Installing a green roof

Ideally, to ensure proper coordination, a green roof project should be entrusted to a specialized firm, whose expertise will be especially valuable in medium and large scale projects. Specialists also have access to proven materials that are not readily available on the market.
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Types of planting
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Smaller projects however, are relatively simple to carry out, and homeowners can easily design and install a green roof as long as certain basic principles are respected. At the outset, it is important to remember that every installation will be unique because the structure of every building is different. A flat roof, for example, is easier to plant than a sloping roof and is also more suitable as a place where you can sit out in privacy. A sloped roof is an interesting feature and, depending on the plants chosen, can blend in more easily with the house.

If you decide you would like to tackle a green roof project yourself, here is an overview of the steps you will need to take.

At the preparatory stage, two essential steps require the help of a professional in order to avoid any expensive repercussions in the long term.
  • It is important to know the maximum load that the building can support. Certain parts of a roof are stronger because they are supported either by a load-bearing wall or by pillars.

  • It is important to check the roof for water-tightness. It must be in perfect condition because any repairs to the roof will be more costly to carry out once the green roof has been installed.

Consider also the following:
  • Permits. It is very important to enquire about municipal regulations. Many cities treat green roof installation as a renovation and will require a permit.

  • If the roof is not flat, the pitch must be calculated. As a general rule, the growing medium remains stable on a pitch of up to 25 degrees. But if the slope is greater, you may need to fix metal, wood or plastic battens to the roof membrane to hold everything in place and to avoid any erosion by summer rain. Special reinforcing mesh is also available for this purpose.

  • Roof access. The roof must be accessible both during construction and for periodic maintenance. During installation, plan an access that allows the materials, soil and plants to be safely carried up. Later you will need to be able to access the roof regularly for maintenance.

  • Exposure to sun. This determines the type of plants to be used. Check for nearby mature trees, high buildings, etc. Consider the direction of prevailing winds in order to choose the best position for plants. Tall plants need to be protected from winds if they are not to be uprooted.

  • Planting. Try to avoid the roof becoming the focus of your house; it should form a harmonious part of the whole.

Depending on where you live, other factors may need to be considered, such as plant hardiness or snow and ice accumulation.

Types of planting

Planting is divided into three categories: extensive, semi-extensive and intensive. The difference between them lies in installation costs, depth of the growing media and choice of plants. Depending on the building structure, one type will be more appropriate than another.
    Extensive
    This is the least expensive type to install. Extensive green roofs are used in places where access is difficult. Requiring only a minimum of expertise, they are recommended for roofs with a 0 to 40-degree pitch. They make a good choice for large surfaces where building reinforcement would be too costly.

    The thickness of the medium varies from 5 to 15 cm and, when saturated with water, it weighs between 70 and 170 kg per square metre, which is well below the standard limit for Canadian buildings.

    The plants you choose must be resistant to drought conditions, quick growing and ideally give good ground cover, since they will only be watered during dry periods. When planned well, extensive systems look naturalistic and provide splashes of colour that change with the season.

    Extensive green roofs do have some drawbacks. The thin substrate layer reduces energy efficiency and water retention, and because they are not suitable for walking on, they cannot be used for recreational purposes. The choice of plants is more limited.

    Semi-extensive
    Semi-extensive green roofs also have a thinner soil layer, but they differ in that, below the growing medium, there is a drip irrigation system between the geotextile filter course and the geotextile root-repellent. With a drip irrigation system there is no wastage of water because there is practically no evaporation. Also, unlike using a hose, there is no runoff.

    It allows a wider range of plants, and you can introduce relief by planting small hardwoods, spreading conifers or even vegetables and small fruit plants.

    To avoid burst pipes, the system must be purged of water before the winter.

    Intensive
    This is the most expensive type of green roof to install because it does require a specialized firm for a structural analysis of the building and to carry out the installation. Intensive applications are recommended for low pitched or flat roofs. They offer optimum insulation properties and greater water retention.

    The growing medium on intensive green roofs is deeper, 20 to 60 cm (or more), and consequently weighs between 30 and 950 kg per square metre when saturated with water. To this we need to add the weight of the building materials (rockery stones, retaining walls, pavers, stone for pathways etc), not forgetting any furniture and decorations (patio set, bird bath, stone statues, etc).

    A deeper soil layer means a wider choice of plants, which can include espaliered trees and shrubs, thereby creating a more complex ecosystem. Intensive systems demand more maintenance, particularly regular watering. The installation of a drip watering system is recommended.

    If the area is easily accessible, it can become a backyard-style place to relax and play. Add a pergola, have pathways lined with low growing plants, or plant flowerbeds and rockeries to create a little oasis of greenery right in town.

    If you are planning on your roof becoming a place where you get together with a group of friends, it would be wise to check your insurance policy.

Cost

No two projects are the same, and prices can vary enormously. Here are some examples to give you an idea of what is entailed. We are not including do-it-yourself installations because these do not call for a great outlay and prices differ from region to region.

Here are some examples:

Small extensive project
To completely cover a roof area of 100 square metres, allow a minimum budget of $120 per square metre, including labour.

Large extensive project with difficult access
Roof repair
(removal of roof, installation of membrane)		$100 - $160/m2
Installation of materials
(including growing medium)		$55 - $110/m2
Plants$11 - $32/m2
Labour
(including crane rental)		$32 - $86/m2
Maintenance
(years 1 and 2)		$13 - $21/m2
Add 8-15% of total cost to cover design, estimate and project administration.

Large intensive project with easy access
Roof repair
(removal of roof, installation of membrane) 		$100 - $160/m2
Installation of materials
(including growing medium)		$160 - $220/m2
Parapet wall and fencing$65 - $130/linear metre
Plants$54 - $2150/m2
Irrigation$21 - $43/m2
Labour
(including crane rental)		$85 - $195/m2
Maintenance
(years 1 and 2)		  $13.50 -  $21.50/m2
Add 8-15% of total costs as above.

Please note that these suggested prices are meant only as a guide and may vary according to your region and contractor.
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