When It Rains, It's Porous: Permeable Paving

Permeable pavement lets rain drain through your driveway or patio, minimizing flooding and toxic runoff.

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This custom driveway incorporates Grasspave2 sections to absorb rainwater runoff.

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The next time it rains, look at your driveway. Much of the water washes off and into storm drains, carrying oil, grease and chemical residue from cars. This runoff can cause erosion, flooding and contamination of public waterways.

Fortunately, permeable paving materials allow stormwater to soak into the earth, where naturally occurring bacteria help digest contaminants before they reach the water table. Called pervious or porous pavement, these systems keep groundwater clean, help tree roots breathe, reduce the severity of flash-flooding and reduce the urban heat-island effect because they absorb water into the ecosystem and don’t reflect heat back into the environment.

Two-thirds of the rain in urban areas falls on a paved surface, and some local governments encourage pervious paving to improve groundwater. In Chicago, new construction projects are now required to capture the first half-inch of rainfall on site.

Other locales are limiting the amount of a building lot’s land that can be covered by buildings or pavement. Build a house near Maryland’s environmentally sensitive Chesapeake Bay, for example, and no more than 10 percent of your land can be covered by anything that prevents rainwater from reaching the ground directly—a regulation that restricts house size.

If you already have a driveway, does it make sense to rip it up and replace it with something porous? Probably not, if it’s in good shape. However, if your driveway is due to be repaved, or if you’re building a new house, it might be time to look into a pervious system.

Surfaces that allow seepage

The simplest and cheapest way to let water run through your driveway is to build it with gravel, also known as aggregate. Single-size, angular particles—thoroughly washed to remove potentially clogging dust and dirt—create a driveway with as much as 40 percent open space between the stones, allowing practically any amount of rainfall to soak through.

More attractive, but also more costly, are concrete or ceramic permeable paving stones, or permeable interlocking concrete pavement (PICP). These blocks fit together in herringbone, basketweave or other patterns, leaving open space between. The openings, also called voids, are filled with gravel, which allows water to drain through and soaks up fairly heavy rainfall.

Interlocking permeable pavers cost about 25 percent more than conventional ones, and they require that you blow leaves and debris off them periodically to keep the voids from filling. Look for pavers containing a percentage of fly ash, a recycled byproduct from coal burning.

Water-permeable paving

It’s possible to build a driveway that looks essentially identical to everyday driveways but allows water through. Porous asphalt (blacktop) and pervious concrete are versions of traditional paving materials, but they’re made without fine sand. These mixes allow as much as 3 to 5 gallons of rainwater a minute to soak through each square foot. They’re slightly rougher looking than their conventional counterparts, but from a distance, it’s hard to tell the difference.

Porous asphalt is the less costly option, and it can be highly durable if installed correctly over a gravel bed. It also can be the trickiest material to work with. When heated, asphalt can become malleable—think of sticky blacktop on a hot summer day—and early versions of porous asphalt tended to clog as binders in the asphalt filled the open spaces. Today, porous asphalt includes polymer fibers that prevent binders from clogging. Those additives have to be applied at the right temperature; an experienced contractor is a must.

Pervious concrete is less complicated to pour—the National Ready Mixed Concrete Association certifies installers—but it’s more expensive than porous asphalt. Pervious concrete is faster to install than conventional because it doesn’t need to be smoothed with a trowel (finishing would seal the pavement surface, decreasing water penetration). Another benefit is that pervious concrete is lighter in color than asphalt, so it doesn’t absorb heat and radiate it back into the environment like blacktop does.

A solid foundation

For permeable pavers—asphalt or concrete—to work correctly, they need the right foundation. They can’t be laid on compacted soil; lay down fabric if they’re on clay or silt. That lets water through but prevents the soil from moving upward and clogging the paving.

The key is a bed of gravel, usually at least 6 inches thick, beneath the pavement to store the water temporarily while it percolates into the soil. The gravel should be evenly sized, to create voids, and deep enough so the water never rises into the asphalt. It should also be rinsed to remove potentially clogging dust.

Going green

You could also create a driveway that’s literally green—by planting grass. A structural lattice lets healthy grass or other plants grow while providing support to keep cars out of muddy ruts.

Inch-thick plastic grids are installed with a mix of sand and fertilizer on top of a gravel and sand bed. (Look for grids made of recycled plastic.) These systems allow even heavy rain to percolate through the grass roots and into the stone bed below, where suspended pollutants and moderate amounts of engine oils are consumed by active soil bacteria.

DON’T forget about your driveway or patio when looking for ways to reduce your environmental impact.

DO consider what happens to rainfall on your land.

Maintain the Drain

To keep the water flowing through a permeable driveway, vacuum up debris periodically. (Broom sweeping actually drives dirt deeper into the pavement.)

• Permeable pavers require occasional blowing of dust and leaves.

• Pervious concrete and porous asphalt require periodic vacuuming.

• In southern climates, you may need to vacuum every few years. In northern climates—where roads are sanded in winter—you may need to do it twice a year.

• To increase vacuuming effectiveness, pressure-wash the surface to loosen buildup of debris.

• Plant or grade slopes away from the driveway to keep soil from eroding or draining on it.

Permeable Pavers Pervious Concrete Plantable Paver Grids Porous Asphalt
COST $10 to $12 per squre foot (based on 2,500-square foot area) $8 to $10 per square foot* (based on 2,500- square-foot area) $4.50 to $6.50 per square foot (installed, with sand and sod) $4 to $5 per square
foot* (based on a
2,500-square-foot area)
PROS ■ Most conventional appearance
■ Can be laid in winter
■ Snow melts more quickly because water can drain
■ National standards for installers set by National Ready Mix Concrete Association
■ Faster to install than conventional concrete 
■ Looks like lawn
■ Can absorb large amounts of water
■ Grass and soil absorb pollutants
■ Absorbs heat and CO2  CONS
■ Looks like conventional asphalt
driveways
■ Can last 20 years
or more
CONS ■ Fine gravel between pavers can pose a tripping hazard ■ Requires seven days of above-freezing weather to cure
■ Vacuuming and power-washing needed (especially where sand is applied to streets)
■ Grass must be watered and mowed like a lawn
■ Could pose a tripping hazard
■ National standards not well established
■ Vacuuming and power-washing needed (especially where sand is applied to streets)
■ Additives to prevent clogging can be tricky to install

* Prices vary by region and soil conditions.