How much water comes off a roof in a storm?

Each inch of rain on 1,000 square feet of roof produces about 620 gallons of water. In Irving’s 2-year storm, 1.77 inches in one hour, a 2,500 square foot roof sheds about 2,760 gallons in that hour, delivered to just a few downspouts. Rein Irving Drainage Company sizes downspout lines from numbers like these, not guesses.

Where the 620 gallons comes from

One inch of water over one square foot is 144 cubic inches. A US gallon is 231 cubic inches, so each square foot of roof sheds about 0.623 gallons per inch of rain. Multiply by 1,000 square feet and you get about 623 gallons, usually rounded to 620.

The formula for any roof and storm is simple:

Gallons = roof footprint (sq ft) x rainfall (in) x 0.623

A little rain is lost to splash, but the full figure is the safe one for planning.

Irving’s design storms

Drains are sized for short, intense bursts, not for annual rainfall (see how to fix yard drainage problems). The DFW area averages about 37 inches of rain a year, with May (4.78 inches) and October (4.37 inches) the wettest months, but what overwhelms a downspout is how much falls in an hour. NOAA’s Atlas 14 gives these figures for Irving:

A “2-year storm” is not a rare event. It has roughly even odds of happening in any given year, which is why it is a sensible baseline for a downspout line.

Worked table: gallons off a roof

These figures use 0.623 gallons per square foot per inch, rounded to the nearest 10 gallons.

Notice how fast the numbers climb: the 100-year day on a 2,500 square foot roof is more than nine times the water from a single inch of rain.

How it splits between downspouts

Gutters gather all that water and drop it at a handful of points. Divide the roof into the sections that drain to each downspout and work out each one separately.

Example: a 2,500 square foot roof with four downspouts, each draining about a quarter of the roof (625 square feet).

Real roofs rarely split evenly: a long back slope can send half the roof to one corner. Rain also falls harder in the worst minutes than the hourly average, so peak flow is higher than shown. That is why one downspout often floods while the others cope.

What this means on the ground

Next to the foundation

Seven hundred gallons in an hour poured into a few square feet beside a slab is far more than Irving clay can absorb. At a USDA soil survey sample point in Las Colinas, the soil maps as Ferris and Heiden clay, about half clay particles, which absorbs water extremely slowly. A sample point is one spot, not every lot, but on soil like that the water ponds, runs along the foundation or soaks in unevenly. That is the case for foundation drainage that starts at the downspouts.

Splash blocks and short extensions

A splash block moves water a couple of feet, so hundreds of gallons land just past it, often on soil that slopes back to the house. Check the first ten feet with the slope guide.

Buried lines

A buried line carries each downspout’s water in smooth solid pipe to a pop-up emitter, swale or curb. A single 4 inch line suits most individual downspouts; lines that collect several downspouts, or a large roof section, are sized for the combined flow. Perforated pipe is the wrong choice; see solid or perforated pipe. Roof water also should not be tied into a French drain, which floods under this volume.

The outlet

Every gallon in the table has to leave the lot somewhere allowed. University Park requires roof drainage to be piped through the curb or to the alley line. Grand Prairie bars concentrating runoff onto a neighbor’s lot; see neighbor water runoff. For Irving we confirm the rule per address. See the permits guide.

Work out your own roof

1. Measure the house footprint including overhangs. A roofing estimate’s square footage is the sloped surface, which overstates the area that catches rain.
2. Note which roof sections drain to each downspout.
3. Multiply each section’s area by 0.623 and by the storm depth.
4. Look for the downspout carrying the most and check where it discharges.

Next step

As a national budgeting range, buried downspouts run about $150 to $350 each; the downspout cost guide and the cost tool help set a budget. For a layout sized to your roof, see downspout drainage or send your roof details.

Questions people ask

Do I measure the roof's sloped surface or its footprint?

The footprint. Rain falls roughly vertically, so the amount a roof catches depends on the area it covers seen from above, including overhangs, not on how steep or long the shingled surface is.

Does a metal roof shed more water than shingles?

Slightly more reaches the gutters, and faster, because less is held on the surface. For drainage planning the difference is small; the 620 gallons per inch per 1,000 square feet figure is used for both.

How many gallons per minute does one downspout carry?

It depends on the roof area feeding it and how hard it rains. A downspout draining 625 square feet in a 1.77 inch hour averages roughly 11 to 12 gallons a minute, with higher peaks during the heaviest minutes.