Impervious roof area in the United States is on the order of 11,000 square miles, and those roofs shed an estimated 5 trillion gallons of stormwater a year, plausibly 4 to 7 trillion depending on rooftop area and regional rainfall. Roofs are roughly a quarter to a third of all impervious surface, and because they are nearly waterproof they convert about 90 percent of the rain that hits them into runoff that a drainage system, not the soil, has to handle. That physical fact is what stormwater utilities are now pricing, charging property owners by the square foot of roof and pavement they own.
Reviewed by The Roofing Brief Team. Last reviewed: October 2026. The 11,000 square mile figure covers the United States only, not Canada or Mexico, and every number here is an estimate built from public datasets and standard hydrology.
How much of America is roof?
The United States holds roughly 43,000 square miles of impervious surface, and rooftops make up an estimated quarter to a third of it, or on the order of 11,000 square miles. That rooftop footprint is close to the land area of Maryland, all of it shedding rain instead of absorbing it.
The anchor figure comes from Christopher Elvidge and colleagues at NOAA, whose 2004 study in Eos (American Geophysical Union) mapped 112,610 square kilometers, about 43,480 square miles, of constructed impervious surface across the lower 48 states and Washington, DC, with an error band of roughly plus or minus 12,700 square kilometers. Commonly cited land-cover breakdowns put roads and parking at about two-thirds of that total and rooftops at the remaining share, which places national rooftop area in a band of roughly 9,000 to 14,000 square miles. This report uses 11,000 square miles as a central estimate and carries the full range through the runoff math.
How much stormwater runs off American roofs each year?
American rooftops generate an estimated 5 trillion gallons of stormwater runoff per year, with a defensible range of about 4 to 7 trillion gallons. The estimate (5.2 trillion gallons at the 11,000 square mile central case) follows the logic engineers use to size storm drains: a roof is nearly waterproof, so almost all rain that lands on it leaves as runoff rather than soaking in.
The calculation applies a runoff coefficient of 0.90 to rooftop area at roughly 30 inches of annual precipitation, the approximate national average. In the Rational Method (Q equals C times I times A) used across US stormwater design manuals, roofs carry a runoff coefficient near 0.90 to 0.95, meaning 90 to 95 percent of rainfall becomes runoff once the surface is wet. At the central estimate of 11,000 square miles of roof, that yields about 5.2 trillion gallons a year; the 9,000 to 14,000 square mile range spans roughly 4.2 to 6.6 trillion gallons.
For scale, 5 trillion gallons (about 6.7 hundred billion cubic feet) would put more than 20 feet of water over the land area of Rhode Island (about 1,030 square miles). Unlike rain on soil or forest, almost none of it recharges groundwater; it moves fast, picks up whatever is on the roof and the pavement below, and arrives at streams and pipes in a concentrated pulse.
Runoff by roof size: how the load scales
Runoff scales directly with roof area, so a 100,000 square foot big-box store sheds as much stormwater as roughly 58 average 1,700 square foot houses (1.68 million / 29,000 gallons). The table below applies the same method (30 inches of annual rain, 0.90 runoff coefficient) to common roof sizes, and expresses each as Equivalent Residential Units (ERUs), the billing yardstick utilities use, at a 3,000 square foot ERU.
| Roof / property type | Approx. roof area | Est. annual runoff | ERUs (at 3,000 sq ft) |
|---|---|---|---|
| Average single-family home | 1,700 sq ft | ~29,000 gallons | 0.6 |
| Large single-family home | 2,600 sq ft | ~44,000 gallons | 0.9 |
| Strip-mall retail unit | 10,000 sq ft | ~168,000 gallons | 3.3 |
| Big-box store | 100,000 sq ft | ~1.68 million gallons | 33 |
| Distribution warehouse | 1,000,000 sq ft | ~16.8 million gallons | 333 |
Large flat-roofed commercial buildings (see commercial building roof types) dominate a jurisdiction’s stormwater load: a 1,000,000 sq ft warehouse is 333 ERUs, so fees charged by measured impervious area put a large part of the bill on it. A single 1-million-square-foot warehouse roof generates about the same annual runoff as roughly 580 average houses.
How stormwater fees price your roof
Stormwater utilities charge property owners for the runoff their impervious surfaces create, and roof area is the biggest single input. More than 2,200 stormwater utilities now operate in the United States, according to the Western Kentucky University Stormwater Utility Survey led by Warren Campbell, and most bill by the Equivalent Residential Unit.
An ERU is the impervious area on a typical single-family lot in that jurisdiction, including the roof, driveway, and patio. The WKU survey puts the median ERU at 3,000 square feet (mean 3,128, standard deviation 1,413). A home is billed a flat number of ERUs; a commercial parcel is billed its measured impervious area divided by the ERU, then multiplied by the monthly ERU rate, which commonly runs from about $4 to $15.
The fee is a runoff charge, not a property tax, so it lands regardless of assessed value, and it is one of the few utility bills a green roof or permeable surface can measurably lower. The sample below spans about $5 a month in Washington, DC to $49 to $79 in Portland.
| Jurisdiction (2025 rates) | Fee basis | Typical single-family monthly charge |
|---|---|---|
| Washington, DC (DOEE) | $2.67 per 1,000 sq ft ERU | ~$5 to $8 (2 to 3 ERUs) |
| Seattle, WA (SPU drainage rates) | Tiered by parcel size, under 2,000 to 9,999 sq ft | $19.61 to $77.46 (2025: $235.28 to $929.48 a year); $37.26 for a 2,000 to 3,499 sq ft parcel |
| Portland, OR (BES) | Tiered single-dwelling | ~$49 to $79 |
| National (WKU survey) | Median 3,000 sq ft ERU | ~$4 to $15 per ERU |
Charges are not comparable line for line: Seattle bills drainage by parcel size ($235.28 to $929.48 a year in 2025), Portland by tiered single-dwelling charge ($49 to $79 a month), and some cities bundle stormwater into a clean-rivers charge. Where a utility measures impervious area, the roof is the line item.
Can a green roof cut the runoff (and the fee)?
A green roof can retain a large share of the rain that would otherwise run off, and many utilities give a fee credit for it. Across a global synthesis of 75 peer-reviewed studies (about 2,375 samples), green roofs retained an average of roughly 62 percent of rainfall, with extensive (shallow) systems averaging near 56 percent and a wide spread driven by climate, media depth, and season.
Retention is highest in warm months and lowest in winter, when the growing media is already saturated. A shallow sedum roof in central Pennsylvania retained roughly 50 to 60 percent of annual rainfall in Penn State Center for Green Roof Research monitoring, close to 100 percent from May through September but only 20 to 30 percent in cold months. That seasonality matters: a green roof shaves the annual runoff total and blunts small storms, but it is not a substitute for drainage in a large event.
Green roofs, permeable pavement, and cisterns are the standard ways to lower a measured stormwater bill (see the EPA Soak Up the Rain guide to green roofs), and utilities publish credit menus tied to how much impervious runoff a property removes. For a roof that cannot carry soil, the alternative is blue roof detention. For structural load and cost detail, see our green roof guide and the range of sustainable roofing options; reflective roofs, another impervious surface with its own climate math, are covered in our cool roof energy savings report.
Decision table: roof type and what it means for runoff and fees
| If your roof is | Then |
|---|---|
| A 1,700 sq ft single-family home | About 29,000 gallons a year and 0.6 ERU at the 3,000 sq ft median; most utilities bill a flat ERU count |
| A 100,000 sq ft big-box roof | About 1.68 million gallons and 33 ERUs; at $4 to $15 per ERU that is roughly $133 to $500 a month before credits (Roofing Brief arithmetic) |
| Structurally able to carry a green roof | Expect about 62 percent average rainfall retention (extensive systems near 56 percent) and ask the utility for a fee credit |
| Unable to carry soil load | Compare blue roof detention and cisterns against the utility credit menu |
| In Seattle | The fee follows parcel size, not roof area: $447.08 a year in 2025 for a 2,000 to 3,499 sq ft parcel |
Methodology
National rooftop area is derived from the Elvidge et al. (2004, Eos/AGU) figure of 43,480 square miles of total US impervious surface, apportioned to roofs using a commonly cited one-quarter to one-third rooftop share, giving a 9,000 to 14,000 square mile range and an 11,000 square mile central estimate. Annual runoff applies the Rational Method logic: runoff volume equals area times annual precipitation (about 30 inches) times a runoff coefficient of 0.90 for roofs, converted at 7.48 gallons per cubic foot and 27,154 gallons per acre-inch. Per-property runoff in the size table uses the same precipitation and coefficient. Fee and ERU figures come from the Western Kentucky University Stormwater Utility Survey (Campbell) and published 2025 municipal rate schedules (DC DOEE, Seattle SPU, Portland BES). Green-roof retention figures come from peer-reviewed synthesis and Penn State Green Roof Center monitoring.
Limitations
These are order-of-magnitude estimates, not a measured inventory. The largest uncertainty is the rooftop share of impervious surface, which is not precisely known nationally and drives the runoff range. National average precipitation masks large regional variation, and rooftop area is concentrated in wetter, more populated regions, which could push the true runoff figure toward the high end. The 0.90 runoff coefficient is a design-standard approximation; actual roof runoff varies with slope, material, and storm size. Fee figures reflect a few published 2025 rates and a national survey and are not comparable line for line across jurisdictions with different billing structures. No proprietary or unpublished data was used, and no figure here should be read as engineering guidance for a specific site.
Frequently asked questions
How much of the US is covered by rooftops?
Rooftops cover an estimated 9,000 to 14,000 square miles of the United States, on the order of 11,000 square miles, or roughly a quarter to a third of the country’s approximately 43,000 square miles of impervious surface (Elvidge et al., 2004, Eos/AGU). That rooftop footprint is close to the land area of Maryland.
How much stormwater runs off American roofs each year?
American rooftops generate an estimated 5 trillion gallons of stormwater runoff a year, with a defensible range of about 4 to 7 trillion gallons. The estimate applies a 0.90 runoff coefficient to roughly 11,000 square miles of roof at about 30 inches of annual rainfall, the same Rational Method logic used to size storm drains.
How do stormwater fees charge for roof area?
Most of the more than 2,200 US stormwater utilities bill by the Equivalent Residential Unit (ERU), the impervious area on a typical single-family lot, with a median of about 3,000 square feet per the Western Kentucky University survey. Homes pay a flat number of ERUs; commercial parcels pay their measured impervious area divided by the ERU, times a monthly rate that commonly runs $4 to $15.
What runoff coefficient do roofs use in stormwater design?
Roofs carry a runoff coefficient of about 0.90 to 0.95 in the Rational Method, meaning 90 to 95 percent of the rain that hits a wet roof becomes runoff rather than soaking in. That is near the top of the scale, higher than lawns or gravel and comparable to asphalt pavement, because a roof is designed to shed water.
Can a green roof lower my stormwater fee?
Often, yes. Green roofs retain an average of roughly 62 percent of rainfall across peer-reviewed studies, and many stormwater utilities offer a fee credit for green roofs, permeable pavement, or cisterns that reduce measured impervious runoff. The size of the credit and the payback depend on local rates, structural capacity, and program rules, which vary by jurisdiction.