Green Roofs

By Kenton Shepard

Source

New York as it would look with green roofs.

“Green” or “living” roofs are vegetative covers installed on roof decks. Designs vary with purpose and are very site-specific. Visual inspection will be limited, since the main concerns are structural and moisture intrusion problems and components of both may be hidden from view. Always recommend a specialist inspection to transfer the considerable liability offered by potential structural and moisture problems.

Roof decks may be nearly flat or may have some slope. Flat decks may be more vulnerable to moisture intrusion since they may drain more slowly. Sloped decks will drain more quickly but may need to be designed with battens or raised-grid features to prevent the soil or growth medium from sliding off the roof.

EXTENSIVE and INTENSIVE SYSTEMS

The two main types of green roofs are extensive and intensive.

Extensive roofs are lightweight systems not designed for human interaction (walking) and may be installed on steeper roofs than intensive systems. They may hold plants in containers. Soil depths start at 2-3 inches and plants with low water requirements are used.

Source: Kenton Shepard

NACHI TV's Valerie Green films InterNACHI founder Nick Gromicko (right) interviewing Jim Blackledge, EPA building project manager, on the deck of the 9th floor, EPA Region 8 building, Denver, CO.  Photovoltaic array is visible at the left and the roof has vegetation in growing medium trays.

Intensive roofs are much heavier, with the accompanying structural/waterproofing requirements and may have walkways, water features and other features designed for human interaction. Soil depths start at 6-8 inches.

MOISTURE INTRUSION

With roofs which have a growing medium or soil installed directly on the roof, moisture intrusion, root penetration and the ability of the roof to support the moisture-saturated weight are the primary concerns. The system cross section might look like this (starting from the bottom…

• waterproof membrane
• root barrier
•  insulation layer (optional)
• drainage layer such as sand or gravel
• filter fabric for fine soils
• growing medium or soil
• plants

The perimeter should have a band of pebbles or gravel to encourage drainage, limit fire danger and provide access for maintenance.

WEIGHT CONCERNS

As can be seen from the chart below, these roofs may be very heavy. Calculations to determine structural design must take into account the weight of roof materials plus the weight of the water capable of being held by roofing materials or saturated weight.

Extensive Greenroof	150 - 375 lb/yd2
Intensive Greenroof	500 - 1250 lb/yd2

GREEN ROOF STANDARDS

Existing Standards:

• E2396-05 Standard Test Method for Saturated Water Permeability of Granular Drainage Media [Falling-Head Method] for Green Roof Systems
      See also WK12185 proposed revision
  
• E2397-05 Standard Practice for Determination of Dead Loads and Live Loads associated with Green Roof Systems
  
• E2398-05 Standard Test Method for Water Capture and Media Retention of Geocomposite Drain Layers for Green Roof Systems
  
• E2399-05 Standard Test Method for Maximum Media Density for Dead Load Analysis of Green Roof Systems
      See also WK12178 proposed revision
  
• E2400-06 Standard Guide for Selection, Installation, and Maintenance of Plants for

...and has proposed the following Standards:

• WK575 Practice for Assessment of Green Roofs
• WK14283 Green Roof Systems

Sources for More Information

• Organic Greenroof Architecture: Design Considerations and System Components
• greenroofs.com- Green Roofs 101 good explanations of the basics

ASTM International offers the following Standards concerning green roofs...