Gutters built for the rain that actually falls on the Southeast

Rain by the year and rain by the minute

The first column is each airport station's 1991-2020 annual precipitation normal from NOAA's National Centers for Environmental Information; each figure links to the station's data file. The two intensity columns come from NOAA Atlas 14 through the Precipitation Frequency Data Server, read at the latitude and longitude listed.

Atlas 14 states these as recurrence intervals. A "10-year" intensity is one expected about once a decade on average at that spot, which does not stop it from arriving two summers running.

Why the five-minute number is the one that matters

A rate of 11.9 inches per hour, Mobile's 10-year five-minute intensity, works out to roughly an inch of rain in five minutes. In every city in the table, the Atlas 14 ten-year rate for a five-minute burst is more than three times the ten-year rate averaged over a full hour. A gutter sized only for the hourly average can be overwhelmed during the heaviest few minutes of a storm.

Volume is the other half of the picture. The U.S. Geological Survey's rainfall figures put a 1-inch rainstorm at 27,154 gallons on one acre. Scaled to a 1,000-square-foot roof, that is about 623 gallons for every inch. At Mobile's ten-year five-minute rate, the same roof sheds on the order of 120 gallons a minute while the burst lasts.

Sizing starts with the plumbing code

The International Plumbing Code's storm drainage chapter bases the sizing of leaders and storm drains on the 100-year hourly rainfall rate, or on other rates from approved local weather data, and it sizes horizontal gutters by the flow coming off the roof surface. The sizing guide walks through that method with the Atlas 14 numbers above.

When the hurricane arrives

Tropical systems bring the extreme totals. National Hurricane Center reports record 30.78 inches in Busick, North Carolina, during Helene and 29.99 inches at Orange Beach, Alabama, during Sally. What those totals do and do not mean for a gutter is covered in the hurricane rain guide, and the five Gulf cities get their own page on Gulf Coast gutters.

Seamless runs and storm prep

A seamless gutter is formed as one continuous piece between corners and outlets, so there are fewer joints along the run to seal and maintain. Whatever the material, it only works if it drains. Ready.gov's hurricane page lists de-cluttering drains and gutters among the steps to strengthen a home before a storm. At ground level, the plumbing code's disposal rule allows storm water from one- and two-family homes to discharge onto flat areas such as lawns, provided it flows away from the building.

Finding help with the roof edge

Gutters hang on the fascia at the roof's edge, so storm damage at the eaves can involve both. You can find a roofer in any state on the National Roofing Directory, check local rules on its roofing licensing page, and follow active weather on the NRD Storm Center.

Frequently asked questions

Which Southeast city in this list gets the most rain?

Pensacola, with a 1991-2020 annual normal of 68.31 inches at its regional airport station. Miami and Mobile follow at 67.41 and 67.08 inches.

How much water comes off a roof in an inch of rain?

About 623 gallons per 1,000 square feet of roof, based on the USGS figure of 27,154 gallons per inch of rain on one acre.

What does a 100-year rainfall rate mean?

It is a rainfall intensity expected about once in 100 years on average at a given location, though nothing stops it from recurring sooner. The International Plumbing Code uses the 100-year hourly rate, or approved local data, as the basis for sizing storm drainage.

Why do gutters overflow in summer storms?

Because the rain comes in bursts. Atlas 14 shows five-minute intensities in these cities that are more than three times their hourly rates for the same return period.