One of the most important parts of site development is grading for drainage. Driveways cross public sidewalks and intersect with traffic, so the slopes for driveways have to drain efficiently and allow safe driving without interfering with adjacent sites. Each jurisdiction has particular requirements for site work, such as driveways, but a few general recommendations for slopes apply to most situations.
One of the critical factors in driveway design is the degree of slope. Driveways need to drain water to avoid deteriorating, but steep grades are difficult to drive. Minimum and maximum slope recommendations can change according to the location. Hilly locations may have to accommodate steeper slopes, and cold, snowy climates need flatter grades because of icy conditions. As a guideline, no more than 4 inches of water should pond on the surface longer than 15 minutes after a rainfall.
Recommended Slopes for Drainage
Outdoor paved surfaces should be sloped with a minimum of 2 percent slope, or 2 feet of rise over 100 feet. Lower slopes of 1 percent, or 1 foot rise over 100 feet, may be acceptable, but in locations with heavy precipitation, runoff drains slowly and is more likely to collect. The maximum grade for a driveway is 25 percent, or 25 feet rise over 100 feet. Transition zones are recommended for driveways that steep.
Typically, driveways drain toward swales or gutters, requiring a cross, or double, slope, where the water drains in two directions. The most common drainage pattern for driveways with curbs and gutters is crowned, where water is directed both to the sides and downhill. Valley drainage is used most often for asphalt; the water is drained toward the center and downhill. This isn't recommended where driveways cross a public sidewalk. Cross slopes should drain with a minimum 2 percent slope.
Where slopes are steeper than 12 percent or vehicles are low to the ground, a transition zone may be necessary. Slopes are moderated to a less-steep slope for a few feet in a transition zone. Adjacent to the garage is another location where steep slopes should be lessened. A critical transition is where the driveway crosses a public sidewalk. Check with the local authority for specific requirements for the transition to the driveway slope.