A Green Roof Primer: Goat Optional

(By Dana Weeder, AIA, LEED AP)

What’s a Green Roof?

Goats on a Green Roof (photo courtesy Jypsee, Stone Lake Productions)

A green roof is an engineered vegetated roof system consisting of a high quality water proofing and root repellant system, a drainage system, filter cloth and a lightweight growing medium to foster plant growth.  Or, more succinctly, a contained ‘green’ space on top of a human-made structure.  This ‘green’ space can be below, at or above grade, but in all cases the plants are not planted in the ‘ground’.


Green Roof Diagram

Green’s green, what’s this whole ‘type’ thing?

Green roofs can be categorized as extensive, intensive or semi-intensive depending on the depth of the planting medium and the type of plants.

  • An extensive green roof is generally thinner, lighter, less expensive and is considered the ideal approach for an ecological roof cover with no or limited human access.  Typical plants are alpine grown succulents, flowering herbs, bulbs, grasses and mosses that are heat, drought, wind and frost tolerant.  An extensive green roof can be installed as a modular system, with drainage layers, filter cloth, growing media and plants already prepared in movable, interlocking grids, or it can arrive as a fully vegetated,  lightweight thin matt and rolled out like sod.  Extensive green roofs are designed to be virtually self-sustaining and should require only a minimum of maintenance.

Extensive Green Roof

  • An intensive green roof is generally installed on a relatively flat roof that has a much thicker growing medium.  This allows for a greater variety of plants such as flowering shrubs, vegetable gardens and even trees which allows for a more ‘park-like’ or traditional roof garden setting look.  Intensive are generally more labor-intensive, requiring irrigation, feeding and other maintenance.
  • A semi-intensive (or semi-extensive) green roof, as the name implies, is often designed with features of both.
  • Intensive Green Roof

Why?

The benefits of a green roof are many…

  • Green roofs are a form of low impact development that helps mitigate the negative effects of a building’s footprint by recreating lost green space and reintroducing flora and fauna into an urban environment.
  • A green roof will filter pollutants and carbon dioxide out of the air along with pollutants and heavy metals out of the rainwater.
  • The soil and plants on green roofs help to insulate a building for sound; the soil helps to block lower frequencies and the plants block higher frequencies.
  • Green roofs plantings usually consists of various types of sedum, but they also allow for the growth of fruits, vegetables and flowers.
  • Reduces glare reflecting onto adjacent buildings.
  • Creates new real estate by taking advantage of normally unutilized space.
  • A concentration of green roofs in an urban area can lower the urban heat island effect and even reduce the city’s average temperatures during the summer.

And the biggie:

  • Green roofs are used as storm water management and act like sponges by absorbing rainwater.  Any overflow is then slowly released thereby decreasing stress on sewer systems at peak flow periods.

Hydrograph

Economic benefits include:

  • Green roofs protect the roof membrane from sunlight and temperature fluctuations, which breaks down the roofing material.  A longer lifespan means reduced roof maintenance costs and reduced material in the landfills.
  • Since green roofs are cooler in the summer and warmer in the winter, both heating and cooling loads/costs are reduced.  Additionally, HVAC equipment sizes can be reduced saving upfront building costs.

Why not?

  • Green roofs have more demanding structural standards.  Some existing buildings cannot be retrofitted with a green roof because of the weight load of the soil and vegetation.
  • Depending on what kind of roof it is, the maintenance costs could be higher.  Intensive green roofs can generally be considered as elevated parks and therefore they require similar maintenance.  An extensive green roof requires less maintenance and generally only adds about $1/sq yd per year more than the cost of maintaining a standard roof.
  • Green roofs also place higher demands on the waterproofing system of the structure due to both the increased amount of water retained on the roof and the possibility of roots penetrating the waterproof membrane.

OK, so why is my Green Roof brown?

In a word, maintenance.

Given that extensive green roofs do not require as much maintenance or irrigation, they tend to ‘grass over’ after a few years and ‘brown-out’ during dry periods.  A rule of thumb states, ‘…one year sedums, two years sedums some grass, three years grass some sedums, four years grass…’  Aesthetics aside, a ‘brown’ green roof is still effective.

Brown Green Roof

So how much ‘green’ does my brown roof cost?

The cost depends on what kind of roof it is, the structure of the building, and what plants can grow on the material that is on top of the roof, but a properly designed and installed green roof system can cost 5 to 10 dollars per square foot for an extensive roof and up to 45 dollars per square foot for an intensive roof.

Additional costs can also be attributed to maintenance.  Extensive green roofs have low maintenance requirements but they are generally not maintenance free.  Maintenance of green roofs often includes fertilization to increase flowering and succulent plant cover.  If aesthetics is not an issue, fertilization and maintenance is generally not needed.

So then, how much ‘green’ does my brown roof save?

Community cost savings opportunities include decreased need to expand or rebuild related infrastructure.  In areas with combined sewer-storm water systems, heavy storms can overload the wastewater system and cause it to flood, dumping raw sewage into the local waterways.  In some cases, this can help reduce the size of storm water pipes, and the amount of storm water that needs to be treated by municipal water treatment.  In addition, green roof installation can reduce the number of drainage outlets which can significantly offset the initial cost of a green roof.

green roof goats are all the rage

Do I need a goat?

No.

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Comments
3 Responses to “A Green Roof Primer: Goat Optional”
  1. Gavin says:

    Wow!))))) That’s amazing. What green roof growing media are you using? Rooflite – http://www.skyland.us? CitySoil – http://www.citysoil.ca?

  2. Lisa says:

    Can you please tell me what articles you viewed that discussed the storm water runoff delay? I’m doing my master’s thesis on green roofs and would like to cite the article.
    thanks

  3. I delight in, result in I found just what I was having a look for. You’ve ended my four day long hunt! God Bless you man. Have a nice day. Bye

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