How to convert your driveway to a permeable surface

Jun 09, 2023

Q: How can I convert my driveway to make it permeable?

A: Permeable paving lets water flow through. It not only prevents puddles, it also offers great environmental benefits. In a rainstorm, water washes off regular paving and flows untreated into storm drains. In many communities, these drains empty directly into local streams or rivers. And even where storm drains empty into sewage treatment plants, large storms can overwhelm the systems, forcing them to release water that has not been fully treated. Either way, any spilled oil or other contaminants from driveways winds up in waterways, which can harm fish and other wildlife.

But with permeable paving, the rain sinks into the driveway, allowing the soil and organisms in it to filter out contaminants. After the water percolates into the soil, some helps replenish underground reservoirs. The rest flows underground and eventually reaches streams or rivers, but that happens slowly, so the soil erosion and even flooding that can happen because of a downpour is less likely.

There are at least five kinds of permeable paving for driveways: pavers, special mixtures of concrete and asphalt, and grid-supported gravel and lawn. Pavers made of concrete or clay work well, provided the installer leaves gaps between pieces and fills them with fine pea gravel rather than sand or mortar. Concrete pavers molded to interlock and create the ideal spacer width work especially well. To create a permeable driveway from concrete or asphalt, suppliers omit the sand that is usually in these paving materials. Instead the mixes consist of just gravel and glue, either Portland cement or asphalt.

The gravel pieces stick together, but gaps between pieces remain open so the paving is porous. For gravel to work alone as permeable paving, it must be installed with a grid support system to shoulder the weight of vehicles, as plain gravel compacts too much over time. Using lawn as paving for a driveway involves a similar issue: To keep the grass from compacting and dying, it needs to be planted in the openings of a support grid. Nevertheless, grid-stabilized grass works best for driveway or parking areas with only occasional traffic.

The key to all these systems is that they need to be installed over a deep gravel bed, often 15 inches or more, because spaces between the stones act as a reservoir that holds water during a downpour until it can slowly percolate into the soil. In an especially heavy storm, the gravel spaces might fill up. Then additional water would run off, as it would on a regular driveway, or go into an overflow pipe, if the design included that feature.

Because of the need for the deep gravel layer, switching to permeable paving is not really an option unless you are completely redoing your driveway. However, you might still be able to achieve some of the same environmental benefits by changing the landscaping that sits alongside your driveway.

If the driveway slants so water runs into landscaping, you can install one or more rain gardens, which are designed with fluffy soil that can absorb large amounts of water and plants that thrive when their roots are soaked but still survive without additional irrigation during dry spells. The list of suitable plants varies depending on where you live. The Environmental Protection Agency offers a good primer on how to build a rain garden and offers links to plant lists suitable for each area of the country.

If your driveway slopes so all water runs to the street, try temporarily lining up sandbags to see where you can divert the water to a place for a rain garden. Once you determine that, rent a concrete saw or hire someone to cut one or more tidy slots across your driveway and install drain pipe to carry water there. Across the driveway, cover the pipe with gravel or bricks.

If your driveway needs to be rebuilt, installing a pervious driveway makes a lot of sense, even though it probably will cost more. In some areas, local governments provide rebates to cover some of the cost. In parts of the District, for example, the website of the Department of Energy and Environment promises rebates of $10 a square foot for installing permeable pavers on surfaces that were previously impermeable. The site estimates the cost of installing permeable pavers at $30 to $40 a square foot.

This Old House has a good overview of what is involved in converting a driveway to one surfaced with pervious pavers. Even this do-it-yourself site, though, warns that installing a pervious paver driveway is a job best left to pros. Actually setting the pavers is the easy part. The job also involves removing and hauling away the old paving, excavating to the required depth, and trucking in and spreading gravel. Other kinds of pervious paving also involve these steps, which makes them jobs for the pros as well.

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