Cost of Green Roofs: What Is the Payback Period?

Green roofs are roofs with vegetation, which have many known environmental benefits. These include:

• They reduce heat island affect, because the green roof absorbs sunlight (the plants use it for photosynthesis), rather than radiating it to its surroundings.
• They can cut a building’s heating and cooling costs because the plants absorb the sunlight, which reduces temperature fluctuations inside the building and makes it easier for the HVAC systems to air condition the space.
• The green roofs absorb and filter stormwater, which prevents stormwater runoff.
• The vegetation adds biodiversity, and they absorb carbon dioxide from the atmosphere (decreasing climate change).
• Green roofs have been shown to improve solar panel performance because they reduce the ambient temperature of the roof, which increases the panel’s output.

Green Roof: What Is the Payback Period?

A green roof can cost between $15-35 per square foot, depending on the cost of the materials and labor.

The size of your roof is usually about 25% greater than your house square footage because the slope. Flat roofs are an exception - they are about the same square footage as your home.

The median size of a new US home was 2,169 square feet in 2010, according to the National Association of Homebuilders. Therefore, the cost of a green roof for this “median home”, if it had a flat roof, would be about $32,535 ($15 x 2,169) to $75,915 ($35 x 2,169).

Green roofs can decrease annual energy demand by over 50%. Studies from both the Heat Island Group and the Florida Energy Center showed that green roofs cut energy use for cooling by up to 40% (compared to dark roofs).

According to the Department of Energy, heating and cooling accounts for 54% of the average American household's energy bill, and the average total energy bill is $2200. Therefore the cost of heating and cooling is $1,188.

Let's say the roof of a "median home" is replaced with a green roof, and the green roof costs $54,225 (an average of $15 and $35 per square foot). If the annual energy savings are 40% for heating and cooling, then the green roof would save $475.20 per year, which would result in a payback period of 114.1 years.

Of course, the actual energy savings could be higher or lower depending on the climate, the building's energy use, and other factors. However, the payback period is likely to remain pretty high. Truthfully, green roofs are most popular for green building certification projects (because they score a lot of points in rating systems like LEED) or in places with supportive programs, like Chicago. Some companies with higher budgets might add a green roof because it looks appealing and gives the company some extra PR. However, because of their high costs, they are not a popular solution for the average American home.

Green Roofs vs Cool Roofs

A cool roof is much cheaper than a green roof, at only about $0.75 to $3 per square foot, or $1,626 to $6,507 for the median-sized home we discussed. Cool roofs can also reportedly save energy on heating and cooling, so they can pay for themselves in as little as 2-3 years, which is much shorter than a green roof. Cool roofs also require much less maintenance.

However, green roofs last 2.5 - 3.5 times longer than a conventional roof, so it would outlast a cool roof. This improves the financial benefits of the green roof compared to the cool roof, because a cool roof would have to be replaced 2-3 times within the lifetime of the green roof.

Also, there has been some debate about the effectiveness of cool roofs. As I mentioned in a previous article, their effectiveness at reducing energy use for heating and cooling most likely depends on the climate. In cooler areas, the building’s HVAC could actually use more energy because the building is not absorbing as much heat.

A Stanford University study also showed that cool roofs can contribute to global warming, because the reflected sunlight becomes trapped in the atmosphere, which warms the earth.

The advantage of green roofs here is that they absorb the sunlight for photosynthesis, rather than reflecting it back into the atmosphere, which eliminates this possibility. They also have added benefits, such as increased biodiversity, stormwater management and acting as a “carbon sink, which could make up for a potential lack of energy savings.