What slope classification defines a flat roof?

A flat roof is defined as having a slope that is less than 3:12. This classification means that for every 12 horizontal units, the roof rises less than 3 vertical units. Roofs with such a slight slope are considered flat because they do not achieve the minimum incline required for effective drainage and are typically designed to minimize water accumulation.

In contrast, roofs with slopes of 3:12 or greater (including the options of 3:12, 4:12, and 7:12) are categorized as pitched roofs, which have significant slopes that facilitate water runoff. Pitched roofs generally employ a steeper slope to encourage drainage and prevent issues such as standing water or water pooling, which can lead to leaks or structural damage over time. Therefore, the classification of a flat roof as less than 3:12 is critical for understanding design criteria, drainage solutions, and maintenance strategies specific to flat roofing systems.