Purpose

Having a vegetative green roof on a building is more than just a novelty and a landscape feature.  Green roofs are very effective tools that can be used to solve several different problems in a sustainable manor.Green Roof

So what does a green roof do exactly?  Aside from being a novelty for neighboring taller buildings to look down upon a green roof is a multi-function tool that can be quite effective.  First and foremost a green roof can be
used as a stormwater management tool.  Storm water that is typically collected into roof drains and sent away in the storm system can be used by the green roof without overtaxing our sewer systems.  Green roofs are typically classified into two different categories; extensive and intensive.  Extensive green roofs have typically 6” or less of growth media (soil).  Intensive green roofs are roofs with more than 6” of growth media and can be several feet deep supporting a much greater variety of plantings.  The green roof on Chicago City Hall would be an example of an intensive green roof.  In fact the entire Millennium Park in downtown Chicago is a green roof over a formerly open train yard.  It is considered one of the largest green roofs in the world.

Energy Savings

In addition to providing a use for stormwater, green roofs also serve other functions.  For one green roofs help keep buildings cooler in the summer and help to reduce energy use from cooling.  Exactly how much energy green roofs save is an evolving science, but from research that was conducted in Michigan we know that on the hottest days of summer there is a 40 degree temperature differential between the surface of the growth media and roof membrane.  The less heat that can enter a building reduces the amount of cooling required to combat its effects.

Lowered Replacement Reserves

Green roofs also help to extend the life of a roof membrane.  A typical rubber or PVC membrane roof has a life expectancy of approximately 20 years.  The two biggest factors that cause roof membrane deterioration are excessive heat and UV.  We’ve already seen how green roofs help to reduce excess heat and the growth media on top of the membrane effectively blocks the UV rays from taking their toll on the membrane.  A roof membrane under a green roof could last 50 years or more thus reducing the amount of money a building owner needs to set aside for its replacement.

Urban Habitat

One of the last and often most overlooked benefits that a green roof provides is habitat for urban wildlife.  It is amazing how many creatures a green roof can support including birds and bees.  As we continue to develop urban areas more and more open space is lost.  A green roof helps to provide some balance to our rapid developments.