What Is Permeable Pavement?
Permeable pavement is a hard surface that has enough gaps in it to allow rain to run through it. When rain runs through permeable pavement, it collects in a base layer of gravel, then gradually soaks into the ground. Permeable pavements are sturdy enough for vehicles and heavy pedestrian traffic.
Permeable pavement is also called pervious pavement, porous pavement, pervious concrete, and porous asphalt. Below are descriptions of several choices of permeable pavement.
Porous Asphalt and Pervious Concrete
Porous asphalt and pervious concrete are like conventional asphalt and concrete but contain less fine aggregate content leaving open spaces for water to pass through and soak into the ground. Porous asphalt and pervious concrete are the most suitable for large areas including residential driveways and parking lots.
Permeable Paver Systems
Permeable paver systems have gaps between the pavers that allow water to pass. A layer of gravel under the paver system acts as a reservoir, holding rainwater while it soaks into the ground. Pervious paver systems are the most versatile type of permeable pavement and are suitable for residential driveways, patios, and parking lots.
Turf Block Systems
Turf block systems are pavers with empty spaces filled with soil and planted. Turf block systems are suitable for residential driveways.
At a Glance: When to use which permeable paving solution
| Property uses | Porous asphalt | Pervious concrete | Permeable pavers | Turf block |
|---|---|---|---|---|
| Residential driveways | Yes | No | Yes | Yes |
| Patios | No | No | Yes | No |
| Small parking lots | Yes | Yes | Yes | No |
How Permeable Pavement Works
Permeable pavement is an effective way to manage rain on your property without limiting use of space. Imagine eliminating the sheets of rain running across parking lots and driveways. Permeable pavement slows runoff and reduces the amount of rain that flows into pipes. This can reduce the risk of sewer backups and recharge groundwater.
The gravel, known as subgrade, layered beneath the asphalt, concrete, or pavers also filters pollutants to help protect water quality both in our rivers and groundwater.
Summary of Permeable Pavement Design Requirements
These site and design requirements can help you decide if permeable pavement might be appropriate for your project.
- Permeable pavement is allowed on surfaces with slopes no greater than 5 percent.
- Underlying soils should have a minimum infiltration rate of 2 inches per hour.
- There are no setback requirements for permeable pavement.
- There must be 5 feet between the high groundwater level and the excavated bottom.
- The subgrade next to structures should slope away from the structures.
- Use a minimum of 6 inches of washed, crushed 2- to ¾-inch or No. 57 rock under concrete or asphalt.
- Consult the Stormwater Management Manual regarding required edge restraints.
For best results, keep in mind the following construction considerations:
- Protect the subgrade from over-compaction during excavation.
- Do not excavate or compact the native subgrade in wet conditions.
- Consider the sequence of construction activities to protect the subgrade from traffic. Protect the paving from construction traffic and sediment after installation.
When to Call a Professional
Call a professional designer if you have more vehicle traffic than a residential driveway.
Also, if your soil infiltration rate is less than 2 inches per hour, you will need to hire a designer to help you.
Costs and Permits
Permeable pavement installation ranges in cost depending on the size and type of system.
Pervious pavement systems used to replace public parking or walkway areas require a building permit from Permitting & Development.
Commercial building permits are required for non-residential projects.
Maintenance Requirements
Clean the surface once or twice per year to maintain permeable pavement.
Permeable pavement is less prone to cracking or buckling from freezing and thawing. Studies indicate it requires less frequent repair and patching than conventional paving.
Properly installed permeable pavement systems can last for more than 20 years.
Stormwater Management Manual
Consult the City’s Stormwater Management Manual for the complete set of requirements on how to safely site, build, and maintain pervious pavement or other stormwater management solution on your property.
Portland’s Clean River Rewards Program
Residential and commercial property owners who install qualified stormwater management solutions may be eligible for discounts on the stormwater charges of their sewer, stormwater, and water bill through Clean River Rewards.
Permeable pavement that meets the safety and space requirements outlined in the stormwater management manual is eligible for Clean River Rewards for commercial, multi-family residential, and industrial accounts. Visit Clean River Rewards to learn more.