails and then you will have to start over. The less seems, the better. Roofers would do the roof membranes, where some roofers also do decking if needed, if it has leaked for a while, then there may be some rotted decking.
Find the source of where the leak is coming from to know who to call to fix the problem. The membrane should also put on with a drain just like a shower drain where the membrane is locked tight on top of the drain with bolts as the membrane is cut with a hole in it only at the drain opening where it can not leak.