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Green Building Resources for Existing Affordable Housing



Building Shell : Roof


Roof Replacement

When replacing your roof, it is important to consider:

If you replaced the roof on your building, did you install light-colored or green roofs? Let us know.

Light-Colored Roofs

Considerations:


Selecting a Light-Colored Roof:

  • Be sure to select an ENERGY STAR product. To qualify for the ENERGY STAR label, roofing products must meet the following criteria:
    • For low-slope roofs (surfaces with a slope of 2:12 inches or less), the initial solar reflectance must be at least 0.65 (maintain a 0.50 after 3 years)
    • For high-slope roofs (surfaces with a slope of 2:12 or greater), the initial solar reflectance must be at least 0.25 (maintain a 0.15 after 3 years)

  • Appropriate cool roof technologies include:
    • Reflective coatings or membranes for a flat roof
    • Metal, tiles or shingles for a sloped roof

  • Review the solar reflectance and emittance values of common roof systems.

Green Roofs

PrincipalGreenRoofTechComponents.gif
Source: National Research Council
A green roof is a roof that is partially or completely covered by vegetation. It includes a growing medium that is installed over a waterproof membrane.

Types and Considerations:

Before determining which type of green roof you would like to implement:
  • Think about your programming goals. Perhaps you are in a dense urban area and your only open space is your green roof. If this is the case, an active/intensive roof might be a better choice.

  • Do a roof assessment to determine:
    • slope
    • structural loading capacity
    • existing materials of the roof
    • nature of any drainage systems, waterproofing, and electrical and water supply in place
    • who would have access to it, who will do maintenance, and what kind of sun and wind exposure the roof gets

  • Read more about green roof FAQs and considerations.
    GreenRoof.jpg
    St. Polycarps, Somerville, MA

Active/Intensive:
These higher-profile green roofs look like roof gardens due to their wider variety of plant material as well as focus on access/enjoyment space for people. Growing media starts at about 8” and can be as deep as 15" depending on load capacity and plant requirements. Fully saturated weights range from about 80 to 120 lbs/sf.

Passive/Extensive:
These green roofs use thinner and fewer number of layers which makes them lighter, less expensive and lower maintenance. They provide limited access for people. They include a minimum growing media of about 2-1/2” to 6” at most and use low-growing, horizontally-spreading root ground covers. This type of green roof can be installed on slopes up to 30-degrees. Fully saturated weights range from 10 to 50 lbs/sf.

Comparison of Common Features of Extensive and Intensive Roofs

Extensive
vs.
Intensive
(Low-Profile/Ecoroofs)

(High-Profile/Roof Gardens)
Low growth media: 2-6"

6-15" and deeper
Lightweight: 13-50 lbs/sf

Heavier weights: 50+ lbs/sf
Low growing plants: 1-24" H

Trees, shrubs and more
Less variety of plants: Alpine types, succulents, herbs, some grasses and mosses

Huge variety of plant selection/architectural features depending on loads, designs & budget
Usually non-accessible and non-recreational

Designed for human recreation: gardening, socializing, etc.
Slopes up to 30 degrees & higher

Relatively flat
Less expensive: $5-25/sf

More expensive: $25-40+/sf
Low water requirements

Irrigation usually necessary
Low maintenance

Higher maintenance
Source: U.S. Environmental Protection Agency

Benefits:

  • Decreases roof temperature and thus decreases cooling costs.

  • Minimizes stormwater runoff.

  • Filters pollution.

  • Protects underlying roof material from UV and temperature fluctuations.

  • Provides habitat for small animals, absorbs CO2, attractive, reduces noise, can be alternative to open space for residents, publicity for development.

  • Read more about the benefits of green roofs.

Challenges:

  • Expensive.

  • Complex system to maintain.

  • Increased load considerations.

Cost-Benefit Analysis of Green Roofs

Benefits/Costs
Energy, Hydrology, and UHI Benefits
Other Benefits
Private Benefits
  • Reduced energy use
  • Extended service life
  • Noise reduction
  • Aesthetic value
  • Food production
Public Benefits
  • Reduced temperature
  • Reduced stormwater
  • Reduced installation costs (from widespread technology use)
  • Reduced air pollutants
  • Reduced greenhouse gases
  • Human health benefits
Private Costs
  • Installation
  • Architecture/Engineering
  • Maintenance
N/A
Public Costs
  • Program administration
N/A
Source: U.S. Environmental Protection Agency

Share your story about incorporating a green roof on your building.

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