Vegetated Roofs

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WHAT IS A VEGETATED ROOF?
Vegetated roofs (also called green roofs) are a green stormwater infrastructure technique for managing stormwater. A vegetated rooftop is comprised of a growing medium planted over a waterproof membrane and is partially or completely covered with vegetation. A green roof also contains a root repelling membrane and drainage system. Layers of engineered soil and vegetation are constructed on top of conventional flat or sloped roofs. More details on design standards can be found in the City of Tacoma Stormwater Management Manual (SWMM) and in the Green Stormwater Infrastructure detail plan.

WHAT ARE THE BENEFITS OF A VEGETATED ROOF?

  • Reduces the volume of stormwater flowing into streams and drainage channels while absorbing pollutants.
  • Provides better building insulation – a shaded roof surface helps cool a building in the summer and acts as a blanket to conserve heat in the winter.
  • Reduces the urban “heat island” effect.
  • Provides habitat for diverse plant and animal species, including beneficial insects and songbirds.

MAINTENANCE REQUIREMENTS

  • Clogging by leaves and other debris can be minimized through proper design and maintenance.
  • Any standing water removed during maintenance operations must be disposed of according to city regulations.
  • A detailed maintenance checklist is available in the SWMM. Green roof manufacturer maintenance standards are also available.

DESIGN STANDARDS
Green roofs shall be designed and installed under the direction of a professional with demonstrated expertise in the design and construction of such facilities.

Vegetated roofs shall conform to best available technology standards, such as those published by Leadership in

Energy and Environmental Design (LEED), and be designed in accordance with the City of Tacoma SWMM.

HEIGHT BONUS
If a building provides a vegetated roof that covers at least 60 percent of the building footprint, a 10-foot height bonus may be granted (TMC 13.06.040).

FLOOR AREA RATIO (FAR) BONUS 
The allowable FAR can be increased by 0.5, up to the Maximum with Design Standards, for incorporating landscape covering at least 15 percent of the surface of the roof and/or the use of vegetated roofs. Access by building occupants is encouraged (TMC 13.06.050.F.6).

RESOURCES

 

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