Asphalt & paving · Guide
How to spec asphalt thickness and base depth
Depth decisions are the difference between a driveway that lasts 8 years and one that lasts 25. Here is how the spec is actually set.
6 min read · Updated August 2026
Start with the subgrade, not the surface
The subgrade is the native soil under everything. Well-drained sandy or gravelly soil supports load directly and needs less base. Clay and silt hold water, soften, and pump fines up into the aggregate — those soils need more base depth and a separation fabric.
The test is simple: after rain, does the site drain within a few hours or hold puddles? Standing water means a drainage problem that no amount of asphalt fixes.
Base depth by soil and load
Dense-graded crushed aggregate — DGA, crusher run, 21A depending on the region — is the standard base. It compacts to an interlocked, load-spreading slab because it contains everything from 3/4 inch stone down to dust.
Compact in lifts of no more than 4 inches. A 8-inch base placed in one pass compacts on top and stays loose underneath, which shows up two winters later as a dip.
Base aggregate weighs about 1.4 tons per cubic yard loose and compacts roughly 20-25%, so order for the compacted depth plus a quarter.
- Sandy, free-draining soil, cars only: 4-6 in base
- Loam or mixed soil: 6-8 in base
- Clay or freeze-thaw climate: 8-12 in base plus geotextile fabric
Loose vs compacted depth
Asphalt loses roughly 15-20% of its loose depth under the roller; aggregate loses 20-25%. Every spec you write should say 'compacted', and every quantity you order should be calculated from the loose depth.
For asphalt: to finish at 2 inches compacted, the crew spreads about 2.4 inches. For base: to finish at 8 inches, place about 10 inches of loose material across two or three lifts.
One thick lift or two courses?
Above about 3 inches total, asphalt is placed as two courses — a coarse binder course and a finer surface course. The binder carries load, the surface sheds water and takes the wear. A single 4-inch lift cannot be compacted uniformly through its depth with residential equipment.
For a 2-inch residential drive, one lift is correct and normal. Adding a second thin lift on top of a failing base is the classic expensive mistake.
Frequently asked questions
+Is 2 inches of asphalt enough for a driveway?
Yes for passenger cars over a properly compacted 6-8 inch aggregate base. Add a binder course if trucks, RVs or delivery vehicles use it regularly.
+How much does aggregate base compact?
Dense-graded aggregate compacts about 20-25%. Order roughly 1.25 cubic yards of loose material for every compacted cubic yard you need.
Part of a series
The complete asphalt driveway guide
Everything that decides what an asphalt driveway costs and how long it lasts — written from the numbers a paving crew works to, not from a brochure.