With a green roof, an additional factor is the depth of soil or growing medium on the roof. Some plants can do well in thin soils, and others require more depth for rooting. On a green roof, deeper soils mean more weight, which in turn means a stronger structure to support the roof.

All these factors should be taken into account during the design phase of a new green roof: What plants would you like to see on the roof? How much soil is then required? And how strong a structure is required to carry that weight of soil? For retrofits or installation on an existing roof, the process might go in reverse: How much weight can my roof carry? How much soil does that mean I can have? What kind of plants do well in those conditions?

PLANTS SUITABLE FOR 5 CM OF GROWING MEDIUM

On shallow green roofs, the growing medium is usually quite inorganic. This is to reduce the overall weight. As a result, the plants suitable to this type of roof are hardy, shallow-rooting varieties that can survive in poor, dry conditions. These types of roofs are often used in inaccessible places, so low maintenance plants are preferred.

Sedum (stone-crop) and delosperma (ice plant) are common varieties. These are particularly common since they are succulents and retain water during dry spells. As a result, a green roof with mostly sedum species can usually survive 2 to 3 weeks without irrigation.

Sedums come in a variety of colours, both leaves and flowers, and mature at different times of the year. This will provide year-round interest on your green roof.

PLANTS SUITABLE FOR 10 CM OF GROWING MEDIUM

With deeper soil, a wider range of plantings become possible. Several species of grasses, alliums, herbs and wildflowers can thrive in 10 cm of growing medium.

PLANTS SUITABLE FOR 15+ CM OF GROWING MEDIUM

At 15 cm, a much larger variety of plantings are available. Grasses, columbines, asters, and Black-eyed Susan are all viable at with this deep a roof.

A green roof garage, especially in dense downtown areas can also effectively double the size of your green space. That should improve your property value as well!

Reduced Heating and Cooling Costs

Studies show that you could save up to 75% of cooling costs and 25% of heating costs with a green roof. The layers of plants and growing material not only provide additional insulation to the structure beneath, but the respiration of the plants actively cools the surrounding air, helping mitigate the urban heat island effect.

When the outside temperature is 25 to 30°C, a gravel roof can reach 60 to 80°C. Green roofs remain at the same temperature as the outside air, and rooms under a green roof would generally be 3 – 4°C less than that.

Increased Roof Lifespan

Reduced temperature variations and no UV radiation mean that the waterproofing won’t deteriorate over time as it would in a typical roof. You could expect your roof membrane to last 2 to 3 times longer than usual … 50 to 100+ years.

Reduced and Filtered Stormwater Run-Off

A green roof will retain 50 – 75% of summertime precipitation. Any run-off is filtered through the plants and soil, and is therefore delayed, reducing combined sewer overflows. The filtering process also significantly reduces the pollutants present in the run-off.

Filtration of Pollutants out of the Air and Water

A green roof on a typical one-car garage will remove 8 kg of airborne particulates per year and generate enough oxygen to supply 25 people with their yearly requirements.

Sound Insulation

Noise can be reduced by 40 – 50 decibels in a room under a 6 inch deep green roof.

Other Benefits

Green roofs provide additional habitat for birds, butterflies, and other wildlife. They can provide extra amenity space for residents. The environmental benefits are also combined with increased aesthetic appeal … leading to an increased property value.