Flat roofs came into use in the Americas in the 19th century, when new waterproof roofing materials and the use of structural steel and concrete made them more practical and economical. Flat roofs soon became the most commonly used type to cover warehouses, office buildings, and other commercial buildings, as well as many residential structures.
The level contour of a flat roof displays better wind resistance than angled systems that have shingles or panels. They also make it possible to locate condensers, air handlers, and other HVAC equipment on top of a building instead of beside it or inside it. Placing HVAC equipment on top of a building can be a valuable noise reduction strategy for buildings where HVAC equipment cannot be located on the ground or mounted on exterior walls. Due to its minimal slope, a flat system is easier to access for unit maintenance as well.
Compared to an angled system that has shingles or metal panels, a flat system requires fewer construction materials. Consequently, it typically costs less to purchase than an angled roof. Fewer construction materials can also translate into reduced labor cost and waste removal cost during construction. And where gabled roofs that have a steep pitch waste valuable space, a flat roof allows every square foot of a facility to be utilized in some fashion.
Commercial districts are designed so that every inch of space is made useable. Flat roofs are much more appealing in these areas. The flat roof allows for closer spacing, which looks better in any city or business district.