Are Green Roofs Worth The Extra Cost?

9/29/2014

I admit that my experience is limited. We designed just one green roof as a demonstration project for a storm water authority, circa 2004. The utility especially likes the decrease in rate of run off. In fact we have two spouts side by side to demonstrate the difference between the vegetated roof and an equal area of typical roof. The difference is dramatic.
Lately I've been wondering, "Is it just me or has the din about green roofs died down?" The touted benefits list is fairly long.

Benefits

The roofing lasts longer
Provides extra insulation
Moderates the temperature differential between inside and outside
Slows time of concentration of storm water runoff
Absorbs CO2
Eliminates the heat island effect of the building
Provides habitat

During the course of that project, I learned that the benefits if a green roof isn't the only thing to consider.

Liabilities

Requires materials and workmanship that are far above the norm, between two and three times more costly
Requires more maintenance of the ’green’ part
Roof leaks are hard to find and fix, which tears up the ’green’ part too. (So far I don't have any firsthand experience, thankfully.)
Although roofs protected by vegetation last much longer, they have to be replaced eventually. That installation will be expensive, too.
You can't inspect the roofing for signs of deterioration, so an alarm system is common to give early warning of water being where it shouldn't be.
Requires a stronger roof structure
Design and specs need to be perfect to defend against litigation because if there is a problem it will be larger than the normal roof leak.

Add your opinion...
Is Green Roof Use Growing or Not?

Personally I am not impressed by some of the benefits.

Questionable Benefits

Extra insulation - energy consumption in buildings over 10,000 SF is not driven by envelope.
Moderates the temperature differential between inside and outside - see extra insulation comment
Slows time of concentration of storm water - this works, however this isn't an energy issue and there are cheaper and more maintainable ways to achieve this benefit.
Absorbs CO2 - The amount of CO2 sequestered is likely to be measured in hundreds of pounds per year. Compare that to the 10,000 pounds that each auto produces on average per year. This isn't helping much and may barely offset the CO 2 generated in making the stronger roof structure. (Here's a handy resource.)
Eliminates the heat island effect - heat islands are not ’natural’ so they must be bad! Unless the structure is in an urban location the benefit is almost immeasurably small. They do seem to keep mild tornadoes away from cities.
Adds habitat - this is another way to say ’Attracts insects and vermin’.

My vote is that green roofs are not worth it; and my personal opinion is that they will go the way of Pet Rocks. But read up on Wikipedia and make up your own mind.