A flat roof requires a curb mount skylight design. This design prohibits water on the roof from submerging the skylight and causing leakage. A plastic dome skylight is also less likely to leak on a flat roof.
The mounting for skylights on flat roofs must be completely watertight as flat roofs do not shed water as pitched roofs do. This means any openings in the mounting can be the cause for leaking. A curb mount is designed specifically for flat roofs and sits proud of the roof, with no corner openings.
The dome skylight's design also lends itself well to flat roofs. The shape allows for quick shedding of the water down and away from the skylight itself and from its base. Shedding of water also means dome skylights stay clean and free of debris better than flat-topped skylights.
Curb flashing should be used when installing a skylight on a flat roof. A flashing made of copper should surround the base of the skylight. It should be well attached to the rubber roof to avoid leaks. The curb flashing is then attached to the copper flashing to create the final seal. A skylight that sits above the roof's surface by several inches is less likely to leak.