Porous Asphalt: How Permeable Pavement Works and What It Takes

Porous asphalt is hot mix with most of the fine particles left out. Without sand-sized material filling the gaps between stones, the finished pavement is full of connected voids, and rain soaks straight through instead of running off. Underneath sits a deep bed of clean, uniform stone that holds the water until it seeps into the soil or leaves through an underdrain. People also call it permeable or pervious asphalt.

Driveway rainfall runoff calculator → How many tons calculator →

How Porous Asphalt Differs From Regular Asphalt

Standard dense-graded hot mix is designed to be nearly watertight. Its blend of coarse and fine aggregate packs tightly, leaving only a small share of air voids after rolling. Porous mix uses an open gradation: mostly one size of stone, very few fines. The result has far more air space, commonly cited in the mid-to-high teens as a percentage of volume, which is enough for water to pass through quickly.

Feature	Dense-graded hot mix	Porous asphalt
Water	Sheds to edges and drains	Passes through the surface
Gradation	Coarse plus fine aggregate	Mostly coarse, few fines
Base	Compacted dense-graded stone	Deep open-graded stone reservoir
Sealcoat	Common maintenance	Not used; it would clog the pores
Typical density for planning	About 145 lb/cu ft	Lighter, often around 125 lb/cu ft

How It's Built

The surface is only one layer of the system. A typical design, from the bottom up:

1. Uncompacted or lightly prepared subgrade so water can soak in. Heavy compaction of the native soil would defeat the purpose.
2. Geotextile on the bottom or sides in many designs, to keep soil from migrating into the stone.
3. Stone reservoir. Clean, uniformly sized crushed stone, often a foot or more deep depending on rainfall and soil. The empty space between stones stores water.
4. Choker course. A thin layer of smaller clean stone that gives the paver a stable surface.
5. Porous asphalt surface, commonly a few inches thick.

Reservoir depth is an engineering decision based on design storms, soil infiltration rates, and frost depth. Many towns treat porous pavement as a stormwater control, so local permits and design standards may apply.

Tonnage Math for Planning

Because porous mix has more air, a cubic foot weighs less than dense hot mix. If your supplier gives a density, use it. Otherwise, here's a rough planning example at 125 pounds per cubic foot:

1. Area: a 20 × 30 foot parking pad is 600 square feet
2. Volume at 3 inches: 600 × 0.25 = 150 cubic feet
3. Weight: 150 × 125 = 18,750 pounds
4. Tons: 18,750 ÷ 2,000 = about 9.4 tons

The same pad in standard hot mix at 145 pounds per cubic foot would be about 10.9 tons. The tonnage calculator uses 145 by default, so scale its result by about 125/145 (0.86) for porous mix. The stone reservoir is a separate, much larger order; the base calculator helps with that.

Where Porous Asphalt Makes Sense

• Parking lots and driveways where runoff causes puddles, erosion, or stormwater fees.
• Paths and trails with light loads.
• Sites with soils that drain reasonably well, or where an underdrain can carry water away.

It's a poor fit where heavy trucks turn sharply, where lots of sediment washes onto the surface, on steep slopes without special design, or where spills of fuel or chemicals could reach groundwater. Local rules may restrict infiltration near wells or in some soils.

Maintenance Rules

Porous pavement works only while its pores stay open. The main rules:

• No sealcoat. Sealers fill the voids.
• No winter sand. Sand clogs the surface. Plowing is fine; many owners find ice forms less readily because meltwater drains through, though that varies by climate.
• Keep sediment off. Stabilize nearby soil and mulch beds so dirt doesn't wash on.
• Vacuum sweep periodically. Regenerative air or vacuum trucks pull debris out; plain brooms can push it deeper.
• Check infiltration. Pour a bucket of water on the surface now and then. If it ponds, cleaning is overdue.

Patching can be done with porous mix. Small patches with ordinary dense mix are sometimes accepted if the area is small, but large dense patches reduce the drainage area.

Cost Considerations

The surface mix itself isn't wildly different from regular asphalt, but the deep stone reservoir, extra excavation, and design work usually make the total project cost more than a conventional pavement. On the other side of the ledger, porous pavement can shrink or replace detention ponds, pipes, and inlets on commercial sites. Whether it pays off depends on your site and local stormwater requirements, so get design and pricing from a contractor who has built it before.

How Much Runoff Are You Dealing With?

A quick way to size the problem: 1 inch of rain on 1,000 square feet of hard surface is about 623 gallons. Our driveway runoff calculator runs that for your area, and our runoff guide covers conventional drainage fixes if porous paving isn't practical.

Bottom Line on Porous Asphalt

Porous asphalt is an open-graded mix laid over a deep stone reservoir so rain drains through the pavement. It can solve runoff problems, but it only works with the right soil, a proper design, and maintenance that keeps the pores clear. Treat it as a drainage system with a paved top, not just another asphalt mix.