Roof slope guidelines

Minimum and maximum slopes vary by roof system and project type


Proper roof slope can be an important consideration when designing high-performance roof systems. Minimum and maximum roof slope limits vary based on specific roof system and project types. Following is a review of code requirements and NRCA's roof slope guidelines, as well as some additional factors designers should consider.

Code requirements

Building codes generally provide prescriptive minimum roof slope requirements for various roof system types. For example, the International Building Code,® 2018 Edition (IBC® 2018) prescribes a 1/4:12 minimum roof slope for asphalt built-up, polymer-modified bitumen, thermoset and thermoplastic single-ply, and liquid-applied membranes in new construction. This 1/4:12 minimum roof slope requirement also applies to spray polyurethane foam (SPF) roof systems in new construction.

For metal panel roof systems with nonsoldered seams with applied lap sealant, IBC 2018 prescribes a 1/2:12 minimum roof slope. For standing-seam metal roof panel systems, a 1/4:12 minimum roof slope is prescribed.