What slope per foot is commonly prescribed for new construction on flat roofs?

The correct answer is 1/4". This slope is generally recommended for new construction flat roofs to ensure proper drainage. Flat roofs, despite their name, should not be completely level; a slight pitch is necessary to facilitate water runoff and prevent ponding.

A slope of 1/4" per foot translates to a 2% grade, which strikes a balance between being gentle enough to maintain a usable surface while being steep enough to effectively channel water away from the roof's surface. Adequate drainage is critical to the longevity of the roofing materials, as standing water can lead to leaks and structural damage over time.

Other slope options, while they might be sufficient in certain contexts, are not commonly prescribed for standard flat roof applications. For example, a slope of 1/8" may not provide adequate drainage, allowing water to accumulate, whereas a steeper slope like 1/2" might create challenges for roofing systems that require a more level surface for installation. Thus, the 1/4" per foot slope is widely accepted as a standard in the industry for effective water drainage on flat roofs.