Things are not always as they seem. Below are some photos I came across on Truck Floor Training. It shows a commercial flat roof. Seems normal right??? As we get into it there are many layers and depth to this roof. You see a commercial roof with a slope for drainage to the edge and a center ridge line. Roof is EPDM membrane (rubber) over 0.5" fiberboard over PVC membrane over 0.5" fiberboard over 1" expanded polystyrene over 0.5" fiberboard over 0.75" Tar and Gravel built-up-roof on a wood deck. Inside the building there was a drop ceiling, nothing too remarkable.
Turns out the sloped framing was installed over more than one flat roof level, no one onsite remembers how long ago. The flat roof is a 1" Tar and Gravel over the wood deck.
There was a large space between the two with doors knocked between walls to access all the attic spaces, but it appeared to have sprinklers. This leaves a huge common opening for fire travel. Thankfully this one is sprinkled, but many may not be. Interior crews may not see anything. Meanwhile, a raging fire could be burning between these two roofs above their heads. Ventilation, and fire attack become difficult and dangerous.