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How Thick Should Asphalt Be?

July 29, 2022

One of the most common questions we ask is how thick the layer of the asphalt is needed. Many people wonder that, but you'll be surprised about the answer because unlike concrete thicker is not always better. Asphalt serves as a smoothing driving surface and a waterproof coating over the base. The base (material below the asphalt) carries the weight of the load on the asphalt. A thick layer of asphalt over a poorly laid base, will fail much faster than a thin layer of asphalt over a properly installed base.

Driveways, Parking Lots and Roads Have Different needs

A major highway has significantly different needs than a typical residential driveway. A highway will have more traffic pass over it in one day than a driveway will have in its entire lifetime. Not all parking lots are the same also. A high-volume strip mall has different needs than an office building or an industrial parking lot, with heavy trucks frequently traveling on the parking lot.

Asphalt is installed in "lifts" by a paving machine. The paving machine lays down asphalt in a uniform thickness. The thickness of each lift is typically 2" to 3" thick uncompacted. If there is a need for a thicker asphalt surface, we will use 2 and sometimes 3 lifts.

After the asphalt is laid large rollers pass over the fresh asphalt causing the asphalt to compact—asphalt compacts approximately 1/4" per 1" of asphalt. So a 3" lift of asphalt will be right around 2.5" once compacted.

Is 2" of Asphalt Enough for a Driveway?

Yes, 2 inches of asphalt is enough for a typical driveway. If you plan to park larger vehicles such as an RV or trailer, you will want to consider using a double lift and having 4"-5" of asphalt. We do recommend installing 3" asphalt because it will be more durable.

How Thick Should My Asphalt Parking Lot Be?

As mentioned before, this will depend on the kind of traffic your parking lot will have.  Most asphalt parking lots are done in 2 lifts with 5"-6" total asphalt. If you have a low-volume parking lot, a single 3" lift might be suitable with no heavy loads.

If the parking lot requires 2 lifts, it will typically be a hot mix asphalt binder made up of 1/2"-3/4"rock. The final lift will be a dense grade surface mix.

It Is All In the Sub-Base

Regardless if you have a parking, driveway or roadway, the most crucial part of these driving surfaces is how well the subbase has been built. The subbase provides 80-90% of the strength of the road or parking surface.

The soil below the site must be sufficiently compacted before the asphalt is installed. The compacted soil is the foundation that gives the pavement its strength and flexibility. Depending on the soil, it may be necessary to add amendments to achieve proper compaction. In case soil amendments are not enough, an aggregate foundation should be installed and compacted carefully.

The soil under the parking surface will determine how much and what kind of aggregate is used. Broadly speaking and drive will need 6"-8" of a base aggregate. Parking lots typically have about 8" of base aggregate.

If the underlying soil is loam, clay or softer soils that don't drain well, additional aggregate may be needed to be used to create adequate drainage. Also, a geotextile fabric may be used to help prevent the base from loosening.

Asphalt is a great material used for driveways, parking lots and highways because it's durable, waterproof, smooth driving surface. Finding out how thick asphalt and base material for a particular purpose can be a challenging task. We are here today and every day ready to answer any questions you may have about installing new paving surfaces on your property that meet all your requirements!