Old commercial flat roofs are a mix of different types. Ours was an old rubber roof with heat-welded seams. They had been tarred over many times. There were patches all over the place. An old AC unit that was removed had a patch where the pipes went through and where the legs on the unit were at. It needed replaced to stop the many leaks inside the building. None were really bad, but you do not want any water dripping in. We called a company that does roof replacement in Manhattan NY to come out for an estimate.
The previous tenants of the building would just set up buckets here and there in the building when it rained. They just used it as a repair shop, and they were not concerned about a little rain getting on things. We needed a dry space as it was being converted into a gallery and workspace for our art. A lot of older buildings in New York have had their interiors fully renovated to bring them up to code for modern use while maintaining the look and feel of the old structures.
The new flat roof is bright white. This will help a lot in the summer to keep the building cool. Those old black rubber roofs absorb a whole lot of heat. The white reflects it. There is actually a recommendation for cities to have white rooftops to save massive amounts of energy in the summer. It was a few weeks before inside renovations were begun, and the old buckets sitting out to catch the rain were dry as a bone when a week of rain hit. There was not a drop of water coming in through the roof. We were really looking forward to having the inside fully renovated now so we could start using the space.