Cool Metal Roofs vs. Green Roofs

Published October 5, 2015 by Whirlwind Team

If you're looking for a green roofing option, you might be able to take that term to the literal level. Twenty-first century architects, city and urban developers, business owners and homeowners are starting to wake up to the benefits of true, green roofs, which incorporate an infrastructure that allows vegetation to be grown on the rooftop.

In addition to providing a more sustainable and energy-efficient building model, green roofs are an excellent way to increase urban agriculture, allowing more space for densely packed cities to grow their own food.

So, how do these green roofs compare with their cool metal roofing counterparts?

Cool Roof or Green Roof - Is One Better than the Other?

When all is said and done, they are both attractive options if sustainability is important to you. Which one makes the most sense for your metal building product depends more on your climate, resources and willingness to maintain an overhead landscape.

While the terms may seem interchangeable, both types of roofs are considered "green" in the eco-friendly building movement, cool metal roofs and green roofs are not exactly the same.

Differences between a Cool Metal Roof and a Green Roof

Cool Metal Roofs. Cool metal roofs are specifically designed to reflect the sun's UV rays - responsible for passive solar heat gain - back into the atmosphere. Typically, they are comprised of specialized coatings that simultaneously protect the metal roofing materials from corrosion, while increasing the roof's reflectivity and emissivity.

Separate studies have been conducted and outcomes are consistent: on average, buildings with cool metal roofs spend 40% less on cooling costs than their shingled counterparts. Cool roofs also mitigate the collective urban heat island (UHI) effect, common in areas with heavy concentrations of asphalt, concrete and shingled roofs.

A cool metal roof works well in any climate. Even colder climates can benefit by using darker colored roofing options to enhance solar heat gain in the winter months. Any level of snow load or quantity of rainfall can be accommodated for in their design.

Green RoofsGreen roofs are constructed with a specific infrastructure that accommodates living plants on all or significant portions of its surface. This requires a strong foundation, frame and roof sub-system as the weight of a green roof is significant via the combined weights of earth, plant materials and water.

Green roofs provide cooling benefits in several ways. First, the engineered soil and vegetation prevent the roofing material from direct exposure to sunlight. The processes of transpiration (via plants) and evaporation (via sunlight) further acts a cooling agent. One study conducted in New York City demonstrated the temperature on green rooftops during peak summer temperatures averaged 60-degrees cooler than nearby asphalt rooftops. As a result, green roofs are also adept at reducing the UHI effect.

In addition to reducing cooling costs, green rooftops also prevent significant storm water runoff and minimizes the amount of pollutants that make their way into storm drains, particularly important in city and urbanscapes where there is more asphalt and concrete with less square footage of permeable landscape.

A green roof makes the most sense in climates with moderate winters and warm summers, and where a lack of permeable ground surface combined with a proliferation of concrete/asphalt, makes it difficult to create green spaces.

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Both roofing options are considered sustainable and, if properly designed, will earn you points in green building certification programs.

Pros and Cons of Cool Metal Roofs

Pros:

  • Easier to design and install
  • Require less maintenance over the course of their lifetime.
  • A single installation of high-quality cool metal roofing products can yield a low-maintenance roof for 50-years or more.
  • Dollar for dollar they are a more affordable option in every way.

Cons:

  • They don't provide the lush, green look of a green roof.
  • Do not yield the same reduction of storm water runoff.
  • Lack the ability to improve air quality.
  • Are not typically designed to accommodate rooftop green spaces that can be enjoyed by building occupants.

Pros and Cons of Green Roofs

Pros:

  • Beautiful to look at and can incorporate peaceful outdoor environments for building occupants.
  • Facilitate storm water runoff, absorbing storm water and filtering the water that does make its way into gutters and downspouts.
  • Natural air cleaners, by capturing airborne pollutants and atmospheric deposition as well as filtering noxious toxins from the environment.

Cons:

  • More complicated and costly to engineer and install.
  • Plants and landscape need water during the dry months.
  • Require significantly more maintenance throughout it's lifetime.
  • They are more expensive from the beginning to the end of their lifetime than a metal roofing system.

Deciding to Go Cool or Green

Your climate may decide for you. If you live in a region that is excessively hot/dry during the warm months, or experiences extended freezing temperatures during the winter months, a green roof probably doesn't make the most sense.

The good news is you can incorporate elements of a green roof on top of a cool metal roof by using reduced slope and green living roof tray systems. Keep in mind that metal building materials are vulnerable to water so you will want to work with an experienced metal building manufacturer and/or contractor to make this option work.

A review of your project's application, budget and desire to perform long-term maintenance will determine which roofing system makes the most sense for you.

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