The lowdown on green roofs

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September 10, 2012 // UPDATED 5:48 pm - December 27, 2012
By: Sarah McKenzie
Sarah McKenzie

Green roofs have been sprouting on Minneapolis rooftops for several years now, but the rise of urban farming has made them even more popular these days. We checked in with Kim Havey, a principal at Sustology, a Minneapolis-based sustainable consulting firm, to learn about the latest trends in green roof design. Here are highlights from a recent interview: 

What are the major benefits for a homeowner who decides to pursue a green roof?

Green roofs are beautiful. The greatest benefit of having a green roof is that it looks gorgeous, adding a valuable amenity to what would normally be considered blight. Unlike rock ballasted and tar roofs, green roofs make people feel grounded and connected to the natural environment — a very hardwired human emotion.  

Anyone with a green roof knows that it will quickly become the conversation piece and favorite part of any home or building. 

Green roofs provide recreational space. Intensive green roofs offer the opportunity for home owners to increase their outdoor recreation space.  Just like a large outdoor balcony, outdoor green space provides a great place for kids’ activities and social gatherings.

They reduce storm water run-off. Green roofs can be used to manage storm water run-off from the roof through retention, detention, filtration and evapotranspiration. -In Minneapolis and St. Paul, homeowners can receive a credit off their water bill for retaining storm water on site, commonly referred to as the storm water utility fee credit.

The roofs improve air quality and reduce pollution. Green roofs add a significant amount of green space to dense urbanized areas. The additional green space removes CO2 from the air and traps fine particulate matter thereby improving the air quality of your property’s microclimate.

Green roofs extend the life of your roof. A green roof provides a barrier that protects the roof membrane from temperature extremes and from ultraviolet radiation.  Green roofs can double the life expectancy of a typical roof from 20 to 40 or more years. … 

They can increase the value of your property. Green roofs are an attractive amenity and are beautiful outdoor spaces, which increases the value of your home or building. Along with the facts that a green roof reduces electric costs and extends the roof’s life expectancy a green roof adds value to a home or business. 

Green roof promote fire retardation. 

Green roofs also reduce noise levels. Green roofs have excellent noise attenuation, especially for low frequency sounds. An extensive green roof can reduce sound from outside by 40 decibels, while an intensive one can reduce sound by 46 to 50 decibels (Peck et al. 1999).

What makes a home suitable for a green roof? 

Although it is possible to install green roofs on a variety of roof types and pitches, an ideally suited home or business for a green roof would have a flat or low pitched roof with enough weight capacity to add an additional 15 to 20 pounds per square foot of weight.  This is the additional weight that is added when the green roof is fully saturated with water. 

Deck, patio or entrance overhangs are also great areas for a green roof.  

Some of the best roofs are on commercial buildings because they offer a large flat space that is generally underutilized. 

Any new trends in green roofs locally?

Green roofs being used more than ever for urban agriculture. Numerous commercial buildings especially in high density urban communities are utilizing roof space for urban gardening. The desire for locally sourced produce has combined with the green roof concept to create gardens in the sky. A local example would be the Bachelor Farmer restaurant rooftop garden.

Another evolving trend related to green roofs is the use of green walls. Green walls can be done inside or outside and can transform an empty wall space or fence into a beautiful and energy saving amenity.  

What kinds of plants work well on a green roof?

Generally there are three types of plants that are utilized on green roofs: Succulents, Herbaceous Perennials and Grasses. The soil depth has a lot to do with the type of plants selected.  The deeper the soil the larger the taller the plants. With soil depths of 4 to 5 inches numerous plants will grow to approximately 18 inches in height. The most typical green roof plant is a succulent known as sedum. Sedums come in a variety of shapes, sizes and colors to suit anyone’s color palette. 

How much does a green roof on a home typically cost? 

There is a wide range of costs depending on the soil depths, type of plants, type of roof and size. On average, larger green roofs cost less per square foot. Typical costs range from $10-$30/square foot. 

How long do they last and what are the maintenance requirements?

Green roofs are perennial so they last a lifetime. A typical green roof will last 40–50 years assuming that the roof was constructed as a green roof rather than trays of plants placed on a roof that was not originally intended for a green roof. Green roofs do require regular watering in dry periods until they become established.Ideally, plants are chosen to handle dry and wet cycles and require little maintenance. It is a good idea, as with all gardens, to put a green roof “to rest” in the fall and spruce it up in the spring. Plants do die periodically so it’s beneficial to add new plants as needed, just as you would in your yard.  

Sustology designs and installs green roofs for all types of property owners and offers maintenance agreements with all the green roofs it installs. With a little annual maintenance, owners of green roof can look forward to years of beautiful plants.