Water conservation is vital to the concept of sustainability. We want to avoid transporting water from long distances, and we want to hang on to the water we already have as much as possible. Of course, one of the ways water is lost is through runoff.

Runoff didn’t exist before people started building things. Human activity has created many millions of miles of impermeable surfaces, mostly concrete and asphalt. Highly developed areas can be as much as 50 percent impervious.

When rainwater hits these surfaces, it has nowhere to go but into the gutter, carrying with it a big load of pollutants—petroleum products, animal wastes, fertilizers and pesticides. These eventually find themselves in our streams, lakes and oceans.

Fortunately, we now have many different green ways of mitigating runoff. One of these is by making formerly impermeable surfaces permeable.

Enter the precast permeable interlocking concrete paving stone, an increasingly important player in the sustainable landscape and hardscape arena. Each stone itself is not porous; rather, water seeps through the joints between stones.

These systems allow stormwater to hang around instead of running off and creating environmental havoc. Rainwater can now infiltrate the ground below and percolate down, recharging the aquifer. Or, it can be collected in below-ground storage media and recycled to irrigate landscapes or to run water features.

And they work. “When properly designed, permeable paving systems can cut runoff by 100 percent,” says Robert Goossens, president of Precise Paving, Inc., in West Palm Beach, Florida.

In a typical installation, permeable pavers sit on a base and subbase composed of different grades of aggregate. It’s the aggregate that retains the water. How much water is actually retained by a system depends on the design. “We don’t get LEED points for the top five inches; we only get it for what’s below,” notes Goossens.

Cavan Howard, owner and president of Denver, Colorado-based Rocky Mountain Hardscapes, LLC, describes the process of layering the different grades of aggregate. “You start with your biggest courses of aggregate at the bottom, and then build up. The three-quarter-inch rock won’t slip through the two-to-four-inch rock. Then the three-eighths-inch aggregate sits on top of the three-quarter-inch and doesn’t fall through the cracks there.”

Avoid ‘fines’

“If you have a lot of fines (particulate matter), like dirt and sand, the pavers will sink down into it,” warns Howard. “You want big pieces of clean, washed rock sitting on top of each other. And you don’t want to contaminate them by driving your Bobcat over them with muddy tires.”

“The compaction equipment has to be very specific,” adds Goossens. “We’ve seen a lot of contractors who didn’t allow for that extra compaction. Then you’ll see finished product a half-inch to an inch below the surrounding structures. That’s not a good sign.”

Plastic block catchment systems

There are some innovative products made to be used with permeable pavers. Among these are modular, lightweight catchment systems consisting of grids made of recycled poly-propylene.

The grid panes are shipped flat and put together onsite to form three-dimensional blocks or groups of blocks of any size, depending on the water storage requirements. The blocks serve a dual purpose; not only do they store water, but they are very strong, and add additional support.

“They are load-bearing, space-filling products that can be used for a lot of different applications,” explains James Lavery of Atlantic Water Gardens in Mantua, Ohio. “If somebody wants to harvest the rainwater that’s coming down, the blocks will store that water and allow it to seep through the ground at a steady pace, as opposed to flooding an area.”

Lavery describes how the modular units are installed. “You dig a reservoir and put rubber pond liner in the bottom. The blocks go in on top of the liner, which is also wrapped around the sides of the blocks to help keep the water in. When the water seeps through the joints in the pavers, it goes down into the blocks’ voids for storage.”

Since 95 percent of the blocks’ area is void space, they can store a lot of water. “If you take a ten-gallon aquarium, fill it with gravel and pour water into it, you’ll get a water capacity of about 2.2 gallons per cubic foot,” says Dave Kelly, vice president of product management and product development at Aquascape, Inc., St. Charles, Illinois. “But with the blocks, you can store 7.4 to 7.5 gallons of water per cubic foot. The storage capacity is about three to four times greater than gravel.”

Kelly says the blocks were originally designed for parking lots. “If a developer is building a giant parking lot with an impermeable surface, then he’ll have to build a retention pond for all the water that’ll sheet off. Instead, he can use permeable pavers and put a large number of these plastic blocks beneath the ground. It’s kind of like having an underground retention pond.” In a case such as this, the blocks would be put in without using pond liner, to slow the water down so that it can gradually be reabsorbed into the ground.

The blocks are a bit more expensive than aggregate, but the additional cost may be offset by not having to purchase additional land to build swales. “It’s a cost advantage to put these in below ground, and have the ability to use that same space for multiple purposes,” says Kelly.

An expanding market

Despite the growing popularity of permeable pavers, Goossens estimates that only about five percent of his paver business is in permeable product.

“Florida seems to be one of the late risers. The potential is great here, but as for actual bricks on the ground, it’s not as much as we’d like.”

Howard has seen much more acceptance of permeable pavers in his area—Colorado. “My client base is two- thirds commercial and one-third residential. Over half of my commercial work is in permeable pavers. Governments and colleges are all heavily into permeables now.”

Some of the demand is legislatively driven. “I think Fort Collins passed a law last year specifying that twenty percent of all concrete laid in new commercial developments must be permeable,” Howard says.

Residential demand for permeable pavers lags behind commercial, probably because they are more expensive than other options. Even with the ability to recycle rainwater for irrigation, there isn’t that much in savings to be realized for average homeowners.

“I think it’s an industry that’s expanding, and will continue to expand; hopefully, our market is going to be one that catches on,” Goossens concludes.

It’s time to take permeable pavers seriously. Clearly, they’re the wave—or should we say, the wave reducers—of the future when it comes to sustainable design.