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Why does call them “greenroofs” and not “green roofs?”

For a variety of reasons: a compound one-word term is more descriptive and exact in its meaning, like dishwasher or microwave; these are household words included in the dictionary, meaning the combination of their parts.  A “dish washer,” for example, is something or more commonly, someone, who washes dishes – but a “dishwasher” is obviously a machine that washes the dishes.  A “green” roof could refer to the fact that any of the roofing material is ecologically sound, or “green” in nature, such as a recycled or naturally harvested product, or uses sustainable design elements in some way.  "Green" can be applied to energy-efficient components, such as highly reflective roofing membranes (e.g., white single plies or modified bituminous roof systems topped with reflective coatings), or roofs of sprayed polyurethane foam.

A “green roof” could also refer to the color of the roofing material, be it green ceramic tiles, metal, or asphalt shingles.

We did so much original information searching in German, that it was just natural that we should call them “greenroofs,” as they are referred to as “Dachbegrünungen” – roof greenings or greened roofs.  Of course, the German language always joins nouns for descriptive purposes, but it really does convey the meaning of a particular word.  I don’t think anyone could confuse the meaning of “greenroof” – a vegetated roof cover, be it extensive or intensive.

OK, so what is a greenroof?

Basically, greenroofs are living roofs or 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 three main types of greenroofs – extensive, semi-intensive or intensive -  although a greenroof can be designed with a combination.

I see you call yourselves "The Greenroof Industry Resource Portal."  What do you consider to be the "Greenroof Industry?"

We consider the green roof industry to be those who contribute in any capacity at large - including those who choose to not be associated with any particular group or regional, non-profit, professional, etc., organization.  The "greenroof industry" is not self-describing other than that these individuals are active in the study, promotion, construction, design or implementation of ideas and ideals within the growing community of people interested in furthering the wide spread implementation of this form of living architecture.  "Industry" is meant to describe the burgeoning commercial and professional arena, and is certainly not limited to one particular "professional" organization's definition or membership's restriction of what "the industry" should construe.  Industry can also mean "Community" or a free and unencumbered "MarketPlace," for that matter.

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. 

Based on these, the depth of the growing medium therefore determines the plant palette.  Although a building may be constructed with a variety, there are three main types and so we have the following definitions:

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 5 or 6” - approximately 13 – 15 cm - 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.

Semi-Intensive – A combination of an extensive greenroof with areas of higher plant depths, the semi-intensive living roof will have both areas of lower than 6” of growing media and higher, ranging from 8-12” or 20 – 30 cm.

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 12” - 30 cm - 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.

IntensiveYes, 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 & Greenwall 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 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! 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, does not recommend specific people or companies, but you will find manufacturers, suppliers & designers listed in The Greenroof & Greenwall Directory and related products and accessories in The MarketPlace.

How many LEEDpoints 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 information.