Flat roofing is generally reserved for commercial buildings, but that doesn't mean you can't use it for residential roofing. In fact, there are some interesting advantages as well as disadvantages to consider here. The maintenance aspects would be another thing to consider if you did decide to go with a flat roof over a sloped roof on your home.
Advantages of a Flat Roof Water can be channeled to the four corners of the roof and directly into downspouts for dispersal away from the foundation of the home. With some flat roofs, it may also be possible to funnel precipitation into a main drain situated on the roof. In this case, the roof is slightly downward sloped toward the middle to get the water to funnel toward the drain. The drain then empties out and away from the house.
Roof design is key. If the roof is flat and you live where ice may accumulate, you can easily step onto the roof and remove the ice. There's less risk of falling off the roof too because you can't slip down a slope that doesn't exist. Repairs also tend to be easier for the roofing contractor.
With less attic space (no attic, really!), your home will heat and cool more efficiently. The heat or cooled air won't need to rise into the peaks of the attic or upper story. The air stops at the ceiling below the flat roof.
You can convert a flat roof into another story later on. Many people who start out with a flat roof on part of their homes use this space to build onto their homes. Sometimes it's just another room with a roof. Sometimes it's an entire one or two stories for extra living space.
Disadvantages of a Flat Roof Drainage can be a big disadvantage if the roof isn't designed well. Standing water can occur, which may create water damage problems eventually. Hail can hit a flat roof and cause greater damage because it provides a huge target for hailstones. The hail won't bounce off the flat roof like they would off of a sloped roof.
Additionally, birds and other animals may choose to live on your roof or nest there. Their activity and their droppings on your roof can create structural damage, which requires vigilant preventive measures. Certain roofing materials, like asphalt shingles, are typically not required under building codes, which is both a plus and a minus in terms of cost and protection respectively.
Maintenance Flat roofs need frequent cleaning. If building code says you don't have to have any sort of protective layer beyond rubber under-roofing, then you may need to replace that rubber layer every couple of years. If you go ahead and put shingles on that flat roof, it needs to be cleaned often to prevent accumulation of leaves, debris, moss, algae, and animal waste. In winter, ice needs to be removed or you will need to install heating coils to prevent ice from forming.
Drains in the roof and/or downspouts have to be cleared and flushed often. Likewise, if you have gutters on your flat roof, they have to remain clear to help with adequate drainage. Poor or no drainage leads to water damage under this kind of roof.
Structural integrity of a flat roof has to be checked regularly. Any areas of weakness can result in a collapse of the roof during a heavy storm or high winds. An annual inspection of the roof is recommended, regardless if the flat roof is commercial or residential.
Hail can cause incredible damage to a flat roof, especially if there isn't a protective layer of shingles or metal. After a hailstorm, check for rips in rubber, cracked shingles, dented metal roofing, etc., as it applies to the flat roof. Have it repaired and restored right away.
If you have other structures on the flat roof (e.g., cooling or ventilation equipment), these structures should be inspected by the proper professionals. A roofing contractor can look at these structures, but it's best left to an HVAC professional or an electrician. (Flat vents are an exception to the rule. They vent air from inside directly to the outside only have to be cleaned and cleared of debris.)