Whether you use concrete or asphalt for your driveway or parking areas, there's no simple answer for what thickness is best. The correct answer depends on the weight it will bear, the kind of traffic it will see, the soil conditions, and the subbase of your project.

Think of the surface of your driveway or parking area as the roof of your house. If your foundation is crumbling, the whole house - including the roof - is going to shift. Likewise, if a wall collapses, the roof over that wall is going to sag. Like the roof of your house, the surface of your parking lot or driveway exists to drain off water, snow, dirt and other materials. It protects what is underneath it.

Just like your home props up your roof, your subbase is critically important in the longevity of your driveway and parking area. It must be smooth, level and free of all vegetation. The last thing you want is dandelions growing through the cracks in your driveway!

When building a roadway, the Portland Cement Association recommends preparing the subgrade and using a base layer. The illustration here clarifies the idea of an untreated, a treated, and a compacted level base beneath the concrete surface.

Your driveway likely won't endure the same traffic as a major highway, but the goal is to have a uniform thickness of concrete mix on a prepared surface with the appropriate slope and elevation to allow water and melting snow or ice to drain from the surface.

RECOMMENDED CONCRETE THICKNESSES

The Michigan Concrete Association recommends following these guidelines when determining the thickness of your concrete parking area or driveway:

• Light Use: 4" concrete - Parking for passenger cars, light trucks and occasional use by heavier trucks.
• Medium Use: 5" to 6" concrete - Driveways and parking areas for light to medium trucks plus occasional use by heavier trucks.
• Heavy Use: 7" or thicker concrete - Driveways and parking areas for heavier commercial and industrial trucks. (Pavements for heavy industrial trucks should be designed on an individual basis.)

RECOMMENDED ASPHALT THICKNESSES

The guidelines for asphalt are very similar:

• Residential Driveways: 6-8" granular base aggregate with 2-3" asphalt. Parking for passenger cars, light trucks and occasional use by heavier trucks.
• Commercial Drives and Parking Areas: 8" granular base aggregate with 4" asphalt for light use.
• Heavy-Duty Use Parking Lot: 6-inch thick hot mix asphalt layer on top of a 8-inch aggregate base layer.

The exact thickness of asphalt needed will depend on the type of subgrade soil and the thickness and type of aggregate base placed between the subgrade and asphalt. Without the correct subgrade and base preparation and asphalt thickness, the parking lot or driveway will quickly degrade and could be rendered unusable or require significant repairs.