Residential gutter sizing — Houston rainfall intensity (NOAA Atlas 14 + SMACNA)

Gutter sizing on a residential roof in Houston is driven by four inputs: the tributary roof area each gutter run serves (the slope-projected area shedding water into that run), the short-duration rainfall intensity for the location, the gutter profile geometry, and the number of downspouts on the run. SMACNA's Architectural Sheet Metal Manual (7th edition, 2012) is the industry-standard reference for the calculation; SMACNA also publishes a free Downspout and Gutter Sizing Calculator that automates the methodology using a Rainfall Wizard for the 5-minute duration at either the 10-year or 100-year storm return interval. NOAA Atlas 14 Volume 11 (2018) is the underlying rainfall frequency data source for Texas — Houston's 100-year rainfall depths were revised upward in that publication, with the Lake Houston area reaching 17-18 inches at the 24-hour duration. The short-duration rainfall intensities that drive gutter sizing are correspondingly elevated for the region. The K-style profile (sometimes called ogee) is the dominant residential profile and the format the dispatch-grade aluminum coil is rolled in. Standard 5" K-style aluminum is adequate for many Houston residences; 6" K-style is commonly specified when the tributary area per run is large, the roof pitch is steep enough to deliver water to the gutter quickly, the run length means a small number of downspouts must carry the volume, or the homeowner is preemptively sizing for the heavier end of Atlas 14's revised intensities. Adding a downspout to a run reduces the volume each section of gutter must carry, so the sizing calculator can sometimes resolve an undersized condition with a downspout addition rather than a larger gutter. The calculator and full methodology should be the basis for the spec on any home with unusual roof geometry rather than a blanket "5-inch is fine" assumption. [Source: SMACNA Architectural Sheet Metal Manual 7th edition; SMACNA Downspout and Gutter Sizing Calculator; NOAA Atlas 14 Volume 11 — Texas precipitation frequency]