Quick answers
Why is my roof leaking when it has been waterproofed?
Water rarely comes through the middle of a roof. It gets in where the waterproofing is interrupted: seams, the corner with the parapet, and the bases of posts, pipes and drains.
A roof rarely leaks because the whole thing has failed at once. Water finds the places where the waterproofing stops or changes material: the seams between sheets of bitumen membrane, the corner where the floor meets the parapet, the bases of posts, railings, solar water heaters and air conditioners, the pipes coming up through the slab, and the drains. Materials move differently there as it gets hot and cold, so that is where they open up first.
On this roof the membrane was mostly in place, but around a rusted post it had torn and come loose. A small opening like this is enough for water to reach the ceiling below. The stain on the ceiling can even show up some way from where the water gets in, because water travels under the membrane until it finds a way through.
On another membrane roof the seams between the sheets had opened and the finish at the foot of the parapet had gone. The loose pieces were cut out, and every seam was sealed with a white strip of waterproof coating, as was the whole parapet. On a third, the coating at the bases of the metal posts was reinforced with fabric and taken up onto the metal as well.
If the old waterproofing is still holding over most of the roof, repairing these spots may be enough. If it has come away over a large area, patches won't do. Which case you are in can only be seen by someone looking at the roof up close.
Jobs that show it
Sealing Pipes, Posts and Parapets on a Roof
The places where pipes and steel posts pass through the membrane are usually where water gets in. Here each one was sealed with a white waterproof coating.
See the job step by step
Membrane Roof: Sealing the Seams and Parapets
The membrane had torn at the edges and its seams had opened. The loose pieces were removed, and the seams and parapets were sealed with a white waterproof coating.
See the job step by step
Sealing the Bases of Metal Posts on a Roof
The old seal around the posts had cracked. Each base was covered with a white waterproof coating reinforced with fabric, taken up onto the metal as well.
See the job step by step