what is a greenroof? 
Basically, greenroofs are vegetated roof covers, with growing media and plants taking the place of bare membrane, gravel ballast, shingles or tiles. The number of layers and the layer placement vary from system to system and greenroof type, but at the very least all greenroofs include a single to multi-ply waterproofing layer, drainage, growing media and the plants, covering the entire roof deck surface. There are two main types of greenroofs – extensive or intensive - although a greenroof is often designed with features of both and then are referred to as either semi-extensive or semi-intensive.
What’s the difference between an extensive and intensive greenroof?
Commonly, the roof function or objective of the roof space determines the design – is it just an ecological cover or is it intended for human recreation, vegetable gardening, etc.? The limiting factors for greenroofs include: the roof loading capacity or maximum dead and live weight loads, determined by a structural engineer; the slope of the roof and perhaps the client’s budget.
Extensive – Also referred to as eco-roofs, and low-profile – They have thinner and less numbers of layers, so therefore they are lighter, less expensive and very low maintenance. Extensive greenroofs are built when the primary desire is for an ecological roof cover with limited human access. The minimum growing media or soil substrate starts at about 2 1/2” to 6” at most (although vegetative mats can actually have even less than 1" of growth media); the engineered soil media contains 70 – 80% inorganic or mineral material (or higher) to 20 – 30% organic (or less). Low growing, horizontally spreading root ground covers with general maximum plant heights of 16 – 24” are ideal. Alpine-type plants are successful because they are high drought, wind, frost, and heat tolerant, all necessary attributes for greenroofs. Plants include sedums and other succulents, flowering herbs, and certain grasses and mosses. Fully saturated weights range from a low of about 10 – 50 lbs/sq. ft. Compare that to common river rock ballast which weighs about 12 lbs/sq. ft. Extensive greenroofs can be constructed on slopes up to 30°, and steeper ones can be installed with raised grids or laths to hold plants and soil media in place.
Intensive – Also referred to as high-profile – They look like traditional roof gardens because a much wider variety of plant material can be included since growing media depths are increased. The growing media starts from about 8 - 12” and can range up to 15 feet or more, depending on the loading capacity of the roof and the architectural and plant features that the building owner desires. The engineered soil media usually contains about 45 - 50% organic material to 50 - 55% mineral, and fully saturated weights range from about 80 -120 lbs/sq. ft. and up. Architectural accents such as waterfalls, ponds, gazebos, etc. are possible and these greenroofs provide recreation spaces and encourage interaction between people and nature. Maintenance requirements are also more intensive, and of course, these roofs are relatively flat.
What’s the difference between an intensive greenroof and a roof garden?
A roof garden usually consists of containerized plantings of various sizes placed on top of a roof. In an intensive greenroof system, all the various layers are applied on top of the entire roof deck surface, allowing unimpeded drainage and a more even weight distribution over the whole roof. The vegetation is planted directly into the soil, not in planters or containers.
Explain inorganic and organic growth media.
Inorganic material refers to a high porosity natural mineral element such as expanded slate, shale, extruded clay, rock wool, lava or pumice, etc., which provides aeration (and could also provide water retention capabilities), and prevents total compaction of organic matter through settling over time, and acts as a good drainage medium. The inorganic medium maintains void or air space necessary for the plant roots to breathe and for the excess water to drain properly.
Organic means well-rotted humus material (hen manure, guano, mushroom compost, etc.) augmented with organic fibrous material and a small amount of clay particles. This mixture holds and slowly releases essential trace elements necessary for the health of the soil community.
Is an extensive greenroof really maintenance-free?
No! Every roof needs to be checked periodically, and extensive greenroofs are no different. It is recommended to do a semi-annual maintenance review, at which time you can look for invasive weeds, disease, stray tree seedlings, etc. Plants, no matter how low growing and drought tolerant, are still living, breathing beings and should be monitored.
Do I have to water my greenroof?
Extensive – Yes, occasionally during the first year of establishment just like any landscape. Drip irrigation is ideal for large projects plus it is inexpensive and delivers the right amount of water to the best area – the base of the plants. But then the answer should be no, if you have chosen the correct drought tolerant plants wisely for your area, except in extreme periods of drought. Then the plants would certainly benefit from occasional watering during extreme periods of duress – that’s one reason a water source should be close by.
Intensive – Yes, since an intensive greenroof can accommodate a large variety of plants, shrubs and trees, their watering requirements are higher than succulents and herbs. Treat an intensive greenroof like any garden or landscape at ground level, but take into account that high winds can be very drying. Usually large intensive greenroofs have an irrigation system installed.
So what about irrigation or supplemental water – can it still be environmentally friendly?
Yes. You can install a traditional active irrigation system or a solar powered system. Pair this with a recycled rainwater collection system, harvested in cisterns at roof deck or at ground level, and you’ve got the ideal self-sustainable answer to supplemental water and how to power it.
Should I fertilize my extensive greenroof?
There is some dissention about whether it is recommended or not to fertilize an extensive greenroof – I believe the answer is yes, as most German roof greening professionals recommend. Use encapsulated 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. At that point, the natural cycle should take over and enough organic material should have composted itself back into the soil substrate to provide sufficient nutrients to the plants. Don’t use soluble N fertilizer as it can get into the runoff.
Do I have to worry about a root-resistant protective layer if I only have an extensive greenroof with nothing but sedums?
Yes! Just because you design a greenroof with tiny horizontal roots, it doesn’t mean you can’t get a surprise plant with a huge taproot looking for water however they can get it. Seeds come in from many sources – the wind, and from those beautiful birds that you love seeing on your greenroof so much. I have seen oak and maple seedlings growing in ½” of rotted compost on a pitched roof, for example. And, believe me, those roots will seek water wherever possible in times of drought – way into the asphalt membrane! If your roofing membrane is organic in nature – asphalt, asphaltic bitumen, etc., you need a root barrier, it doesn't matter how low growing your extensive greenroof 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 atop the drainage layer.
Does the soil really have to be “engineered?” Why can’t I just put top soil on my greenroof?”
Yes! Regular garden soil is heavy, can contain pathogens, undesirable insects, and WEEDS! Basically, you don’t know what’s in it. Think potting soils in that really they are engineered soiless media. Greenroof “soils” need to be lightweight to conform to roof loading weight restrictions, drain properly and yet 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.
Should I worry about flammability issues?
Yes! Always have access to an adequate water supply just in case of fire, which can occur to any type of building. First of all, choose plants that are inherently non-flammable – succulents or others that store water in their stems, and stay away from ornamental grasses and certain mosses that could become kindling material in an extremely dry situation. For example, Miscanthus can be very flammable. Succulent plants and a high inorganic soil media can actually act as a fire barrier. But make sure that your roof has a 12-24” 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.
Who should maintain my roof?
It depends on the type and size of the greenroof. Most homeowners could check a flat or low pitch greenroof. A roofing professional should if it is too large or intensive – you can include a maintenance agreement of at least twice per year with the greenroof contractor/greenroof company. Or you can work out an arrangement with the roof maintenance staff of your building. And if you are referring to an elaborate hotel roof garden, for example, there will be a garden staff for maintenance needs already.
What about costs?
Extensive – I’ve seen them listed as low as about $9/sq .ft. for 3” of growing media and sedums. More commonly the range is between $14 - $25/sq. ft., (including roofing membranes).
Intensive – $25 - $40 and up.
But every project is unique, and certainly there are ways to lower the costs. Economy of scale is also very relevant. The Ford Motor Company River Rouge Plant greenroof in Dearborn, MI, for example, came in around $4/sq. ft., but we’re also talking about an extensive greenroof that’s almost 500,000 sq. ft in size.
Which is the most expensive component of a greenroof?
Just like any roof, the waterproofing membrane (or membranes) is the single highest cost item of a new roof.
Why shouldn’t you plant just sod or grass on a roof? They did it that way in Scandinavia and northern North America for hundreds of years.
Sod roofs resulted from a lack of natural resources, so people had to use the only materials they had at hand. But a monoculture of plant species is never considered healthy nor is desired in a land or roofscape for a 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 – a plant community should be ecologically diverse – with many types of vegetation to be vibrant (just like the human world – think The Boys from Brazil). Grass or sod needs to be watered, fertilized, and cut regularly, so they would incur greater costs be considered high maintenance.
Are greenroofs necessarily green in color?
No! Many greenroofs 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, greenroof 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. A virtual living carpet or tapestry varies from season to season as plant communities naturally migrate in their random regenerative patterns.
Where can I find info about different greenroof projects?
Search The Greenroof Projects Database, which is international in scope, growing daily, and a free community resource. Submit your projects for free by using the easy online form in English, Spanish or German, and send us your logo for placement at the top of the Project Profile as the "Information Partner." You can refine your search using over 15 fields.
So if I do a particular defined search in a variety of fields, does that mean that all the greenroof projects in the world are truly listed here?
No! Greenroofs.com continues to gather profiles from various sources across the globe, and many are in the works from various organizations, manufacturers, designers, owners, etc. It will probably take several years to catalogue all these projects properly, but that is one of our missions!
Can you recommend a greenroof designer or specific company? Where can I find this information?
No, Greenroofs.com does not recommend specific people or companies, but you will find manufacturers, suppliers & designers listed in The Greenroof Directory and related products and accessories in The MarketPlace.
How many LEED™points can a greenroof qualify for?
Greenroofs can contribute to at least 6 LEED™ points (more are possible) up to a possible 15 or 16. New info shows that greenroofs can contribute up to 14 credits with LEED-NC, Version 2.2. See more info here, and consult a LEED™ Accredited Professional for specifics.
See also Concept and Greenroofs 101 for more general info.
03:07, 17 April 2008 (EDT)
Check out these very informative sites to see the answers to their respective following FAQ's:
Green Roof Plants:
What's the difference between an Extensive green roof and an Intensive green roof?
What are the qualities of a good extensive green roof plant?
What plants should I put on an extensive green roof?
Can people walk on the green roof?
What will my green roof look like in the winter time? Will the foliage disappear?
What about allergens added to the air by the plants on the roof?
I would like to use native plants on my green roof project. Which varieties are appropriate?
Relative to a green roof, what are the differences between using hearty succulents and grasses or perennials?
What about sloped roofs?
Can I put a green roof on my existing residence?
The Green Roof Centre:
How much does a Green Roof cost? Do Green Roofs leak? Do the plants roots grow into the original roofs? How do Green Roofs affect the life span of a roof? How do Green Roofs affect water drainage from the roof? How do Green Roofs affect the buildings they are on? How do Green Roofs affect wildlife? How do Green Roofs affect the quality of life for people? Does a Green Roof need planning permission? Do I need to consult a Structural Engineer? Where can I buy seeds to build my own green roof?
Green Roofs for Healthy Cities:
What do I need to know about my building before I initiate a green roof installation?
What kinds of landscape design should I use and what plants can I grow on my roof?
How much does a green roof cost?
How can I purchase a green roof system for my home or building?
Who can design and install a green roof?
Are there any financial incentives to help offset the greater initial cost of a green roof?
Where can I learn more about green roofs?
Earth Pledge Foundation and www.greeninggotham.org:
What is a green roof?
Why green roofs for NYC?
Robert Herman, www.greenroofpro.com
 What is a roof top garden?
Rooftop gardens are man-made green spaces on the topmost levels of industrial, commercial, and residential structures. They may be designed to grow produce, provide play space, give shade and shelter, or simply be there as a living, green area.
 History of Rooftop Gardening or Greenroofs
 What are the types of rooftop gardens ?
Two main divisions of garden types exist: extensive and intensive. Extensive gardens require minimal maintenance and behave as another from of roofing material. They are not intended for heavy foot traffic nor do they need to meet any additional safety standards.
The other extreme includes intensive gardens created with the intent of active human use. These gardens require landscaping and regular upkeep. In some cases, the roof structure must be reinforced through the addition of decking or additional bracing to accommodate the combined weight of soil, plants, and precipitation. Furthermore, intensive gardens may also need to comply with safety regulations regarding decks and public areas on raised structures. These regulations may require some kind of fencing or barrier to be installed with the intent of preventing people from slipping over the edge of the roofline.
 What are the benefits?
- Urban Island Heat Affect Phenomenon (ACROS Fukuoka step garden creates winds)
- Overall Air Quality - Rooftop Gardens Reduce Smog, Improve Water Quality and More 
- Storm Water Removal System Overload
- Building Efficiency
 Structural Considerations of Rooftop Gardens
 What are the barriers or limitations of rooftop gardens?
- Lack of water. if one lives in a semi-arid high desert climate, where the annual precipitation does not often exceed sixteen inches. A typical garden requires much higher levels than what is naturally given.
Solution 1: Several construction, use, and maintenance methods can be employed to minimize the necessary levels of irrigation. Firstly, the use of growing mediums that have a high retention level would work to maximize what water is delivered to the garden and thus reduce the need for irrigation. Secondly, non-potable water may be used for irrigation, not affecting the drinking water supply. Drip method irrigation can be used to minimize the level of water lost to evaporation. Finally, xeriscaping, or using plants that flourish in dry environments will aid in water use reduction.
- Current roofs cannot support the weight added by soil and plants.
Solution 2: Where physical roof reinforcement is impractical or too expensive, roofs may use lighter weight plants such as mosses and seeders along with a thinner layer of growing medium.
- What about roof damage caused by leaking water?
Solution 3: In addition to using drip method of low-level water irrigation, drainage systems would need to be installed between the growing medium and the roof membrane. This system will channel out excess water during rainstorms and snowmelt preventing accumulation of water upon the roof surface. These are currently part of construction and commonly used by rooftop gardeners.
- This program will be too expensive.
Solution 4: If reducing the temperature of the city is the primary goal, then there are other options including the use of lighter colored roofing materials, however this method eliminates all environmental benefits derived from natural plant life. The question really is, which costs more: the complete repair of the stormwater removal system, plus the higher temperatures within the city, plus having to live with air polluted by smog and dirt OR the construction of green spaces on the majority of building in the Springs?
- Green roof directory 
- Forum at the Green Roof Centre