Building a lot
How thick should an asphalt parking lot be?
The section a lot needs is set by the soil, the traffic and the frost depth, and on anything of size it is an engineer's number rather than a rule of thumb.
The numbers
- The base carries the load. The asphalt spreads it and keeps water out of the base.
- Car areas and truck lanes are different pavements even where they look the same.
- Soil decides more than anything else: drained sand and clay that holds water want different sections.
- In northwestern Wisconsin the frost depth is part of the same calculation.
- On a new build or a rebuild of any size, the section comes from an engineer, not from a website.
Why the stone matters more than the black
A lot built for cars only is commonly two to three inches of asphalt over six to eight inches of compacted stone. That is a description of what gets built around here rather than a specification for your property: the soil under it, the trucks that will use it and the frost depth all move that section, and on a new build it is an engineer who sets it.
People buy asphalt by the inch and think of the base as fill. It is the other way around. The compacted stone under the surface is the structure, and the asphalt is a waterproof wearing course that spreads a wheel load across it.
A thick surface over a thin or poorly compacted base fails the same way a thin one does, only later and more expensively. That is the single most common reason a lot that is only a few years old is already alligator cracked in the drive lanes.
Where the traffic decides it
A stall a car sits in and a lane a loaded truck turns in are not the same pavement even though they look the same. Garbage trucks, delivery trailers and fire lanes are the loads that break lots, and they always run in the same places.
So the useful question when a lot is being built or rebuilt is not how thick the lot is, it is which parts of it take trucks and whether those parts were built for them. On many properties a dumpster approach belongs in concrete rather than in more asphalt.
What decides it on a new build
Soil. A lot on well drained sand and a lot on clay that holds water need different sections, and in northwestern Wisconsin the frost depth is part of that math too. On anything of size that is a design number from an engineer, not a rule of thumb off a website.
On a repair or a rebuild of part of a lot, the section that goes back is matched to what is already there and to what actually failed.