It surprises some people to hear that flat roofs aren’t entirely flat. They do need a slight slope to move water, which is normally 3:12 and lower in terms of roof geometry. Anything less than 1:12 can be built up from insulation to add a slope.
Slopes 3:12 and below are too low for shingles, so we use other products like torch on or tar & gravel. Torch on is a type of roofing material that gets applied with a flame. It covers the entire roof’s surface without needing nails or screws. This means there’s no breaks or protrusions in the roof (over time these protrusions would wear out and let water through with this design). Water is instead allowed to collect and is then directed through a drainage hole.
Flat roofs are commonly installed on commercial buildings but are also a good option for many residential homes, too.
