Green Roof Facts

What is a green roof? 

You can time travel to the future and see how awesome a green roof is or you can read this hopefully not to dry essay on the subject. Basically, green roofs are living roofs with growing media (basically the "soil plant system") on the outside of the roof. The number of layers and the type of layers vary from system to system and the type of greenroof. All green roofs include a single to multiply waterproofing membrane(s), drainage system, growing media and the plants, covering the entire roof deck surface. Generally, there are three main types of green roofs – extensive, semi-intensive or intensive - Even though they can be a combination.

How much does a green roof cost?

Extensive – For 3” of growing media and sedums. Most commonly the range is between $14 - $25/sq. ft., (including roofing membranes). So for example a 100 squares would cost around $150,000 on average with all the works.Intensive – $25 - $40 and up.... it would cost for a 100 squares around $250,000 on average with all the works.But every project is unique. Economy of scale is also very relevant. The Ford Motor Company River Rouge Plant green roof in Dearborn, MI, for example, came in around $4/sq. ft., but we’re also talking about an extensive green roof that’s almost 500,000 sq. ft in size.

Which is the most expensive component of a green roof?

Just like any roof, the waterproofing membrane (or membranes) is the single highest cost item of a new roof.What’s the difference between an extensive and intensive green roof?The roof space and function determines the design – is it an ecological cover or is it also intended for human recreation, vegetable gardening, etc. Some factors for green roofs include: the roof loading capacity determined by a structural engineer; the slope of the roof and perhaps the budget. 

Broadly speaking, there are three main types of green roofs:

Extensive – Also known as eco-roofs – They have thinner and less numbers of layers, so therefore they are lighter, less expensive and very low maintenance. Extensive green roofs are built when the primary desire is for an ecological roof cover with limited human access. The minimum soil substrate starts at about 2 1/2” to 5" deep Plants include sedums and other succulents, flowering herbs, and certain grasses and mosses. Fully water saturated weights range from a low of about 10 – 50 lbs/sq. ft. 

 Semi-Intensive – A combination of an extensive green roof with higher plant depth with deeper soil, ranging from 8-12”.

Intensive – Also known as high-profile – They look like traditional roof gardens, green roofs can have many different types of plants since the growing media depth is increased. The growing media starts from about 12” - 30 cm - and can rang up to 15 feet or more, depending on the loading capacity of the roof and plants desired. Architectural accents such as waterfalls, ponds, gazebos, etc. are possible and these green roofs provide recreation spaces and encourage interaction between people and nature. Maintenance requirements are also more intensive, and of course, these roofs are mostly flat roofs or have a low slope.

What’s the difference between an intensive green roof and a roof garden?

A roof garden usually consists of plantings in pots or containers of different sizes placed on top of a roof. In an intensive green roof system, everything is built up on top of the deck, so the flow of drainage part of one whole system.The vegetation is planted directly into the soil, not in planters or containers.

Explain inorganic and organic growth media?

Inorganic materials are high porosity natural mineral element like expanded slate, shale, extruded clay, rock wool, lava or pumice, etc., which provides aeration and also sometimes provides water retention, and prevents total compaction of organic matter through settling over time, and acts as a good drainage medium. inorganic medium maintains void or air space necessary for the plant roots to breathe and for the excess water to drain properly.Organic media growth contains well-rotted humus material such as , hen manure, guano, mushroom compost, etc. It also contains organic fibrous material and a small amount of clay particles. Organic soil of media growth holds and slowly releases essential elements necessary for the health of the soil community.

 Is an extensive green roof really maintenance-free?

No! Every roof needs a periodic check. It is recommended to do a maintenance review every six months or so. Plants, no matter how low growing and drought tolerant a green roof is , are still living, breathing "beings" and that need to be be checked on and taken care of..

Do I have to water my green roof?

Extensive – During the first year of just like any landscape paying careful attention to watering is very important. Drip irrigation is best for large projects plus it is inexpensive and delivers the right amount of water to the base of the plants. If you have installed drought tolerant plants that are naturally in your area, the plants would certainly benefit from occasional watering during extreme periods of duress – even though it would be mostly maintenance free. 

Intensive – Yes, since an intensive green roof accommodates many different plants, shrubs and trees, their watering requirements are higher than succulents and herbs. An intensive green roof like any other garden or landscape on the ground. Also keep in mind, that high winds can be very drying. Usually large intensive green roofs should have an irrigation system installed along with the drainage system

How can we make the green roof's irrigation or supplemental water even more environmentally 

friendly ?

You can install a traditional active irrigation system or a solar powered system. Add to this with a recycled rainwater collection system, gathered in cisterns at roof deck or at ground level, and this gives you an ideal self-sustainable supplemental water system. 

Do we have to fertilize an extensive green roof?

There is some disagreement about whether it is recommended or not to fertilize an extensive green roof. However, companies and organization that have been making green roofs for a long time do recommend it. Especially, in the initial stages a slow release fertilizer twice during the first year of establishment in the early spring and fall, and then yearly thereafter for the next 4 years or so. After that, the natural cycle should take over and enough organic material should have composted back into the soil substrate to provide sufficient nutrients to the plants. 

 An important word of caution--Don’t use soluble N fertilizer as it can get into the runoff.

What about adding a root-resistant protective layer on an extensive green roof with nothing but sedums?

Seeds come in from many sources – the wind, from birds, bees, etc. There have been cases seen where oak and maple seedlings growing in ½” of rotted compost on a pitched roof. Its always safer to install a root barrier, it doesn't matter how low growing your extensive green roof plants are. It needs to be a dense inorganic material that inhibits root penetration, like polyethylene. This protective layer can be a heavy duty pond liner (EPDM, etc.) or other non-organic element that contains an injected root repellent, such as a copper element. Also, many North American drainage products bypass a separate root barrier layer and now incorporate a root-repellent ingredient in their synthetic filter fabric, placed on top of the drainage layer.

Does the soil really have to be “engineered?” 

 How about top soil on a green roof?”

Regular garden soil is heavy, can contain pathogens, undesirable insects, and even weeds! Green roof “soils” need to be lightweight to conform to roof loading weight restrictions, drain properly and at the same time retain a certain amount of rain water. Some designers will approve a certain amount of topsoil to reduce costs, but great care needs to be taken with this option. In this case, a typical mix is to use 1/3 clean topsoil, 1/3 compost, 1/3 perlite or other inorganic material.

What about flammability issues?

Always have access to an adequate water supply just in case of fire, which can occur to any type of building. So, water supply should always be nearby. It is important to choose plants that are inherently non-flammable. In dry climates ornamental grasses and certain mosses could become kindling material in an extremely dry situation. Succulent plants and a high inorganic soil media can actually act as a fire barrier. But make sure that your roof has a 24” perimeter of vegetation free zone around the edges of the roof – both for a fire break and for sure-footed access of firefighters to the roof. This can be crushed gravel, pebbles or pavers.

What is the best way to do roof maintenance in a green roof?

It depends on the type and size of the green roof. If its just a home owner, they can check a flat or low pitch green roof by themselves. A roofing professional should be hired if it is too large or intensive – you can include a maintenance agreement of at least twice per year with the green roof contractor/green roof company/ or a landscaping company. An arrangement with the roof maintenance staff of your building if you already have a maintenance staff. 

How about planting just sod or grass on a roof? 

 Monoculture of plant species is not healthy in a roof scape for a land or variety of reasons: It would be open to plant disease, or an insect infestation could wipe it out. A monoculture is simply not ecological in nature; meaning not natural – a plant community should be ecologically diverse – with many types of vegetation to be vibrant. Grass or sod needs to be watered, fertilized, and cut regularly, so they would incur greater costs and would be considered high maintenance.

Are green roofs necessarily green in color?

Many green roofs appear red, orange, yellow and all colors in between at different times of the year because the fleshy leaves of various succulents change colors throughout the year. Also, green roof plants are not all evergreen, nor should they be. The beauty and anticipation of the change of seasons add to the color palette. And of course, flowers vary, too, in colors from whites to yellow, pinks, deep reds and purples and blues. 

Where can I find information about different green roof projects?

Search google for "The Green roof & Green wall Projects Database", this database is international, growing daily, and a free community resource.

How many LEED™ points can a green roof qualify for?

Green roofs can contribute to at least 6 LEED™ points (more are possible) up to a possible 15 or 16. New information shows that green roofs can contribute up to 14 credits with LEED-NC, Version 2.2.