If this technique is not possible below are other ways to detect a leak on a flat roof and remember to clearly mark the area of the leak when you find it.
Patch repair flat felt roof.
We had a problem with our shed workshop flat felt roof leaking and needed a quick repair.
Press the patch firmly into place and make sure that the edges are well sealed.
Permanent flat roof repair this can either be a patch of torched on roofing felt or a layer on top of your existing.
Apply some bitumen adhesive over the repair and beyond to suit the patch and apply it.
Instead of replacing the felt roof we decided to use a product call.
Firstly measure from the outside wall of the room along the ceiling to the damp spot dripping area.
Sweep away water and debris from the damaged area.
In addition sweeping the area clear gives you a better view of the damage.
To clear the roof use a tough broom.
Cut a piece of new felt to use as a patch big enough to give above a 50mm 2 inch overlap beyond the repaired blister.
Any leftover dirt gravel and water in the area can prevent the repair material from bonding to the roof.
Always clean up any areas you wish to repair.