Roof drainage sizing calculator.
A monsoon dumps a year of Seattle rain in ten minutes. Undersized gutters overflow, undersized downspouts back up, and flat roofs pond. Use IPC-2021 rainfall intensity for your city — not guesses.
Albuquerque · 1800 sf plan area · effective 1890 sf
Each 3×4 handles ~1043 sf at 4.6"/hr. Space no more than 40 ft apart.
6" K-style handles up to 3339 sf. You have 1890 sf.
Each 4×4 scupper drains ~1739 sf. Always add an overflow scupper 2" above the primary per IPC 1108.
IPC 1108: every flat-roof drainage system needs a secondary (overflow) drain or scupper sized equal to the primary. NM code enforces this.
Why NM homes drown their own foundations.
Overflows in the first 90-second cloudburst. Water sheets down stucco, saturates the foundation, cracks slab.
Even 6-inch gutter overwhelms because water can't exit fast enough. Add a mid-run outlet.
Primary clogs with cottonwood catkins, water ponds 4 inches, membrane fails at seams. Overflow is code — enforce it.
Water pooling or overflowing? Free drainage inspection — (505) 595-7203.
Book the free on-site inspection, or send a photo and we'll tell you what to do next. No sales pitch — we don't install roofs.