Green Roofs

What is a Green Roof? A green roof, also known as a “living roof” or “vegetated roof,” is a planted roof top garden that offers an attractive and energy-saving alternative to a conventional rooftop. Green roofs are covered with a waterproofing membrane, a growing medium and vegetation.    

Benefits of Green Roofs:seedling drawing

  • Reduces stormwater runoff and helps filter and retain rainwater on-site
  • Improves comfort and energy efficiency by adding a layer of insulation to roofs
  • Provides urban wildlife habitats
  • Reduces heat island effect
  • Adds distinctive architectural element
  • Increases longevity of roof by protecting it from UV rays

Types of Green Roofs:
Green roofs can be intensive, extensive or a combination of both. Choosing the right living roof depends on your building, your objectives and your budget.

Extensive Roofs are less than 6 inches deep, less than 35 lbs/sq ft when soil is saturated, and covered with a light layer of drought tolerant vegetation. Extensive roof are low maintenance, require minimal roof access and are self sustaining. Plants used successfully on an extensive green roof system are specialized hardy plants, such as sedums, that are wind, frost and drought resistant, can tolerate direct sunlight, and have a shallow root system with good regenerative capabilities. Due to the low maintenance requirements, extensive green roofs are more common and less expensive than intensive roofs.

Intensive Roofs are 6 inches deep or deeper, approximately 50 – 300 lbs/sq ft when soil is saturated, and can support a wide variety of plants. Intensive roofs require substantial maintenance at regular intervals, including irrigation, planting and weeding. Because of this, intensive green roofs must be designed to accommodate limited access for maintenance activities, such as stairs to the roof, walkways and guardrails.

Green Roof Building Factors to Consider:
It is important to consult a structural engineer to review your green roof plans and evaluate the building’s load capacity to support added weight. Some existing buildings cannot be retrofitted green roofs because of weight load, roof slope, and seismic considerations. Other factors to consider when designing a green roof include: drainage, fire hazards and access, safety, wind uplift, visual impacts and maintenance. An irrigation system will be needed for fire suppression and to establish the plantings.

Green Roof Assembly Components:
Green roof systems may be modular, with drainage layers, filter cloth, growing media and plants already prepared in movable, interlocking grids, or, each component of the system may be installed separately. A green roof system consists of two major groups of components; the roofing base assembly and a vegetated roof system. Green roof components can include: root barrier, lower filter fabric, water retention mat, upper filter fabric, growing medium, and sedum.

Building Permit Submittal Requirements:

  • Site map showing location of green roof and property lines
  • Approval of Application for Alternative Materials or  Methods of Construction (AMMR) showing equivalency of fire protection for roof surface
  • Structural calculations including saturated weight of materials
  • Must meet Factory Mutual Global Standards for engineering and membrane testing
  • Green roof specifications (system type) and planting schedule (including plant type & location) 
  • Roof drainage plan and specifications on how drainage will be kept protected from sediment build-up
  • Safety features, such as stairs and guardrails for intensive green roofs
  • Special Inspection Agreement (as determined by plan checker)
  • Owner’s Operation and Maintenance Guide

Required Permits: The permits required depend on roof type, use of green roof, and the structure of the building.

Type of Roof 

 Building Permit  Zoning Requirements
Extensive Required for all green roofs. An extensive green roof, with no added roof access or additional height, does not require Zoning Division staff approval.

Intensive

Required for all green roofs. An intensive green roof requires Zoning Division staff review based on design requirements and safety features, such as walkways and railings. These elements are subject to zoning consideration including height limits, views, solar access, and other potential neighborhood impacts. The designed use should be for limited landscape maintenance only. Any other use may require additional zoning approval.

Resources:

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