Cross Slope for a Parking Lot

A parking lot is sheet-drained on a gentle slope so cars stay put and water does not sit. Accessible stalls have a tighter flatness limit, so those bays often sit on a leveled area. Ponding in the aisle means the grade is wrong. More sealer will not fix it.

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Gentle sheet flow, not a ramp

A parking lot needs enough slope that rain moves to an inlet or a curb cut, and little enough slope that a car in neutral does not roll and a shopping cart does not take off. That compromise is the cross slope and the slope along the stalls. Designers often talk in percent. One percent is about 1/8 inch per foot. Two percent is about 1/4 inch per foot. A lot can use a gentle sheet slope in the drive aisle and still be too steep for an accessible stall. Write the percents on the grading plan before asphalt arrives, because the paver will follow the stone you shaped. A level does not replace a survey at a large lot, but it will show you a birdbath the eye missed.

Accessible bays are the flat shelf

A slope that is comfortable for drainage can be too steep for an accessible stall and its access aisle. Those bays are often placed on a leveled area, with the rest of the lot draining around them. Check the current ADA Standards and the local official for the flatness limit and the markings before you pave. Paint will not flatten a ramp. If an overlay will change the slope, mill or redesign so the accessible bay stays within the limit and still connects to the walk. The asphalt tons follow the paved area, including that bay and the aisles. At 145 pounds per cubic foot, 1,000 square feet at 2 inches is about 12.1 tons as a planning estimate. The plant ticket is the real weight.

A pond is a grade problem

Water sitting in the aisle after a shower means the grade is wrong, or an inlet is high, or a curb is acting as a dam. It does not mean the lot needs another coat of sealer. Sealer is a film. It will not fill a birdbath that holds an inch of water. Find the low spot with a string or a level, and fix it with milling, a leveling course, or a drain. Leveling mix is hot mix at variable thickness. Plan any uniform area you repave near 145 pounds per cubic foot and treat the ticket as the real weight. Keep the overlay from burying a gutter that used to carry the water. A blacker aisle that still ponds will ice in winter in the same place.

A practical sequence

Set a gentle sheet slope so water moves and parked cars stay put.
Put accessible stalls on a leveled area and confirm the limit in the current ADA Standards.
Treat ponding as a grade or inlet problem, not a sealer problem.
Plan overlay tons at 145 pounds per cubic foot, and use the plant ticket as the real weight.

The mistake to skip

Sealing a birdbath in the aisle and expecting the film to make the water leave.

Questions people ask

How steep should a parking lot be?

Steep enough to sheet-drain, gentle enough that cars do not roll. People often talk in the range of about 1 to 2 percent for general pavement, which is about 1/8 to 1/4 inch per foot. Accessible stalls are held to a tighter flatness limit. Check the current ADA Standards and the local official for those bays. This page is not a drainage code.

Will sealcoat fix standing water?

No. Standing water means the grade, the inlet, or a curb dam is wrong. Sealcoat does not fill a birdbath. Correct the slope or add a drain, then pave. New asphalt plans near 145 pounds per cubic foot as an estimate, and the plant ticket is the real weight. Keep accessible bays within the flatness the official requires.

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