What are Permeable Pavers?

Understanding Permeable Pavers

Permeable pavement is a porous alternative to the conventional pavers and pavements we use on our walkways and driveways, patios and pool decks. Some permeable materials look nearly identical to the traditional hardscape materials we’re used to, but these porous cousins sure don’t act the same!

Characteristics of Permeable Pavers

If something is permeable, something else can pass through it and, in the case of these pavements, that something is water. Our homes’ typical hardscapes are constructed to be water-resistant. At best, they work like your roof, relying on a grade to send rainwater to the periphery to flow onto a storm drain or into your yard. Without an appropriate grade, a conventional pavement allows water to collect on its surface, relying on overflow and evaporation to disperse it.

By contrast, a permeable pavement lays out the welcome mat to water! Rain falls on its surface, flows through tiny holes in the pavement and from there seeps into a carefully constructed rock bed below for absorption, filtration and storage. The holes might be in the poured pavement or in gaps between the pavers, but all permeable surfaces share a commonality: The surface is just the visible portion of an entire system that includes an absorbent subsurface, which is critical to the installation.

Why do we want a hardscape to be permeable? Easy! All the jobs a typical residential hardscape does not do — absorb, filter, store — are to the detriment of our landscapes, the greater environment, our personal health and maybe even our bank accounts. We’ll explain this part in a minute. First, let’s look at the types of permeable pavements you might consider.

Types of Permeable Paving Materials

We better take a moment to mention that when shopping, you’ll often see permeable, pervious and porous used to describe the same pavement. Don’t get too hung up on the adjectives. We’re going to stick with the most common term for each material here, but prepare to encounter them all during further research.

Pervious concrete is a poured surface like the conventional concrete used for driveways, walkways and patios, and the permeable version can be mixed by the same suppliers. The traditional formula is tweaked to eliminate (or reduce) the fine-grain stuff in the mix, which creates small voids and makes the product permeable. Often this surface has a coarse appearance.

Porous asphalt is another poured pavement in which the traditional formula is tweaked to create small voids. Porous asphalt can also be mixed by conventional ready-mix suppliers, and it's most frequently considered for residential driveways.

Permeable concrete pavers are sometimes called permeable interlocking concrete pavers (PICPs). They are laid in a pattern like conventional pavers and are even made of the same material. As such, the pavers aren’t porous themselves. Instead, they have little spacer bars on the sides to maintain a gap between them that is filled with small aggregate (tiny stones). These gaps (joints) are what let water through. The pavers are used on driveways, walkways and patios. They come in a multitude of colors, finishes, shapes and sizes. Greater thicknesses will support more weight.

Permeable clay pavers are the brick besties of PICPs, with similar uses and features. They, too, have those spacer bars on the sides and are made of the same fired clay as conventional brick. They are offered in many of the colors of the landscape — and texture options too. Permeable concrete and clay pavers are often marketed as green pavers, environmental pavers and eco pavers to set them apart from the company’s conventional paver line when it offers both.

Concrete grid pavers essentially serve as a concrete joint for a paver made of grass or gravel, like an opposite-world version of your typical PICP. Concrete grid pavers are sometimes called aqua pavers or turf pavers and their primary purpose is erosion control and ground stabilization while allowing for a natural appearance.

Permeable paving grids are a similar concept to the concrete grid pavers but are made of recycled plastic and have internal patterns. These are less expensive and their smaller scale joints can make them nearly 100 percent porous. The paver joints can be noticeable as a pattern or nearly invisible within the fill you’ve chosen.

You’ll encounter additional products for more specific applications. For example, permeable resin-bound paving is a sheet pavement in which recycled glass, stone aggregates or even recycled rubber chips are bound in a natural or polyurethane resin, in a mix that leaves voids in the final product. These specialty aggregate products might be troweled over an existing walkway as a decorative veneer, used to stabilize a natural running path or to prevent erosion around tree roots.

Pervious concrete, porous asphalt and the permeable concrete and clay pavers are frequently considered for the same home projects, so our discussion will largely focus on them.

Permeable Pavers vs. Traditional Paving Options

Fundamentally, permeable pavements differ from traditional paving options in the way they handle water, sucking it up it like grown-up ground instead of shrugging off the chore like a petulant teen.

• With conventional pavement, rainwater overflow from the hardscape can result in erosion problems and flooding. This will damage landscaping, create a mosquito breeding ground and lead to algae growth on your hardscape in warm weather. With permeable pavement and pavers, your patio, walkway or driveway accepts the water it receives, meaning no erosion, mosquito habitat or waterlogged plants nearby.
• With conventional pavement, pollutants collect on paved surfaces and are left to accumulate or are washed into your yard. The permeable system filters up to 99 percent of pollutants, eliminating them from the environment.
• With conventional pavement, excess water heads en masse to storm drains, taking those unfiltered pollutants with it to tax local waterways. The filtered water from a permeable system is kept in your residence’s ecosystem.
• With conventional pavement, poor moisture handling creates an unhealthy cycle of freezing and thawing in cold weather, compromising your hardscape and risking injury to you. In passing water through, permeable pavements have superior freeze-thaw resistance, providing a safer surface for walking and needing repair less frequently.

In large part because of these superior water-handling capabilities, a permeable installation can have a longer lifespan than a conventional hardscape when installed correctly and maintained —sometimes double (EPA).

Installing permeable pavement is frequently very different than a conventional pavement, starting from the top for concrete and brick pavers. Traditional pavers are laid with a joint width of 1/16 inch to 3/16 inch, with brick pavers on the tighter end of that. The joints are filled with dry or polymeric sand, and the base of the installation is a 1-inch bed of sand over compacted aggregate and a compacted subgrade. By comparison, permeable pavement joints are larger (around 3/8 of an inch); and boy will you be sorry if you pull out the bag of sand! The bedding layer and joints are filled with a tiny aggregate, which is what allows the water to flow through.

It doesn’t stop there because the permeable surface is the visible portion of an entire subsurface system that manages the water. And if you get that wrong, the system isn’t permeable. Whoops! For this reason, a permeable installation is a pro job, rarely a DIY opportunity. This will make more sense when we discuss next how the system works.

How Permeable Pavers Work

The of Behind Water Drainage and Absorption

In a natural system, some of the rainfall that reaches the ground’s surface runs off and some is absorbed into the subsurface through the pores in the ground. How much water is absorbed and how quickly (the soil infiltration rate) is essentially about how much sand or clay is in the soil (the soil composition). These soil characteristics vary from region to region, even between your home and your neighbor’s. When you cover the ground with a conventional hardscape — an uninterrupted expanse of pavement or pavers with sealed joints — you’ve turned a water-absorbent surface into a water-resistant one.

A permeable pavement system mimics the water-handling of an ideal natural system. Instead of the soil infiltration rate of inches of water per hour, the permeable pavement has a surface infiltration rate. This time it’s telling us how efficiently the pores in the pavement surface or the voids between the pavers are sending water through. And permeable pavements have over-the-top surface infiltration rates when installed, often hundreds of times more than you’d ever experience in a storm.

But it’s only part of the equation. Because if your soil infiltration rate is really poor, what good is it to have a fabulous surface infiltration rate? The water has nowhere to go. A permeable pavement surface depends on permeability below it to do its job. More than the visible layer you went to all that trouble to select, what happens below will be what makes an installation work or fail.

Permeable Pavement Is a System, Not a Surface

This is why it makes sense to think of permeable pavement as a system instead of just a surface, and this is where the subsurface in a permeable system comes into play — as a middle (water) manager, so to speak, between the ground and the surface. Imagine that it’s raining buckets one morning and your permeable pavement is welcoming all that water with open arms. But your soil is not so friendly: Whoa now, wait a second, buddy! I can only handle half that! This is when the subsurface layer calmly steps in with a hand on the arm: It’s okay, it’s okay. I’m going to slow that down for you.

In fact, if your soil is very slow draining or if you want to reclaim the filtered water the system produces, you can also put an underdrain here. The subsurface is where the magic happens.

For poured materials, the subsurface magic may be a simple card trick — several inches of crushed rock. For a paver installation, it’s often a Houdini-level performance:

• the permeable concrete or brick pavers with joints filled with tiny aggregate
• a bedding layer (choker course) of small aggregate
• a base layer of compacted medium aggregate
• (sometimes) a subbase layer of compacted coarse aggregate
• (sometimes) a geotextile liner
• the subgrade layer of your uncompacted soil.

The subsurface for permeable grid products (if needed) can resemble either of their cousin pavements.

Bottom line: The nature of the subsurface depends on which surface you choose, your soil characteristics and what you are asking of this hardscape.

Installation Techniques for Optimal Permeability

Well-planned permeable hardscapes can easily outlast conventional hardscapes if you start with good permeability and then maintain it.

• Know your soil to plan your subsurface adequately. Your soil composition will determine how thick subsurface layers will be, among other things.
• Know your surface. You'll discover that even some conventional pavers can be appropriate for permeable installations under the right circumstances. Paver manufacturers have specific recommendations for joint width, aggregate size and subsurface construction across different applications to ensure permeability from the start and in perpetuity.
• Mind your border. Permeable surfaces must be exposed and the pores kept clear for the system to absorb water. An edge restraint is important for pavers for structural reasons, but incorporating a border can also keep adjacent landscaping from knocking dirt onto your pretty pavement. Poured pavements are frequently laid a couple inches above planted borders with the landscaping sloping away for the same reason.
• Account for the grade. Many permeable surfaces perform best on a grade of less than 5 percent because otherwise rainwater may drain off before it can seep in. A steeper grade isn’t the dealbreaker it was in the early days, but it’ll require additional design features and knock some pavers out of contention.
• Plan for nonpermeable surfaces nearby. Another no-no when permeable was young was allowing nonpermeable surfaces to freeload off your pristine permeable for drainage. Now, this water-handling altruism is recognized as one of the perks of a permeable hardscape, but you must plan for it and be savvy about it. If you get the math wrong — BIG house roof into tiny permeable driveway — you can overload the system. If it’s a conventional surface that contributes a lot of debris, it’ll clog your permeable.

Which brings us to the number-one technique to make sure your permeable pavement will do its job for many decades: Hire an experienced professional to design and install it.

Maintenance Requirements for Permeable Paver Systems

Maintaining permeable pavement is easy: Pull out the broom or leaf blower regularly to remove interloping mulch, lawn clippings and leaf fall. This is especially important for paver systems because weeds really like to move into joints. In the winter, avoid traction products with sand that could compromise your permeability. You might not need as much de-icer since water isn’t sitting on the pavement surface!

Annual maintenance

Vacuuming or pressure washing once or twice a year is often sufficient to keep the pavement pores unclogged and your system flowing. Inspect any drains or pipes that are part of the installation to make sure they’re clear too. If you have a paver installation, inspect the joints at this time to ensure you don’t need to add more aggregate. Your paver manufacturer or pavement contractor will have thorough instructions on annual upkeep.

Over the lifespan

David Liguori is president and founder of Bay Area Pervious Concrete, a green hardscape designer and installer in Northern California. Liguori likes to call his company’s work putting the puddle below the pavement, but he adds that we shouldn’t be alarmed if that puddle appears up top at some point! Liguori explains that any type of permeable pavement in any environment will lose some of its mojo over time as sediments build up. You can call your contractor to come out and perform an infiltration test to check for reduced permeability, but Liguori has an easier assessment: “Look for pooled water on the surface when it’s raining. If permeability is reduced enough to create a puddle, it’s definitely time for a refresh.”

Again, vacuuming or pressure washing should do the trick for pervious concrete and porous asphalt. Paver systems are a little more involved: They’re regenerated by using a vacuum to suck up some of the joint aggregate and then replacing it.

Benefits of Using Permeable Pavers

The permeable paver aisle attracts homeowners of all motivations — environmental health, personal health, budgetary, energy independence, aesthetics — you name it. Here are a few reasons permeable pavement should appeal to you!

Environmental Advantages, Including Stormwater Management

The environmental benefits of permeable pavements start with manufacture. Many are constructed of locally sourced natural materials, recycled materials or both. Once the pavement is installed, it provides a healthier local environment and benefits your greater environment, avoiding many problems endemic to conventional hardscapes. We mentioned before how a permeable pavement can prevent erosion and damage to plants. It also recharges your groundwater — a science-y way of saying that the rain is kept locally so you won’t have to use the hose as much after the storm.

Filtering pollutants is one of the coolest things a permeable pavement system does! It can handle oil from your engine, heavy metals from your brake pads and chemicals in rainfall that entered the weather system miles away. Specific pavement types are standouts for different filtering capabilities: Paver systems excel at eliminating nutrients that cause algae blooms (up to 72 percent), whereas poured pavements are the best at removing petroleum products and heavy metals — up to 99 percent in some cases! The chemistry of the pavement and aggregate and the microorganisms living in the subsurface process the pollutants (EPA).

Compare that to a conventional pavement, where those unfiltered pollutants collect so you walk through them, and then wash into your lawn during storms or head to the storm drain unfiltered.

Cost-Effectiveness and Long-Term Savings

A permeable installation is usually more expensive upfront than a conventional hardscape even if the pavement itself is comparable, which it frequently is not. This is because of the added cost of the subsurface. But permeable surfaces can be expected to last longer than their conventional counterparts and require less maintenance and repair, which translates to savings over the long term (EPA). Some cities incentivize residents with tax breaks, grants and utility credits to install permeable hardscapes at their homes.

Permeable hardscapes can spare you additional water handling infrastructure for the pavement and other impermeable parts of the property. And you can design your permeable pavement to move filtered water to areas of the property that need it or store it for later use, saving on irrigation costs.

Aesthetic Appeal and Design Flexibility

The great thing about permeable surfaces is that there is something for everyone. Because permeable pavers look so much like their conventional cousins, all the textures, colors and shapes that have traditionally defined our landscape aesthetics are possibilities. A weathered-brick driveway or a cobble walkway, a patio of smooth and modern rectangles, squares or hexagons. The coarser texture of pervious concrete can be celebrated if rustic is your vibe, or a finer grain pavement might be an option. An experienced pervious concrete designer can even customize the aggregate mix to fit your unique vision.

The permeability of the hardscape creates novel opportunities too — an overflow water feature on a pool deck, a fountain into a patio, a splash pad or an outdoor shower.

And for homeowners who love the idea of a grass or pebble hardscape but need to prioritize ground stability, the answer could be concrete grid pavers or permeable paving grids. Their primary purpose may be erosion control and water management but because you fill them with natural substrate, these turf pavers can disappear into the landscape or offer a decorative pattern to a driveway or a side yard.

Choosing the Right Permeable Pavement for You

When shopping for a permeable system, it can be hard to make sense of surface infiltration rates and porosity percentages (also called void space and surface open area) because performance data won’t match up across products. The porosity of the pavement is really just how hole-y the whole thing is, and this varies widely — from 7 percent to 98 percent. Pavements on the lower end of the range can still accept hundreds of inches of rainwater over an hour — more than you’d ever see in a typical storm. Then, remember, surface performance depends on the subsurface performance. Bottom line: If a professional is designing the system, don’t worry about it too much. A well-installed permeable pavement system will accept what nature gives you plus additional water you’ve planned for, and it will continue to do so if well-maintained.

There are practical reasons to choose one surface over another, and cost certainly is one of them. Pervious concrete is frequently the most expensive choice but the subsurface requirements of a concrete or clay paver installation can make it cost even more sometimes. Porous asphalt is usually the least expensive option but has fewer residential applications.

Accessibility is an important consideration for everyone. Porous asphalt and pervious concrete are most easily installed as level and stable ADA-compliant surfaces, with traction year-round. Permeable concrete and brick pavers can also meet the requirements, but you must ensure a joint size of 1/2 inch or less. For grid pavers (concrete and plastic), the requirements are more of a challenge. They may all be doable, but harder with the natural filler.

Finally, there’s your unique home environment, which you may worry excludes you from even entertaining permeable pavement. But the old limitations on use are being discarded one by one as the surfaces regularly outperform study expectations, installation practices are refined and the systems are adapted to accommodate challenging settings.

Bay Area Pervious Concrete’s David Liguori has been designing and installing pervious concrete pavements in Northern California for more than 25 years and at this point there simply aren’t many residential conditions he’d say no to. “It’s all doable, if it’s done right,” Liguori says.

Liguori used to install all three of the most popular permeable surfaces — concrete, asphalt and pavers — and he’s now happily gone exclusive, making an honest pavement out of pervious concrete for what he feels is its superior performance. Though he is devoted to his choice, he does not recommend it in all cases. This is because of the three surfaces, pervious concrete is the hardest to install. His recommendation? “If you can find an experienced pervious concrete installer in your area, do it,” Liguori says. “Otherwise, opt for permeable pavers that any landscape contractor will be qualified to handle.”

How to Begin

• The National Ready Mixed Concrete Association (NRMCA) has a searchable list of certified pervious concrete craftsmen, installers and technicians. As a NRMCA pervious concrete–certified craftsman and instructor, David Liguori advises us to focus only on those individuals with craftsmen and installer level–certification because they have field experience. Technicians have just passed a written exam.
• Unfortunately, porous asphalt doesn’t have the equivalent certification yet but a Google search for porous asphalt in your area might direct you to installations nearby.
• Your successful permeable paver project begins with a search for qualified installers on the manufacturers’ websites. Conventional paver companies Belgard, Pine Hall Brick, Techo-Bloc and UNILOCK all have permeable offerings.
• Learn more about how to ensure your pavement is ADA-compliant on Belgard’s helpful page on ADA Standards For Accessible Design as it applies to paver surfaces. The Brick Industry Association also has a useful Technical Note on ADA-compliance.
• Uni-Group has a helpful list of incentive programs organized by state that you can check for grants or credits available for permeable technology.