Green Roofs

Components of green roofs


  • The plants are often specially selected for particular applications.
  • The load bearing capacity of the roof structure will suggest the use of an engineered growing medium, which may not include soil, use up to 30% BRF (bois raméal fragmenté, green wood chips) and 60% compost.
  • Use a landscape or filter cloth to contain the roots and the growing medium,
    allowing for water penetration.
  • A specialized drainage layer, sometimes with built-in water reservoirs depending on the design purpose.
  • The protection of the roof is made by using a waterproofing / roofing membrane, with an integral root repellent.
  • Finally the roof structure, with traditional insulation either above or below.

Types of green roofs

gr6Extensive green roofs

These are often not accessible,low weight,low capital cost,low plant diversity and minimal maintenance requirements. The design goal is typically oriented towards energy savings and storm water management.

The growing medium is generally mineral-based mixture of sand, gravel, crushed brick, leca, peat, organic matter, and some soil, varies in depth between 5-15 cm (2-6″) with a weight increase of between 72.6-169.4 kg /m2 (16-35 lbs/sf) when fully saturated.

Due to the shallowness of the growing medium and the extreme desert-like microclimate on many roofs, plants must be low and hardy, typically alpine, dryland, or indigenous. Typically the plants are watered and fertilized only until they are established, and after the first year, maintenance consists of two visits a year for weeding of invasive species, safety and membrane inspections.

Intensive conventional green roofs

gr7Intensive green roofs are often accessible. They use deeper soil which means greater weight and higher capital costs. They feature increased plant diversity, and more maintenance requirements
The growing medium is often soil based, ranging in depth from 20–60 cm (8-24″), with a saturated weight increase of between 290 – 967.7 kg/m2 (60-200 lbs/sf). Due to the increased soil depth, the plant selection is more diverse and can include trees and shrubs, which allows for the development of a more complex ecosystem. Requirements for maintenance – especially watering – are more demanding and ongoing, and irrigation systems are usually specified.

Intensive container green roofs

gr8This technique goes around the high growing medium, capital cost issue: instead of using huge quantities of heavy mineral substrate, containers minimize growing medium quantities and they use light weight, high compost mixtures.
The main alleys of this technique are “Smart Pots” or “SacriCultures” and “Strawbale gardening“.
Read more about the different techniques of container gardening under the header “Methods” on the Pont de Vie Library.

Following a comparison table for the two major techniques:


Design Considerations


The primary functions has a profound effect on its overall design. (designed to retain storm
water may look very different from one whose main purpose is to brighten a hospital courtyard)


  • height of the roof above grade
  • exposure to wind
  • roof ’s orientation to the sun and shading  by surrounding buildings
  • general climate of the area; specific micro climate
  • views to and from the roof


Wet soil weighs approximately 1,597 kg per cubic metre (100 lbs per cubic foot); existing roofs are typically designed for a live load of only 40 lbs per square foot (195 kg per square meter), which includes the snow load.
Various types of lightweight growing media: 35.6 cm. (14″) layer of lightweight substrate made up of sand, pumice, and compost. This weighs only 292.6 kg per square metre (60 lbs/sf) when fully saturated and did not require structural upgrading requirements for snow loading must be accommodated roof slope greater than 20 degrees, the green roof installer needs to ensure that the sod or plant layer does not slip or slump when wet; 30º is maximum slope for normal installation – the greater the slope the thinner the substrate.

Access and exists

This is important for installation and ongoing maintenance, bringing up materials, soil and plants. With a new building, the design of internal stairs.


Consider the ability of plants to survive on a green roof is directly proportional to the amount of maintenance time and budget allocated to the project, particularly in the first two years when they are getting established.
Hardier or indigenous varieties of grasses and sedums are not maintenance intensive
if seeding: seed indigenous seeds end of autumn (seeds need freezing, spread out so late they will not sprout immediately) chive for example is great plant for green roofs.


Plant maintenance and maintenance of the waterproofing membrane are required. Depending on whether the green roof is extensive or intensive, required plant maintenance will range from two to three yearly inspections to check for weeds or damage, to weekly visits or irrigation, pruning, and replanting.
fees for three to five years of this service be included in the original bid regular maintenance inspections should be scheduled as for a standard roof installation, especially just before the warranty period expires (incorporation of an electronic leak detection system between or underneath the waterproofing membrane to pinpoint the exact location of water leaks. Access strategies include keeping the sensitive areas free of plants and growing medium (gravel skirts, etc), and dividing the green roof into distinct compartments for ease of removal.
Membrane replacement typically after 30-50 years


Benefits of green roofs

Increased energy efficiency

gr2Modeling research suggests that the reduced need for air conditioning in the summer is more substantial than the value of the added insulation in the winter. Building type is a key factor in determining overall cost savings (one or two story complexes where the roof is a large portion of the building envelope, the cooling energy savings in the summer have been modeled as high as 25%. A 20 cm  (8″) layer of growing medium and a thick layer of plants has a combined insulative value of RSI 0.14 (R20). Even when the growing media freezes, studies show that 30 cm (12″) of growing medium will not experience temperature drops below 0ºC (32ºF), even when outside temperatures are, -20º. Depending on the climate zone, the implementation of a green roof may also allow for a reduction in the requirements for traditional insulation.

Green roofs help to reduce the ‘urban heat island effect’ – the overheating of cities in the summer which contributes to air pollution and increased energy consumption: Every one degree Fahrenheit decrease in ambient air temperature results in a 1.2% drop in cooling energy use. Cooling slows down chemical processes that produce ground level ozone, nitrous oxides and smog, and help offset the production of sulfur dioxides from coal fired utilities.

Longer roof membrane life

Green roofs help to protect roofing membranes from extreme temperature fluctuations, the
negative impact of ultraviolet radiation, and accidental damage from pedestrian traffic. European  evidence indicates that green roofs will easily double the life span of a conventional roof, and thus decrease the need for re-roofing and the amount of waste material bound for landfill. These are direct operational cost savings for the building owner. Life cycle costing data which includes the cost of deferred maintenance and replacement suggests that green roofs cost the same or less than conventional roof systems.

Sound insulation

The growing medium is blocking lower frequencies of sound, and the plants blocking the higher frequencies. Tests show that 12 cm (5″) of growing medium alone can reduce sound by 40 db.

Fire resistance

Green roofs can help slow the spread of fire to and from the building through the roof
particularly where the growing medium is saturated. plants, if dry, can present a fire hazard.
integration of “fire breaks” at regular intervals across the roof, at the roof perimeter, and around all roof penetrations is recommended.(made of a non-combustible material such as gravel or concrete pavers, 60 cm (24″) wide, and located every 40 m (130 feet) in all directions. Other options would be the use of  “fire retardant plants”, such as sedums, which have a high water content, or a sprinkler irrigation system connected to the fire alarm.

Amenity Space

gr4Green roofs yield the ability to turn wasted roof space into various types of amenity space for building occupants; thus raising the value of the building. Green roofs generate aesthetic benefits.
Important to note that flat roofs constitute an ideal surface for urban agriculture: weed-free, rodent-free, accessible, full exposure to the sun.
Green roofs Provide new opportunities for biodiversity preservation and habitat creation.

Air filtration

Green roofs filter particulate matter from the air. Rain takes a lot of pollutants out of the air which are then washed into the growing medium below. Plants also absorb gaseous pollutants through photosynthesis and sequester them in their leaves. Studies show that treed urban streets have 10-15% fewer dust particles than found than similar streets without trees. Grass roof with 2,000 m² of unmown grass (100 m² of leaf surface per m² of roof) could cleanse 4,000 kg of dirt from the air per year (2 kg per m² of roof).

Retain and cleanse storm water

plants capture and hold rainwater.Water stored in the growing media is released through
evaporation and evapo-transpiration.  A heavily vegetated green roof with a 20-40 cm (8-16″)
thick growing medium can hold between of water 10-15 cm (4-6″).( if half of the buildings in
the downtown area of Portland/Or had green roofs, (219 acres), an estimated 66 million gallons of water would be retained annually. This would eliminate combined sewage overflows by 17 million gallons.Storm water discharges would be reduced between 11 and 15%.