Go With The Flow | Permeable Hardscape Offers Eco-Friendly Outdoor Spaces

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It’s hard to find a home without hardscape. Whether it’s a driveway, walkway, front porch, an entry around an exterior door, or a backyard patio, virtually every home needs some kind of hardscaping to help make outdoor areas more usable. Oftentimes, these zones are constructed with materials such as a slab of concrete or mortared pavers. But if you’re looking to create a new feature on your property, or to replace an existing surface, there’s another option you should consider: permeable hardscape.

Around this time of year, many homeowners are eager to begin preparing their outdoor spaces for spring and warmer weather. If you’ve been experiencing any flooding, muddy or soggy areas, or are envisioning an expanded outdoor living space this season, you might be thinking about installing or upgrading the hardscape on your property. There are many ways to accomplish this, but an increasingly popular choice is permeable hardscape. Like its name suggests, permeable hardscape allows water to pass through the materials and into the soil beneath, and it has several positive attributes.

CV_ES2018_feature_hardscape2The positives of permeable

Permeable hardscape is an excellent way to control the flow of water on your property. Not only does it prevent water from ponding in low spots in your yard, it also prevents a lot of run-off and erosion problems caused by traditional, non-permeable surfaces. If you’ve ever seen water sheeting off a concrete patio during a heavy rain, you know all that water has to go somewhere. This type of run-off can cause problems, not only to nearby structures and gardens, but also to local waterways. Water from residential areas, often full of soil nutrients and chemicals, eventually ends up in streams, which can cause chemical imbalances and sediment build up.

Permeable hardscape, on the other hand, allows water to be filtered back into the soil, which replenishes groundwater and prevents erosion and pollution problems. If that fact alone isn’t enough to convince you, consider the fact that permeable hardscape is already mandatory in commercial applications around Lynchburg, and may be the future in residential areas as well. Mark Maslow, president of Southern Landscape Group, says that the trend is becoming more and more prominent as stormwater run-off regulations develop. “In the city of Lynchburg, you get taxed for non-permeable driveways,” notes Maslow. “When you choose permeable, the major advantage is that you don’t have all of that run-off going into your landscape, which mitigates a lot of problems, and it also limits the amount of water that goes down into storm sewers and ends up in creeks and streams.”

Luckily, there are plenty of attractive choices in permeable hardscape, along with local experts who can guide you through the process.

Materials

As with traditional hardscape, homeowners have options in the finished look that will complement any style of home. According to Trevor Templeton, design and build project manager for CLC, Inc, in Forest, there are hundreds of options—from modern slabs to rustic cobblestones—but what all permeable pavers have in common is a special horizontal interlock. “If you just use regular pavers with gaps in between, your surface won’t be strong,” advises Templeton. “Pavers designed for permeable applications have space keepers on their sides that allow a gap, which you then fill with a very fine, clean draining stone.” Templeton adds that the size of the gap can also vary, depending on the desired flow rate.

CV_ES2018_feature_hardscape3Additionally, the pavers themselves are specially fabricated to be porous. According to Maslow, even concrete used in permeable applications is a porous version. “Permeable pavers have more of a natural hydraulic balance,” he says. “They trap and slowly release water into the ground, rather than having water flow across the surface.” Maslow notes that there are also natural flagstones you can use in certain applications, if preferred, and that in this case, permeability will come from the construction of the base layer. When using freeform flagstone, fitting them closer together will allow more ease of furniture placement, while wider spacing offers a more natural look, especially when gaps are filled with plant material.

The most popular sizes of permeable pavers are 6×6 and 6×9, and they can be arranged in various patterns, such as basketweave, herringbone, running bond (the traditional staggered pattern often seen when using brick) or a simple stacked bond, where pavers are laid side-by-side in rows. Each option varies in cost, ease of installation, amount of material required, and strength of finished product, so be sure to discuss the best choice for your landscape with your installer.

There are also options for the filler material between pavers, which is important for the filtration process. Templeton says small aggregate does a great job of filtering out both particulate and mineral matter that can be undesirable in large amounts when entering your landscape or local waterways. This small aggregate can be left as the finished look, or you can top it with pea gravel or a fine decomposed granite. An attractive option for naturalizing or softening the look of permeable hardscape is to place grass or ground cover plants between the pavers. Any hardy, low-growing plant (such as dwarf mondo grass or creeping thyme) or turf grass can be used, though of course grass will require regular weedwhacking to keep it looking good.

An alternative to permeable pavers is to hardscape an area with a natural stone aggregate, such as pea gravel, or crushed or decomposed granite. All of these options are charming—you’ll see this look everywhere from a country cottage to a European chateau. As you may know, pea gravel is a smooth, rounded stone about the size of a pea, which comes in a variety of hues. Crushed granite comes in various sizes, and will have small textured stones in neutral colors. Decomposed granite, often referred to as “DG”, has a finer grain, more sand- or silt-like in appearance. Each has potential positives and drawbacks depending upon your landscape needs. For example, DG is an excellent choice for a walkway, but can be tracked into the house on shoes, so don’t install it directly outside a door—have a few feet of buffer zone. Pea gravel offers a soft, attractive look and is great for drainage— you can even use it as mulch—but it can be tough to wheel a lawn mower, wheelbarrow, or bicycle across, so consider location and usage before choosing this option.

No matter which finished look appeals to you, keep in mind that the top layer is the tip of the iceberg, so to speak. When choosing permeable hardscape, you are installing a system as much as an aesthetic feature. As Maslow points out, “With successful permeable hardscape, the majority happens underground, where you don’t see it.”

Estimating cost

Because proper preparation of the site is the key to success, you can expect to pay more for permeable hardscape than traditional. Templeton says, “Every application is specific, but for foot traffic pavers, you have about 30 to 40 percent more preparation than in standard hardscaping, because you have to go deeper to install a drainage pipe.” Maslow estimates that permeable hardscape can cost anywhere from 20 to 50 percent more than traditional hardscape, depending on materials and the site, but adds, “Right now, it’s used [in residential spaces] because it’s eco-friendly and helps drainage issues, but soon it will be required.” He notes that while it’s already mandatory in commercial areas, a lot of his colleagues in Northern Virginia are required to have a percentage of their projects done in permeable surfaces, and he expects that trend to make its way here in the next five years or so.

Installation

There are location considerations when installing permeable hardscape. “You can lay permeable on a bit of a slope, but it doesn’t perform best,” says Maslow, because it’s harder to control run-off on an incline. Also, you want to stay away from the ‘drip edge’ of any trees, because the excavation required will have a negative impact on root systems.

With the proper location identified, the first step of the process is to level the site and excavate down to the required depth. “For impermeable or traditional hardscape, the rule of thumb is to dig about four to six inches for foot traffic, and eight to twelve inches for vehicular,” says Templeton. “For permeable vehicular traffic, we’re going to dig about two feet down from the final height.” Maslow echoes those estimates, saying, “We excavate down a minimum of twelve inches below the finished surfaces, but it can be as much as eighteen to twenty-four.”

Once the proper depth is reached, a sub-drainage system may be needed to pipe away excess water, particularly if being installed in clay soil. This under drain (also referred to as a “French drain” or “tile drain”), says Maslow, is built by laying down a geotextile (i.e. permeable) fabric to filter water and stabilize the soil, then placing a perforated pipe that is routed to either a stormwater drain or some sort of bio-retention filter for emergency overflow.

CV_ES2018_feature_hardscape4From there, drainage stone is added to cover the base of the permeable structure. “It has to be clean aggregate,” says Maslow, with no fine material that can clog the system. Templeton says for a two-foot deep system, twelve to fifteen inches of 1 to 1½ inch gravel is added to the base. (For a foot path, probably half that amount.) Above that, a layer of smaller gravel, called bedding stone, is put down. Pea gravel can be used as bedding stone, with pavers placed on top of it, then more pea gravel added to fill the spaces between the pavers.

Both CLC, Inc and Southern Landscape Group offer ICPIcertified installation, a national standard set by the Interlocking Concrete Pavement Institute to ensure that projects will stand up to a lifetime of use. When researching for your landscape needs, it’s a good idea to ask about both certification and prior experience before installing permeable hardscape.

With so many attractive and eco-friendly options available, there has never been a better time to embrace the benefits of permeable hardscape. Alleviating drainage issues and increasing the usability of your outdoor areas are excellent reasons to go permeable. But using permeable hardscape offers the additional advantages of doing your part to filter and recharge groundwater reservoirs, as well as protecting local waterways. It’s a green trend that’s here to stay.

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