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The ins and outs of installing a green roof

If you have ever been up on top of a building with a green roof—we’re not talking a few planters, but rather a large area carpeted by plants—you can appreciate how beautiful a transformation this can be.

Of course, a rooftop covering of plants has many benefits other than visual. Green roofs insulate better during the winter, cool better during the summer, and protect your roof from the sun’s UV rays, potentially adding years to its life. The environmental benefits are obvious: more plants, less CO2 and less of an urban island warming effect, because the sun’s rays aren’t being absorbed by a black roof.

Is your roof suitable as a green roof?
Whether yours is a new build or a retrofit, a green roof can only sit atop a building with the appropriate loadbearing capacity. Determining this involves calculating the dead load of the trusses, roofing membrane and vegetated roof components, as well as the live load of snow, ice and water that permeates and occasionally saturates the growth layer.

Your roofing assembly must also have the right wind uplift, fire rating, drainage pitch and flashing details to support a vegetated roof. You can quite easily integrate a green roof into your new building plans. The best time to retrofit an old building is when roof work is needed in any case.

Types of green rooftop systems
> Extensive: up to 4 inches of soil, with low maintenance plants
> Semi-intensive: up to 6 inches of soil, with low-maintenance plants of up to 18 inches in height
> Intensive: up to 18 inches of soil, with more ornamental garden or meadow-like plants up to 48 inches in height

Types of green rooftop systems
The most common type of green rooftop includes areas with grasses, small shrubs and other low-maintenance plants. This is what is called an extensive green roof, with the least depth of soil needed. The most intensive green roofs include more common garden plants and even small trees, typically up to 4 feet in height. For these, up to 18 inches of soil is required—necessitating a robust load-bearing ability.

An intensive green roof is often the most beautiful and is something that you might consider on a roof that people can see and/or use. But regardless of whether you install a green roof for its aesthetic qualities or not, you will need to consider access, safety and maintenance.

Green rooftop installation methods

Mat systems
After your roof has been properly waterproofed and a root barrier has been laid down, mat system plantings grown offsite are installed in the same fashion as grass sod is installed on a new lawn. Rolled mats of plants are unrolled on the roof to provide instant green coverage. Mats will grow together over time and the seams between individual mats will disappear, leaving you with a beautiful, uniform green roof. This is a popular installation method for large roof areas.

Built-up systems
This method of installation is more like planting a garden than installing grass sod. Start with a waterproof roof membrane, add a root barrier, then growing medium. Individual plants can be added wherever they are desired. The end result is a custom look that can be breathtaking. This method, also called ‘planting in place’, is often used in planting intensive green roofs.

Modular systems
Modular systems are becoming increasingly popular because of their convenience, both in terms of installation, swapping out containers, and in terms of gaining access to the roof. A modular system is composed of large custom-built trays of plants, grown offsite, that are simply placed in pre-ordained locations on a roof. Such systems typically include containers, which can be of varying depths, a planting medium, a filter blanket, a drainage medium and of course the plants. Containers come with drainage holes and channels, and may have holes along the sides so that root systems can spread from one container to another.

Technical considerations
> Roof loadbearing capacity
> Wind uplift
> Fire rating
> Drainage pitch
> Flashing details
> Safe access
> Access to water
> Safe pathways for green roof maintenance
> Safe pathways and room for equipment maintenance
> Tie-in points for safety

Green roof access, safety & maintenance
If your building has a roof that is meant to be used, with tall barriers at the edges, safe walking areas and the appropriate loadbearing capacity, it will make installing and maintaining a green roof simpler and easier.

If you don’t have easy and safe access to the roof, you will need to make that available. Carrying a bale of peat moss up a freestanding ladder to the rooftop is neither easy nor safe, so the best solution is a flight of stairs with direct access to the roof. The next concern is safety while on the roof. Most building safety codes stipulate tie-down points if the parapet doesn’t meet a certain height requirement. With buildings over ten stories high, wind uplift is also an issue, so roping in might be required in any case.

Like any building component, a green roof is not entirely maintenance-free and it will require some upkeep, especially during the first two years, before plants are fully established. Extensive and semi-intensive green roofs need the least maintenance, but intensive roofs have needs similar to that of a garden. Tree seedlings must be culled, as their roots can damage the membrane layer, and all systems will require watering at times, since groundwater is absent, shade is often non-existent and the drying effect of the wind is more pronounced on top of a building. Larger green roof installations often come with automatic sprinkler systems fit with moisture sensors.

Vegetation considerations

  • Mat, built-up or modular system
  • Extensive, semi-intensive or intensive vegetation
  • Depth and composition of soil
  • Self-regulating, drought-resistant plants, or
  • Ornamental plants in need of regular irrigation

Green roof planning considerations
Installing a green roof is not difficult to do, but planning and installation are most successful when dedicated green roof professionals are involved. If your roof has the right drainage pitch and other attributes such as live load bearing capacity (to support the weight of saturated growing medium as well as added ice and snow in winter), you are ready to install. Consider, however, space required for green roof maintenance as well as the maintenance of other roof components like HVAC systems. Plan your green spaces accordingly—the damage incurred by footsteps across a frost-encrusted green roof can take years to regenerate.

Routes for maintenance staff and space around HVAC and other equipment can consist of bare roof, gravel, concrete pavers, or special grass pavers. Grass pavers are plastic or rubber mats riddled with spaces through which plants can grow. As people walk on the mats, only the portion of the plant exposed above the mat is impacted, and not the plant crowns. Such pavers can be laid down as needed, or installed permanently.

A greener world
Green roofs have increased in popularity, especially over the last 15 years, when they have moved from a novelty to a design option for all buildings. Green roofs benefit the environment, improve aesthetics, reduce building maintenance costs and can extend the life of your roof.

To find out more about the technical aspects of installing a roof suitable for green roof cultivation, please contact Yves Bradet, Principal, Atlantic Roofers, at ybradet@atlanticroofers.com or 1-800-563-8648.

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