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For asphalt pavement, a runoff coefficient is commonly in the 0.85 to 0.95 range for streets, although broader engineering tables may use about 0.70 to 0.95 depending on the surface and design method. TxDOT lists asphalt streets at 0.85–0.95, while the Federal Highway Administration gives 0.70–0.90 for paved surfaces generally.
If you mean an asphalt-shingle roof, a commonly listed runoff-coefficient range is about 0.75 to 0.95.
What a Runoff Coefficient Means
A runoff coefficient, often written as C, is the share of rainfall that becomes runoff instead of being absorbed, held on the surface, or otherwise lost.
For example:
- C = 1.00 means essentially all rainfall becomes runoff.
- C = 0.90 means about 90% becomes runoff.
- C = 0.50 means about half becomes runoff.
The coefficient has no units.
Asphalt has a high coefficient because it is mostly impermeable. Water cannot readily soak through ordinary asphalt pavement, so most rain moves across its surface.
Typical Runoff Coefficients for Asphalt
The correct number depends on what type of asphalt surface you mean.
| Surface | Typical runoff coefficient |
|---|---|
| Asphalt street | 0.85–0.95 |
| Asphalt or concrete pavement, broader range | 0.70–0.95 |
| Asphalt-shingle or similar shingle roof | 0.75–0.95 |
TxDOT lists asphalt streets at 0.85–0.95, while another municipal drainage standard uses 0.95 for asphalt, concrete, and rooftop surfaces. These differences are normal because drainage manuals may set coefficients for specific design conditions.
That means there is no single runoff coefficient that is correct for every asphalt surface.
Asphalt Pavement vs. Asphalt Shingle Roofs
The word "asphalt" can cause confusion because these two surfaces behave differently.
Asphalt pavement
A paved driveway, parking lot, or road generally sheds water very efficiently. TxDOT's Rational Method guidance gives asphalt streets a coefficient of 0.85 to 0.95.
The practical guide “How Do I Calculate How Much Rainwater I Will Collect?” shows where debris, overflow, or poor discharge can disrupt collection.
The lower end may better represent conditions where water can collect in depressions, drain toward edges, or encounter other losses. A smooth, well-drained surface will normally be toward the higher end.
Asphalt-shingle roofs
Asphalt shingles are also largely water-resistant, but their rough granular surface can hold a small amount of water. Roof geometry, debris, gutters, and the size of the rain event also affect how much eventually reaches a storage tank.
Published drainage tables commonly place shingle roofs around 0.75 to 0.95.
For rainwater harvesting, remember that the roof runoff coefficient is only one part of the calculation. Water can also be lost through a first-flush diverter, overflowing gutters, screens, leaks, tank overflow, and other parts of the collection system.
How the Coefficient Affects Rainwater Yield
For a roof, a simple U.S. customary estimate is:
Collected water (gallons) = roof area (sq ft) × rainfall (inches) × 0.623 × runoff coefficient
The 0.623 factor converts one inch of rainfall on one square foot into gallons.
Suppose you have:
- 1,000 square feet of roof
- 1 inch of rain
- runoff coefficient of 0.85
The estimate is:
1,000 × 1 × 0.623 × 0.85 = about 530 gallons
Without any losses, that rainfall would represent about 623 gallons. Applying a 0.85 coefficient reduces the estimated roof runoff to about 530 gallons.
Actual water reaching the tank may be lower once first-flush diversion, gutter losses, overflow, and other system losses are included.
Why Asphalt Runoff Coefficients Vary
A coefficient is an estimate rather than a fixed material property.
Several conditions can change runoff.
Rainfall intensity
With calculating a roof runoff coefficient, you can coordinate roof runoff and available drainage capacity.
A short, heavy storm may produce a greater percentage of runoff than a very light rain because initial surface wetting becomes less important.
Surface condition
Cracked pavement, depressions, accumulated debris, and damaged surfaces may retain or redirect some water.
The walkthrough “How to Calculate Roof Rainwater?” helps match the drainage path to the expected rainfall load.
Slope
Steeper pavement generally moves water away faster. Flat areas can hold water in small depressions.
Drainage layout
Curbs, gutters, landscaped edges, drains, and adjoining surfaces can change how much water actually reaches the point where you are measuring runoff.
Previous moisture
A surface that is already wet behaves differently from one that is dry at the beginning of a storm. Iowa DOT notes that runoff coefficients can be influenced by factors including surface type, storage, previous moisture, rainfall duration, and rainfall intensity.
Which Number Should You Use?
For a stormwater engineering calculation, use the coefficient required by your local drainage or engineering standard. Do not substitute a generic internet value when designing drains, culverts, detention systems, or other infrastructure.
For a rough rainwater-harvesting estimate from an asphalt-shingle roof, work within the roof coefficient range rather than assuming every drop will reach your tank. A lower coefficient gives a more conservative estimate of available water.
If you are sizing a tank, it is also important to account for:
- Local rainfall
- Roof footprint
- First-flush diversion
- Gutter and screen losses
- Tank overflow
- How much water you actually use between storms
Do not use a runoff coefficient of 1.0 simply because the roof appears waterproof.
Can You Harvest Rainwater From Asphalt Pavement?
Water can physically be collected from an asphalt driveway or paved area, but pavement runoff needs more caution than roof runoff.
Driveways and roads can collect:
- Oil and fuel residue
- Tire and brake particles
- Sediment
- Animal waste
- Lawn chemicals
- Road salts or deicers
- Other contaminants carried by vehicles and foot traffic
That makes pavement runoff a poor choice for drinking-water collection without a properly designed treatment system, suitable collection practices, current laboratory testing, maintenance, and compliance with applicable local requirements.
The planning note “How Many Gallons of Water Can I Collect from My Roof?” helps match the drainage path to the expected rainfall load.
Even for irrigation, consider where the runoff comes from and what contaminants may enter it.
An asphalt-shingle roof also does not make collected rainwater automatically potable. Drinking-water use requires looking at the entire collection and treatment system rather than relying on the runoff coefficient or a single filter.
Frequently Asked Questions
What is the runoff coefficient of asphalt?
Asphalt pavement is commonly assigned a runoff coefficient around 0.85 to 0.95, although some engineering references use a broader range of approximately 0.70 to 0.95.
What is the runoff coefficient of an asphalt-shingle roof?
Shingle roofs are commonly listed at approximately 0.75 to 0.95. The exact value used depends on the calculation and applicable design guidance.
Does a coefficient of 0.9 mean 90% of the rain can be collected?
It means the calculation assumes about 90% of the rainfall contributes to runoff. For a rainwater-harvesting system, additional losses can occur after the water leaves the roof, so the amount entering your tank may be lower.
Is asphalt more efficient at producing runoff than grass?
Yes. Conventional asphalt is largely impermeable, while soil and vegetation allow much more rainfall to infiltrate or be retained. Engineering runoff coefficients for lawns are therefore much lower than those used for asphalt.
Should I use 1.0 for an asphalt surface?
Usually not. Even very impermeable surfaces have some wetting, storage, evaporation, and drainage losses. Published design values for asphalt are normally below 1.0.
Can I use the same coefficient for a driveway and a roof?
Not automatically. Asphalt pavement and asphalt-shingle roofing have different surfaces and are treated separately in many drainage tables. Use a coefficient appropriate to the actual catchment surface.
Is the runoff coefficient enough to size a rainwater tank?
No. You also need roof area, local rainfall, expected water use, storage capacity, and realistic collection losses. Tank sizing should consider how rainfall and water demand change throughout the year.
