Driveway grade and scraping check

Grade is a percent, not a degree

A 10 percent grade rises 10 feet in 100 feet of horizontal run; it is only 5.7 degrees. People who mix the two imagine a 10 degree driveway, which is 17.6 percent and a very different pitch. Always check which one a rule or a drawing uses.

How do I calculate the grade of a driveway?

Divide the rise by the run and multiply by 100. The run is the horizontal distance, not the length along the surface. A drop of 6 ft over 50 ft of run is 12.0 percent, which is 6.8 degrees, or 1:8.3.

What is the maximum slope for a driveway?

It is set locally, and there is no single national number. One California county, Sonoma, bars a road or driveway steeper than 15 percent unless the county fire chief approves, and bars anything over 20 percent altogether; a stretch between 15 and 20 percent may be approved for up to 300 ft in any 1,000 ft. Other places have lower or higher limits, often lower close to the street. Ask your planning or public works office.

Is 1:12 the right slope for a driveway?

No. The 1:12 slope, 8.3 percent, is the steepest running slope allowed for an accessible ramp under the 2010 ADA Standards (section 405.2). A driveway is a different thing; a car can climb much more, though ice, a loaded truck and a manual-transmission start on a hill become harder as it gets steeper.

What is a breakover angle?

The largest change in grade a car can drive over a sharp crest before its belly touches. It follows from two numbers, the wheelbase and the clearance under the middle of the car. A longer car or a lower one has a smaller angle. Manufacturers publish it for off-road vehicles.

Why does my car scrape at the end of the driveway?

The street is a gutter or a crown that falls away and the driveway rises, or the driveway rises steeply and then levels. In both cases the profile changes angle over a short distance, and the car's belly or its overhang meets the surface. Making the change gradual over a longer distance, or building a longer transition, fixes it.

How is the grade change at a crest found?

Convert each grade to an angle with the arctangent and subtract. Going from a 20 percent rise to level, the change is arctan(0.20) − 0 = 11.3 degrees. Going from a 25 percent rise to level it is 14.0 degrees.

How much concrete does the drive need?

That is a volume job: width × length × thickness, in cubic yards. Use the concrete calculator and add the extra for the sloped length, which is a little more than the run on a steep grade.

Grade, degrees and ratio

Grade is rise over run, written as a percent. The angle is the arctangent of the grade, so 100 percent is 45 degrees and the two do not move in step: 8.3 percent is 4.8 degrees, 15 percent is 8.5 degrees, 20 percent is 11.3 degrees. A ratio such as 1:8 is the same slope, run per unit of rise. The surface you pour or pave is slightly longer than the run, by the factor √(1 + grade²).

Worked example: will the car clear the crest?

Take a car with a 120 inch wheelbase and 6 inches of clearance under its middle. Its breakover angle is 2 × arctan(2 × 6 ÷ 120) = 11.4 degrees. A crest where a 20 percent rise meets level ground is a 11.3 degree change, just under the limit, so it clears if the crest is sharp and has no margin if the car is loaded. A 25 percent rise to level is 14.0 degrees and scrapes. With only 4 inches of clearance the limit is 7.6 degrees.

Limits of this method

The breakover formula is our own geometry for a rigid car, not a published standard: it takes the lowest point to be under the middle of the wheelbase and the crest to be a sharp corner. A rounded crest, softer suspension and where the lowest point really is (an exhaust or a skid plate can be off center) all change the result, and so does a road that is not flat. The page does not check the approach and departure angles of the bumpers, which govern the ends of a driveway, or the legal limit where you live. Local code, and the fire department, can set the maximum grade, the width and the landing near the street.

Common mistakes

Measuring along the slope and calling it the run. Mixing up percent and degrees: a 45 degree slope is 100 percent. Planning only the average grade of a long drive and missing a short, steep step in the middle. Forgetting drainage: a drive that slopes toward the garage needs a drain at the bottom, and a steep gravel drive washes out.

Sources: Sonoma County Permit and Resource Management, Fire Prevention bulletin, Maximum Grades for Roads and Driveways (permitsonoma.org; Sonoma County Code section 13-31(a) and California Code of Regulations Title 14, section 1720.03, as quoted there); 2010 ADA Standards for Accessible Design, section 405.2 (access-board.gov). The breakover formula is our own geometry. Related: slope calculator, concrete calculator, gravel calculator, asphalt calculator, ramp calculator. How we check our calculators.