Managing a property's impervious footprint is becoming more and more regulated.

What that means is that it's important to monitor and minimize how much of the ground is covered with structures and solid coverage such as paving, sidewalks and patios. The rainwater hits these surfaces and is channeled away from the property and into the municipal public storm system.

"In the city, for example, household wastewater and storm water are flowing into the same system and through the treatment plant," said John McGrann, manager of Pennsylvania Stone, Cement and Supply Company, 190 W. Ross St. "With heavy rain, we're taxing the limits of the treatment plant, which is not good for the city or the environment," said McGrann.

If a municipality doesn't have regulations in place yet, it's just a matter of time, according to Dale Paden, paver field manager for the northwest region of Pine Hall Brick Company, Madison, N.C. Some municipalities tax homeowners if too much of their property has impervious cover.

There are two reasons a homeowner might opt to use a permeable paving material. The first is to meet government regulations and meet impervious coverage limits. The second is to encourage a greener approach to handling rainwater and help the environment.

If the reason is government-mandated, engineering skills are often involved to ensure the appropriate outcome.

"The permeable pavers have the exact same properties as regular pavers. They simply have larger spacers or lugs on the sides, and the spaces are filled with gravel," Paden said. The small rice-sized stone filler needs to be coarse and angular to lock in the joint. Once the stone is packed, it is about 40 percent open, allowing water to flow through the voids.

According to Paden, suspended solids are filtered when they run through the granular joint. "The water is now 95 percent clean water. The idea is that it can recharge the ground water," he said.

For the most part, according to McGrann, the system requires between 12 and 16 inches of depth, from the first layer of stone to the top surface of the paver. "With any permeable surface, the most important part is the stone base that is underneath it," McGrann said.

So a crushed stone reservoir is created under the pavement, and rain flows down into the stone base.

"The idea is that, within 48 hours, the water will redistribute itself into the property," McGrann said. "If you have a heavy, clay soil, a deeper base is needed."

According to Paden, most municipalities have charts that indicate soil permeability and determine how well the soil will absorb rainwater. The thickness of the base stone layers then can be modified based on the soil quality.

The first layer of clean stone is a No. 2 stone, the pieces measuring between 2½ - 1½ inches in diameter. That layer is typically 12 inches deep. In order to compact the stone adequately, use a vibrating plate compactor every 2 or 3 inches of stone.

The second layer is a smaller, 4-inch deep layer using a No. 57 clean stone. The final layer is done with a No. 89 stone, which ranges in diameter from ¼ to 3/8 inches.

The pavers are laid on the small stone base, eliminating the customary sand base used when installing a regular paver sidewalk or patio. The same angular stone is swept between the joints of the pavers.

Not only is the water being kept on the property, but it is also being filtered through the various layers of stone, reducing pollutants and other undesirable minerals and chemicals.

According to McGrann, a permeable pavement accommodates more water infiltration than the typical lawn because the base is very often a better system than the typical soil beneath a lawn. He added that another plus to the permeable pavers is that when ice melt is spread in the winter, the melted precipitation flows down into the stone base under the surface, while a regular sidewalk tends to melt and then refreeze as the temperature drops.

PRICE, UPKEEP & OPTIONS

The most common residential pavers used for pedestrian or light vehicle traffic are rated to handle between 12,000 and 15,000 psi (pounds per square inch) and are 2¼-inches thick. The permeable pavers are available in various colors and designs.

The price difference between a basic permeable paver versus a regular paver is $4 versus $3.60 per square foot.

Maintenance of a permeable paver system is minimal. It's important to keep it clean, primarily focusing on the joints. "With time," Paden said, "the cracks will start to filter off suspended solids and filtration will be reduced at some point. At some point, the joints will need to be vacuumed out."

In addition to the permeable systems that use pavers and bricks atop the appropriate stone base, there are porous concrete and asphalt products, which are designed with more voids in the product to allow water to flow through them. The idea is the same as the permeable pavers and they're installed over the same stone base, but the pavers themselves are not porous and do not allow water to pass.

Permeable paving or porous materials should be used as part of an overall site management system for storm water, and is not a replacement for other techniques.

rmcroberts@lnpnews.com