How a Porous Asphalt Driveway Drains

Porous asphalt lets rain pass through an open stone matrix into a stone reservoir and then into the soil. Planning density is often nearer 130 pounds per cubic foot than 145, so a ton covers more area. Sealcoat would plug the voids.

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The path rain is supposed to take

A porous asphalt driveway is built so rain goes through the mat, not across it to a gutter. The mix uses an open stone matrix, which means the rocks touch and leave connected voids. Water drops through those voids into a stone reservoir under the asphalt, then into the soil if the soil can take it. That is a drainage section, not a darker version of a normal driveway. The reservoir has to have somewhere to go. If the soil is tight, or the water hits a buried wall, the stone fills and the surface can stay wet or heave in a freeze. Ask the designer where the reservoir drains before you talk about tons.

Why a ton covers more area

Planning density for porous mix is lower than the 145 pounds per cubic foot used for dense hot mix. A figure sometimes used is about 130 pounds per cubic foot. One ton is 2,000 pounds. Divide 2,000 by 145 and you get about 13.8 cubic feet. Divide 2,000 by 130 and you get about 15.4 cubic feet. At 1 inch thick, cubic feet times 12 is the coverage in square feet, about 166 square feet per ton at 145 pounds, and about 185 square feet per ton at 130 pounds. The lighter mix covers more area per ton because each cubic foot weighs less. Both numbers are planning estimates. The plant scale ticket is the real weight. Do not invent a permeability rate the mix was never tested for.

What closes the voids

Porous asphalt fails when fines clog the voids or when the reservoir has nowhere to send the water. Sand, soil, and dirty runoff packed into the surface turn an open mat into a weak dense one that still cannot shed water the way a tight mix can. Sealcoat would plug it. A sealcoat quote on a porous driveway is a different product aimed at a normal dense surface, and the film would close the paths the rain is supposed to use. Keep winter sand and landscaping fines off the mat if you can. If someone offers to seal it for a uniform black color, ask them to explain how water will still reach the stone reservoir.

A practical sequence

Ask where the stone reservoir sends water after a storm.
Plan porous tons near 130 pounds per cubic foot unless the plant names another density.
Compare that with 145 only to see why a ton covers more area.
Leave sealcoat off the surface so the voids stay open.

The mistake to skip

Sealcoating a porous driveway and closing the voids the rain is supposed to use.

Questions people ask

How do I turn porous thickness into tons?

Use the plant density if they give one. A planning figure sometimes used is about 130 pounds per cubic foot, against about 145 for dense mix. Divide 2,000 by that density to get cubic feet per ton, then multiply by 12 for square feet at 1 inch. That is a planning estimate. The plant ticket is the real weight.

Can I seal a porous driveway to make it blacker?

Sealcoat sits on a dense surface and would plug the open stone matrix. Rain is supposed to pass through the mat into the reservoir and then into the soil. Clogging from fines causes the same kind of failure. Ask how the surface will be kept open. A permeability number should come from a test, not a guess.

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