What Is a Roof Garden?
A roof garden is a heavyweight, planted roof. While ecoroofs are thin, lightweight, and not built to support pedestrians, roof gardens can also provide outdoor building amenities like walkways, terraces, plazas, or seating areas. They are often designed with hardscaping to allow access to the garden.
Also called “intensive roofs,” roof gardens are built using more infrastructure and more soil than ecoroofs.
Roof gardens reduce stormwater runoff flow rate and volume. They also:
- Create usable space on roofs, adding value to buildings.
- Lower heating and cooling costs.
- Reduce outdoor air temperatures and the resulting urban heat island effect.
- Provide habitats for birds and insects.
Roof gardens can outlast conventional roofs by 20 years or more.
How Roof Gardens Work
Roof gardens are living systems with soil and plants that soak up rainwater. The rain nurtures the plants and stays in the soil instead of going into sewer or stormwater pipes. During heavy rains, when more rain falls than the roof garden can absorb, the soil slows the flow of water into the pipes. Reducing the volume and speed of rain runoff in this way helps reduce the risk of flooding, sewer backups, and sewer overflows.
Green roofs are designed to filter pollutants out of rainwater and help minimize the use of conventional roof materials that can have oil or chemical pollutants on their surfaces.
Summary of Roof Garden Design Requirements
These site and design requirements can help you decide if a roof garden might be appropriate for your project.
- Locate roof gardens on flat or shallow-pitched roof structures.
- Building structures must be adequate to hold the extra weight on the roof.
- Include overflow structures such as drains or downspouts.
- Avoid using copper, lead, galvanized metal, and other materials that can introduce pollutants to rain runoff.
- Avoid using chemical root barriers and fertilizers, as they will pollute the water runoff.
- Roof gardens can contain picnic or park areas. They can include walkways for recreation or maintenance.
When to Call a Professional
A professional is required in all cases to build a roof garden.
Costs and Permits
Roof gardens are more expensive than traditional roofs. The exact difference in cost depends on the design, type, amount of waterproofing and drainage material, and the types of plants used. While the up-front costs are greater, roof gardens are expected to last twice as long as a conventional roof. In addition, roof gardens help insulate buildings, which offers savings in the form of reduced heating and cooling costs.
Roof gardens need substantial structural support. The Bureau of Development Services requires designs stamped by a structural engineer for project approval.
Like a conventional roof, roof gardens need regular maintenance.
Ordinary maintenance tasks include:
- Occasional or regular summer watering depending on plant selection.
- Occasional weeding (especially in the first two years while plants are becoming established).
- Occasional cutting of plants and removal of dry materials, especially in the summer.
- Regular inspection to ensure proper drainage.
With proper maintenance, a roof garden can last up to 40 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 roof gardens 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 a discount on the stormwater charges of their sewer, stormwater, and water bill through Clean River Rewards.
Roof gardens that meet the safety and space requirements outlined in the Stormwater Management Manual are eligible for Clean River Rewards. Roof gardens planted for food production will not be credited for stormwater management due to watering and fertilizer needs that could compromise the performance of the system. Visit Clean River Rewards to learn more.
Find More Resources
Suggested plants can be found in the Ecoroof Plant List section of the Stormwater Management Manual.
For more plants to consider, consult the plant lists in the Bureau of Development Services Tree and Landscaping Manual.