What lies beneath

When it comes to permeable pavers, it’s not only what’s on the outside that counts.

Photo courtesy of Belgard

Outdoor living spaces are recognized by their intricate pavers and designs, but what lies beneath is actually more important to the overall success of the design.

Before a contractor begins to lay a section of permeable pavers, a solid base or drainage system needs to be established. The different types of bases depend on the overall scope of the property, the location and the use.

“Within those permeable asphalt (installs) or concrete permeable pavers, there's different base systems that can be used and drains that go within the system,” says Scott Shorrow of Landscape Concepts.

Why it matters.

The job of permeable pavers is often to soak up the water on a property. These types of pavers can also be utilized to help divert water flow from structures like homes and prevent water damage in high-flood areas.

Shorrow says drainage systems and permeable pavements have had the spotlight on them lately because of the push toward sustainability.

“Now, the big thing is getting the rain water back into the ground,” he says. He also notes that proper drainage systems are even a requirement on most structures.

“There’s nothing more frustrating than having a new hardscape or landscape installed that creates drainage issues,” says Joe Raboine, residential business manager for Belgard. “It’s unwanted water that ends up in a basement, garage or pool on the surface can cost thousands of dollars to repair, and sometimes leads to unsatisfied customers and legal implications in some cases.”

“If you have a house, the purpose of the downspout is to get water away from the foundation,” he says. When you consider drainage and permeable pavements, the function is often the same.

Diagram courtesy of Belgard
Under the pavers.

The base layer of a paver project sets the overall design up for success when done properly. Shorrow says it’s important that a contractor first considers what type of property they are working on.

“Is it commercial or residential,” he says. “And what kind of traffic is this going be experiencing?” Raboine says that because some permeable pavers can be used for vehicular applications, the types of material used should be able to withstand that use.

“(Contractors should consider) the appropriate shape and thickness is based on project-specific conditions, including type of loading, base design and subgrade conditions,” Raboine says. “With a complex project, it’s crucial to determine the grades and current route of the water runoff that’s falling on the site.”

Typically, a permeable project is designed to withstand five times its area in water, Shorrow says. But, you don't really want excess water running onto a pervious pavement system, he notes.

“They know they can pull their driveways up, put down permeable pavers, and get their additions.” Scott Shorrow, Landscape Concepts
With the flow.

With that in mind, contractors have to consider the rate that water will flow through the pavement systems. The flow rate also depends on the type of soil underneath.

“It truly depends on the application and soil conditions. If the water table is too close to the surface of the pavers, a drainage system may be required,” Raboine says. “If not, and the soil conditions warrant, a drainage system may not be required.”

Clay soils will absorb water at a slower rate than other types of soil, Shorrow says. And, if the area of application is going to see an excessive amount of water, a larger, more complex drainage solution may be needed. For example, if the pavers are being installed around a pool, it’s expected that more water might be flowing over the paver. Raboine says drainage solutions like catch basins or even a large network of drain pipes might be required.

The direction of the flow is also crucial. Water tends to flow downwards to the lowest point on a property, so diverting it from your client’s home is obvious. However, paying attention to neighboring structures is just as important, Raboine says.

The layers of stone aggregate underneath a permeable paver area act as the basic system of drainage. Different sized stones are layered on top of each other, typically with the larger grade stone at the bottom. While you may have 10 feet of stone to fill, Shorrow says that’s 10 feet for water to flow and not just a big empty hole.

At the base of the install, before any aggregate or pipes are put into place, it might be necessary to lay down some geotextile fabric. The pipes are installed to direct water toward storm sewers or other nearby streams.

Another facet to the drainage system, which acts as a finishing touch, is the polymeric sand that is spread to fill the cracks in between the pavers.

Not a hard sell.

Shorrow says he doesn’t typically go into too much detail about the specifics of a drainage system unless the customers ask. “A lot of customers we do work for like to sit down and chat but they just want their addition built. They don't really care,” he says. “They know they can pull their driveways up, put down permeable pavers, and get their additions.”

The company had a job in the D.C. area, however, and the client was very invested in the work that was being done. “We put a sump system in basically like (you would find) in a basement and then that kicked all that run off water into a system that then ran all the toilets in the house from the driveway,” he says. “So that's a little more on the green side and drainage.”

For some homeowners, creating a permeable area on their property can act as a credit on their property in order to build onto their homes or add another structure.

“If they have a 2,500 square foot driveway, they pull that out and make it permeable,” Shorrow says. “Now, they can add a 2,500 square foot addition to their house.”

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