Permeable paving refers to street and sidewalk paving materials that allow stormwater to filter through to the soil below. Permeable paving materials like porous concrete or unit pavers may look similar to traditional paving materials but allow air and water to pass through the paving material, providing the opportunity for temporary storage of stormwater runoff and/or groundwater recharge into the soils below.

Permeable paving can be used on various portions of the public right-of-way.

Process Overview

Permeable paving may be installed by small property owners, by major new development or as part of corridor-wide improvements.

Applicants are encouraged to apply as multiple property owners along a block or neighborhood to enhance the project’s benefits and lower costs to the applicants. Talk to your neighbors about putting together a joint application for permeable paving.

See Building Neighborhood Support

Permeable paving located in the sidewalk area requires a Special Sidewalk Surface Permit. A Special Sidewalk Surface Permit grants permission to replace standard concrete sidewalk with alternative paving materials, including permeable paving. A permit is required to ensure that permeable paving is properly constructed and maintained in order to maximize environmental benefits, protect public safety and limit conflicts with infrastructure.

Special Sidewalk Surface Permit (sidewalks only)

Permeable paving may be built as a standalone feature, or in coordination with other streetscape and stormwater management features, such as street trees, bioretention planters, or sidewalk landscaping.

If permeable paving will be installed in combination with other streetscape features or constructed in the roadway, parking lane, medians, or other areas, the project sponsor must obtain relevant permits for all features. DPW will generally combine multiple features into a single permit.

See Permit Process for more information.

Official Codes & Documents

• Better Streets Plan (street design guidelines)
• Stormwater Design Guidelines
  (The SFPUC’s guidelines for design of stormwater management features)
• Stormwater Design Guidelines Appendix A
  (Stormwater management tool fact sheets for specific applications)

Design Guidelines

Location

Street types: All

Sidewalk zones: Frontage; Throughway; Furnishings; Extension; Parking lane; Full Roadway on minor streets and alleys

Permeable paving systems are most appropriate on:

• Sloped paving surface of 5% or less (may be greater with appropriate subbase design considerations)
• Sidewalks, driveways, park lands, shared streets, plazas, pedestrian and bike paths, and publicly owned open parking lots
• Parking strips and gutters that are not used as traffic lanes or bus stops
• Alleyway traffic lanes with prior approval from DPW; and
• Low traffic streets

Permeable paving systems are not suitable on:

• Traffic lanes on streets classified as arterials or collectors, concrete bus pads, or curbs;
• Sloped areas or steep hillsides with slopes greater than 20%, unless designed with an appropriate sub-base;
• Areas with a previous history of soil or shallow groundwater contamination;
• Gas stations, car washes, and automotive repair shops, or in areas where there is a possibility of chemical spills;
• Streets with a history of combined sewer overflows unless as part of a project aimed at eliminating such overflows;
• Areas with shallow groundwater or seasonal high groundwater (less than 4 feet) if receiving run-on (stormwater runoff from adjacent areas);
• Within 20 feet of sub-sidewalk basements if receiving runoff; or
• Within 50 feet of domestic water wells if receiving runoff.

Soils: Permeable paving requires an understanding of the prior use of a site. Site tests should be performed to determine soil conditions including: percolation rate and infiltration capabilities, depth to seasonal high water table, depth to bedrock, and soil contamination.

Existing subgrade should be able to demonstrate a minimum permeability rate of 0.5 inches per hour. An underdrain may be used in soils with lower infiltration rate in order to obtain the minimum permeability rate. Percolation testing to identify the infiltration rate of the native soil will determine the depth of base rock for the storage of stormwater, and whether an underdrain system is necessary.

Permeable paving must maintain separation of 4 feet above both the high water table and bedrock for proper performance. Permeable paving should not be installed over new or existing compacted fill.

Schematic Drawing of Permeable Pavement

 

Design

Permeable Paving is appropriate for installation in both small projects sponsored by individual property owners as well as larger block-scale or neighborhood-scale projects involving multiple LID features.

Permeable paving must be designed to appropriate specifications and well maintained in order to drain stormwater effectively. To minimize potential for failure or clogging, design of permeable paving systems should consider:

• Suitability of the selected paving material for the site conditions;
• Strength of underlying subgrade – subgrade should be constructed to support anticipated vehicular and pedestrian loads for the site;
• Required loading depth and storage capacity of aggregate base course;
• Surface and subsurface materials including filter fabrics; and
• Installation method.

Accessibility: Selected paving materials must be ADA-compliant, and not cause tripping hazards or excessive vibration. As with all paving materials, permeable unit pavers should leave gaps of no more than ¼” inch wide, or up to 1/2” wide with beveled edges. Beveled edges should only be used to mitigate vertical change. Vertical changes should not exceed a slope of 1:2.

Materials: Permeable pavement consists of a series of layered elements that allows percolation of stormwater. From top to bottom these layers are: a surface material, a bedding layer (also called a choking layer), a transition (or base) layer and a storage (or subbase) layer of permeable base rock, and the native soil. Common materials for the paving surface include:

• Permeable hot-mix asphalt: similar to standard hot-mix asphalt but with reduced aggregate fines (typically 2.5 inches deep)
• Permeable concrete: similar to standard concrete, but without the fine aggregate (sand and finer) and with optional special admixtures incorporated (typically 4 to 8 inches deep depending on loading)
• Interlocking block pavers: either cast-in-place or pre-cast concrete blocks with small joints or openings that can be filled with soil and grass or gravel

Both permeable pavers and porous concrete have the added benefit of elimination of polycyclic aromatic hydrocarbons (PAHs) from the wastewater stream. Where porous asphalt is used, PAH-free asphalt alternatives should be considered as a source-control measure.

For more detailed information on materials, see Stormwater Design Guidelines Appendix A

Installation

Permeable Paving is most susceptible to failure during construction. Special staging and installation phasing measures should be taken to prevent compaction, sealing, or sediment build-up, including:

• Incorporate appropriate sediment reduction techniques wherever possible. Maintain erosion and sediment control measures until a site is stabilized.
• Remove all compacted sub-base and avoid compacting soils during construction.
• Spread the infiltration over the largest area feasible. Avoid concentrating too much street or sidewalk runoff in one area. Most types of permeable pavement (pervious concrete, porous asphalt, and permeable unit pavers) can receive run-on from adjacent areas: permeable paving on streets and sidewalks should be at least 25% of the total drainage area.
• Avoid smearing of underlying soil, to minimize sealing of soils.
• Avoid contamination with sediment; avoid tracking sediment onto pavement.
• Avoid drainage of sediment laden waters onto pervious surface or into stone sub-base constructed bed.
• Do not allow construction staging, soil/mulch storage, etc. on unprotected pavement surfaces.

The bottom sub-grade should be graded level to allow even distribution of infiltration where soil conditions provide for permeability. Where provided, underdrains should be placed at the pavement edge to provide drainage that prevents ponding.

Existing compacted or otherwise impermeable sub-base should be fully removed to prepare the soil for infiltration.

Maintenance

Maintenance Responsibilities

With some exceptions, fronting property owners are responsible for the on-going maintenance and upkeep of sidewalk paving as well as all sidewalk elements directly fronting their property, such as trees, landscaping, and streetscape furnishings. Generally, the City is responsible to maintain roadway paving and other features in the roadway, such as medians.

Typically, if you initiate street or sidewalk improvements, you will be responsible for maintenance of those features. Specific requirements will be described in your permit.

For a more detailed description of maintenance responsibilities, see Maintenance.

Maintenance Practices

Permeable pavement requires periodic maintenance to retain its infiltration capacity. Permeable pavement should be vaccuumed once or twice annually. Vaccuuming has been found to be most effective when sediments are dry. If routine cleaning does not restore infiltration rates, then partial or full reconstruction of the pervious surface may be required.

Also, broken or damaged pavement needs to be removed and replaced. Once a year, the paving should be tested to determine if it is clogged.

Top of Page