NC Roof Runoff Worksheet, Gutter Sizing Calculator, Rainwater Guide (PDF Download)

A worksheet for North Carolina homeowners. Work out the actual number of gallons coming off your roof, whether your gutters and downspouts are sized for it, and where the water is supposed to go.

THE NUMBER THAT SURPRISES PEOPLE

A 2,000 square foot roof footprint sheds roughly 1,250 gallons in a one-inch rainstorm. Charlotte averages 43.6 inches of rain a year, so that roof handles about 54,000 gallons annually - and every drop of it lands within a few feet of your foundation before anything moves it away.

This is why so many drainage problems are actually roof problems. The yard did not change. The water was always arriving; it just used to leave somewhere you did not notice.

WHAT IS INSIDE (9 pages)

- The conversion, derived step by step so you can check it rather than take my word for it: gallons = roof footprint x inches of rain x 0.623
- A table of gallons by roof size, for a 1-inch rain, a 2-inch storm, and the full year
- NOAA rainfall intensities for the Charlotte area, 2-year through 25-year storms
- Gutter sizing by diameter and slope, and downspout sizing by rainfall rate
- Discharge distance, and where roof water may legally go in NC
- Rainwater harvesting: what NC statute protects, first-flush diverters, mosquito screening, cistern installation figures
- A page on what I could not verify, and why

THE PROBLEM HIDING IN THE SIZING TABLES

The gutter tables in the plumbing code are published at a 4 inch per hour design rainfall. Charlotte's 10-year, 5-minute intensity is roughly 8 inches per hour.

The table scales inversely, so a gutter rated for 625 square feet on paper handles about 313 square feet in a real Charlotte storm. Which means a gutter that is technically compliant can still overflow twice a summer with nothing broken. It was sized for a gentler storm than the one that arrives here.

This guide gives you the correction.

AND THE MOST COMMON FITTING IN AMERICA

NC State Extension, verbatim: Splash blocks are not recommended as they release water too close to the home. The number they give is 10 feet downhill and away from the house. A splash block stops erosion at the pipe. It does not relocate anything.

WHO WROTE IT

Alex Purdy, Carolina Terrain - a licensed North Carolina landscaping, drainage and hardscaping contractor in Waxhaw, Union County. NC Landscape License CL.1872, NCSU SCM Inspection and Maintenance Certification #5970, NDS Certified Drainage Contractor.

SOURCES

NOAA Atlas 14, NOAA climate normals, the USDA-NRCS North Carolina roof runoff standard, the NC plumbing code storm drainage tables, NC State Extension gutter management, NCDA and CS Soil and Water, NC DEQ rainwater harvesting, and NC General Statute 143-138(b). Full numbered source list included.

HONEST ABOUT THE GAPS

Some code tables sit behind a paywall. The guide says which figures I could verify directly and which came from a public reproduction of the identical table, rather than presenting everything with equal confidence. It also tells you that no authoritative table exists converting a 2x3 downspout to a roof area, and what to use instead.

FORMAT

Instant digital download. PDF, 9 pages, printable at home on US Letter. No physical item ships.

PLEASE NOTE

Arithmetic and published guidance, assembled so a homeowner can work out their own numbers. Not a plumbing design, not a stormwater design, and not a code compliance document. I am a licensed contractor - not a professional engineer and not a licensed plumber.