What is an impervious surface?
An impervious surface refers to any structure that blocks rainwater from soaking into the soil and recharging the ground water. These could be houses, sheds, driveways, sidewalks, and patios.
Why are impervious surfaces bad?
Soil is a natural filter. In urban and suburban areas, when rainwater falls on a paved surface like a driveway, much of that water will find its way directly into lakes and rivers. When unfiltered water run-off flows directly into a body of water, it usually carries with it fertilizers and other pollutants that are harmful to aquatic wildlife. Urban and suburban water run-off can create temperature swings that also have a negative impact.
In addition, areas covered with a high percentage of impervious surfaces are more prone to flooding caused by heavy rainfalls because more water reaches the lakes and rivers instead of soaking into the ground.
How can impervious surfaces be reduced?
To mitigate the negative effects of impervious structures, many suburban municipalities have ordinances that restrict the amount of ground surface that can be covered by an impervious structure. Suburban cities in our area, Twin Cities, often target 30% as the total amount of impervious surface any given lot is allowed. This is going to come as a surprise to more and more homeowners when they apply for that shed permit because the size of the average suburban house is growing while at the same time, the size of the average suburban lot is shrinking.
What are permeable pavers?
Permeable pavers are pavers that allow rain water to seep down into the spaces between the pavers. The permeability of these pavers is achieved by the design of the pavers and by the type of base that’s used.
One common installation method (methods vary) requires laying down several inches of 1-1/2-in. rock and then several more inches of ¾-in. rock on top of that. The top layer is 3/8-in. chipped stone. And once the pavers are installed, more 3/8-in. chipped stone is swept into the joints between pavers.
Unlike the compactable gravel or sand installed under standard pavers, the rock base under permeable pavers is, well…permeable. The base acts as a reservoir and allows the water to soak slowly into the soil. Permeable pavers capture almost 100% of rainwater that falls on them, and the rock reservoir under the pavers can hold an astonishing amount of water.
Do cities make allowances for permeable pavers?
Some municipalities make total allowances for permeable pavers, so if a given property has already reached the 30% impervious coverage, the city or county would allow the addition of a patio if it was constructed using permeable pavers.
Some municipalities make partial allowances, so if a 100-sq.-ft. patio was built using permeable pavers, the city or county would allow half, or some other percentage, of the patio’s square footage to be counted against the total impervious coverage.
Permeable pavers need to be maintained
Some municipalities don’t recognize permeable pavers at all, and it has everything to do with maintenance. Permeable pavers need to be maintained on a regular basis or they lose their effectiveness. Over time, debris, silt and organic material can clog the spaces between the pavers preventing the water from reaching the reservoir beneath them. Some cities argue that there’s no simple way to make sure that homeowners, and future homeowners, are keeping up on the regular maintenance schedule.
Are permeable pavers the solution?
Kevin Earley, Director of Commercial Pavers for permeable paver manufacturer Belgard, says, “Years ago it was all about convincing municipalities that permeable paver systems worked. That fact has been established, so now it’s all about convincing them that maintenance is a manageable requirement.” And it looks as if they are winning the debate because permeable pavers are showing up in more and more communities each year. So, it appears that municipalities do believe that permeable pavers are at least part of the solution to reducing impervious surface.