Flat roofs used to be a lot darker than they are today. They’d be tar or they’d be torchdown — usually black or similar.
Then concerns about global heating became common. This was a problem for dark-colored roofs because they absorb heat. White reflects solar rays so much that people began asking for white-colored roofs on their buildings.
This seemed like a good idea at first. It’s easy enough to re-tool factories en masse and sell a bunch of white membranes instead of black ones. But as with all things, there are unintended consequences.
The primary one of our concern is that many of these new white roofs began failing — some in terrible fashion. Leaks would appear, but they weren’t actual leaks. Roofing structures for many commercial buildings would rot from the inside out.
What was going on?
The new white roofs were revealing a weakness in the roofing system that dark roofs didn’t allow to surface that often. That weakness is condensation creation and buildup.
Flat roofs don’t have attics to ventilate moisture. Instead, to control moisture, flat roofs need need a layered system of insulation and vapor barriers. Without that system, moisture will generate inside the system then rot from the inside.
How do black roofs like torchdown help alleviate this issue? By helping to release “trapped” moisture. A white roof doesn’t get that hot because it reflects so much of the solar radiation. But a dark roof absorbs most of the heat. This causes the roofing system to increase in temperature significantly more than a roofing system with a white top membrane does.
This heating can cause moisture inside the system to vaporize and travel through it and release from it. The amount of rot greatly reduces in that case. Many white roofs have suffered tremendous damage because moisture would diffuse into the system then condense and stay there. Over time, this can cause total failure of the roof.
These days, black roofs are very uncommon, and white roofs are still common. The best of both worlds is to go with a dark gray roof together with a system of insulation and vapor barriers. In that case, many commercial buildings will find that not only is their roof not falling apart from the inside, but in 20 years when it comes time to get a new roof, the system is still intact and they just need a new waterproofing layer.