Greening Your Roof to Life

For any environmentally conscious homeowner, there are a multitude of things you can do to your property to make it more ecologically sound. For example, insulation and high-albedo siding can both reduce fuel costs and help fight climate change, and installing smart meters to monitor your energy and water usage can help you cut down on consumption. A growing trend in urban environments these days – pun mostly unintended – is the green roof, where standard roofing materials are replaced with plants to provide either integrated rooftop gardens or just a few environmental benefits. Slate, after all, is so twentieth-century.

Green is Good for You

The advantages of green roofs are manifold, as they:

• improve stormwater management and reduce runoff

• reduce the urban heat island effect through the absorption of heat

• improve air quality, as plants filter out airborne pollutants and take in CO2

• improve energy conservation as they are natural insulators

• can extend the life of a conventional roof by a factor of three as they protect the roof surface from UV light

Greening your roof also has natural benefits in terms of incorporating food gardens and habitat creation, and aesthetic and social benefits that can improve the quality of life in urban areas.

Intensive Rooftop Gardening

Depending on your goals and needs, a green roof can either be intensive, which means that it’s essentially an elevated park with complex structural support, as well as irrigation, drainage and root protection layers; or extensive, which is much lighter and provides native ground cover that does not require heavy maintenance. Like in a traditional garden, the types of plants you can grow on a green roof depend on the climate in your area, the sun exposure, the condition of the coil, the water requirements, etc. One additional thing to think about is the depth of soil or growing medium on the roof.

Depending on whether you want an intensive or extensive roof, the depth of soil will vary; deeper soils mean more weight, which means that the structural support required for your roof needs to be carefully considered. Different types of plants are viable at different soil depths:

• 5 cm of growing medium: hardy, shallow-rooting plants that can survive in poor, dry conditions and are low-maintenance, like sedum (stone-crop) and delosperma (ice plant)

• 10 cm of growing medium: several species of grasses become viable as well as alliums, herbs and wildflowers

• 15 cm of growing medium: many more types of plants such as grasses, columbines and asters will be able to survive

Whatever your choice, careful research about the types of plants and the amount of soil they require is definitely necessary.

Growing Pains

The medium that you choose to grow your plants in is also important. A good growing medium must both be able to retain and drain water in the correct amounts. If you want an extensive roof with plants that only require a shallow growth medium, such as sedum or mosses, even a very small layer of rockwool can support it. For more intensive roofs, which often have more complex layering to support a more varied and high-maintenance ecosystem, specially formulated soil compounds exist that prioritize the correct nutrients, pH, and drainage properties while still being light enough to be placed on a roof. Usually such compounds are a combination of bark, compost, sand and lightweight aggregate.

It’s also necessary to ensure that the roof is properly waterproofed for its own protection, as regardless of how good the drainage is, during a particularly wet spell it is still possible for damage to occur. Since green roofs intercept between 15% and 90% of stormwater runoff (the variation depends on differences in things like temperature, wind, evaporation and transpiration rates, and plant uptake), less water is directed into storm drains but more is absorbed by the roof itself. A roof completely saturated with water is very heavy, and this should be taken into account when deciding on the substrate. The last thing you want after spending money on your rooftop garden is to have to spend more money on roof repairs.

Winter is Coming

In colder climates, rain isn’t the only thing you have to worry about. Rooftop snow removal is crucial for green roofs, as the extra weight may in some cases exceed the load limits of the roof. Ensure that the snow is evenly distributed and that drifts are not allowed to build up. However, removing too much snow can also be a problem in that it can disturb the plants! So caution is advised when performing winter maintenance. Ice dams, caused when snow melts and the refreezes as ice, are a particular issue, and need to be removed before they reach dangerous levels.

So these are just a few things to think about when turning your dull, utilitarian roof into a natural, eco-friendly space. Happy rooftop gardening!