Green Roof

Green roofs have many benefits, including heat island reduction, reduced stormwater runoff, and improved water and air quality. Other benefits may include extended roof lifespan and sound attenuation.

Heat Island Effect

Heat island effect is the absorption of heat by hardscapes such as dark, non-reflective pavement and buildings, and its radiation to surroundings.

This is the reason why many urban areas (or areas with a lot of hardscape and development) typically have higher temperatures than rural areas. In fact, traditional roof surfaces (i.e. those with an untreated, non-vegetated dark surface, such as asphalt tile or shingles) can absorb up to 83% of the sun's heat and UV rays causing significant heat buildup. As anyone who has worn a black t-shirt on a sunny day knows, black surfaces can become up to 70°F (40°C) hotter than the most reflective or white surfaces.

If that dark surface is a roof, the heat collected by the roof is transferred to the inside of the building. These temperature fluctuations can increase energy loads and costs, because the HVAC systems must work to keep the building’s temperature constant.

Green roofs reduce heat island effect because they absorb sunlight for photosynthesis, which prevents surrounding surfaces from absorbing and radiating the temperatures. This means that the heating and air conditioning systems do not have to work as hard to heat or cool the building, which saves energy.

Stormwater Protection and Native Vegetation

Additionally, vegetative roofs can improve stormwater quality and reduce runoff quantity by naturally absorbing and filtering rainwater, which prevents it from collecting and transporting pollutants and suspended solids to surface waters. Studies show that green roofs retain 50-60% of stormwater runoff, on average.

A green roof may have additional environmental benefits if it contains natural or adapted vegetation. Native or adapted plants, according to the United States Green Building Council, are “plants indigenous to a locality or cultivars of native plants that are adapted to the local climate and are not considered invasive species or noxious weeds”. Such plants promote biodiversity and provide habitat, while also reducing the amount of water and fertilizer required for healthy growth.

Green Roofs Improve Solar Panel Performance

Two recent studies have shown that green roofs improve solar panel productivity.

One study in Berlin showed an average of six percent increase in yield from solar photovoltaic (PV) panels that were installed on green roofs rather than bitumen roofs. The increase in energy output from these PV panels ranged from 1-10%.

Similarly, Engineering for Change reported that the Bronx Design and Construction Academy in New York City found that green, or living, roofs boost solar panel energy production by 3%.
The studies suggest that the vegetation on the roof lowers the ambient temperature, which improves the solar panels' output. This means that installing solar panels on a green roof will result in a shorter payback period and higher ROI than installing them on a black, bitumen roof.

Also, it is important to mention that not only do green roofs boost solar panels' productivity, but they also decrease the amount of energy that the building needs to operate - which means the solar panels need to do less work to meet the building's electric load. Therefore, green roofs are a great way to boost the impact of an on-site solar PV system.

Disadvantages of Green Roofs

While green roofs have many environmental benefits, they do have a few disadvantages. Typically, green roofs are more expensive to install than conventional roofs and could have higher maintenance costs.

A vegetative roof is typically not a do it yourself project. It requires a professional green roof vendor (and/or installer) with experience and knowledge of green roofs.

Green Roofs and LEED

Using a cool roof product in a LEED certification project could potentially help earn points in various categories, including:

Sustainable Sites (SS) Credit 5.1, Protect or Restore Habitat
SS Credit 5.2, Maximize Open Space (if the project achieved SS Credit 2, Development Density & Community Connectivity)
SS Credit 6.1, Stormwater Design - Quantity Control
SS Credit 6.2, Stormwater Design - Quality Control
SSCredit 7.2, Heat Island Effect – Roof

Water Efficiency (WE) Credit 1, Water Efficient Landscaping

Energy and Atmosphere (EA) Prerequisite 2, Minimum Energy Performance)
EA Credit 1, Optimize Energy Performance)

Cost of Vegetative Roofs

When considering both materials and labor, green roofs can cost $15 to $35 a square foot.

Comparatively, cool roof coatings vary from $0.75 per square foot for reflective, acrylic paints to over $3 per square foot for PVC single-ply membranes. The payback for a green roof may be much longer than the payback for a cool roof.

Also, the maintenance associated with a green roof, as well as the potential headaches associated with any leaks that may occur underneath the surface of the green roof or its membrane, can add cost. In comparison, reflective roof coatings usually pay for themselves in a couple of years.

However, because of their heat absorption properties, green roofs have been shown to reduce energy demand by more than 50% annually in certain types of structures, geography and configurations. Reduced demand and increased efficiency may also lead to smaller cooling systems and lower capital costs. This means that green roofs pay for themselves over time.

The Heat Island Group, an organization that concentrates on the study and development of more reflective surfaces for roadways and buildings, has monitored buildings in Sacramento with lightly colored, more reflective roofs. The group found that these buildings used up to 40% less energy for cooling than buildings with darker roofs. The Florida Solar Energy Center performed a similar study, also showing up to 40% cooling energy savings.

Additionally, green roofs are shown to last 2.5 to 3.5 times longer than a conventional roof, implying more long-term monetary savings.