Permeable paving, also called pervious paving, is a term used to describe paving methods for roads, parking lots and walkways that allow the movement of water and air through the paving material. The best-known of these are cobblestones and bricks, but there are many newer ones.

Actually, many cobblestone and brick roadways are not particularly permeable surfaces because of the way that they were built. Some newer ones are actually laid over concrete, while some older ones are either mortared or laid over a dense-grade aggregate that prevents absorption of most water.

Advantages of permeable paving

Permeable paving surfaces are highly desirable because of the problems associated with water runoff from paved surfaces. Part of the problem is creating an unnatural volume of runoff from precipitation, which causes serious erosion and siltation in streams and other bodies of waters. Part of the problem is also the washing off of vehicular pollutants into water bodies.

Permeable paving surfaces keep the pollutants in place in the soil or other material underlying the roadway, and allow water seepage to recharge groundwater while preventing the stream erosion problems.

Types of permeable paving surfaces

There are three general types of permeable paving surfaces. The first is loose stone, which tends to become less pervious over time, especially when used as a mixed grade with stone fines. The second involves the use of hard paving materials, either concrete, asphalt or paving blocks, that is constructed to be porous and to allow water to pass through the material.

The third, sometimes called open paving, involves the use of generally impermeable materials, but these are placed to maintain open space between them, as in the "hopsack" method, to allow permeable areas of soils or other fill between the material placed. "Hopsack" is a method of placing oblong blocks so that space is permanently maintained between them. Special spacers are also available on the market. Open-type paving is usually intended for applications where grass will grow in the spaces between the blocks or other materials. This is a highly desirable feature in parking areas and low-impact roadways, being esthetically more pleasing, and has the additional benefit of reducing summer heat buildup due to inert pavement materials absorbing solar radiation and reradiating it as ambient heat.

Outside links

• McCormock & Son's pavingexpert site
• Sustainable Building Sourcebook