The phrase reduce, reuse, recycle is purposely listed in order of sustainability. It is always better to reduce our consumption, then reuse products, and only then to recycle as much as we can.
My last two columns have been about the timbers and posts.
Living with the earth instead of just on it has some unexpected benefits – such as the delight we felt when we sold our lawn mower. Before we built here, the land was maintenance-free woodland. After we built, it was still maintenance-free woodland because of our green roof.
Our green roof also provides temperature regulation in and around the house, improved energy efficiency, reduction of the urban heat island effect, storm water management, and a very long lifespan. A green roof also integrates the house into the surroundings, improves the look of our property, and reduces exterior noise.
If our new house was on the site of our old farmhouse, we would have designed a green roof as a pasture and let the sheep graze it. But, because our house is situated in and among trees, we are designing it to support low-lying forest woodland ground covers. Our intention is to have the roof be part of the forest as well as supporting solar thermal panels for our domestic hot water and in-floor heating needs.
The first step was to carefully place 15 cm of sand on top of the waterproof EPDM rubber membrane to protect it. Then we formed and poured parapet caps to both seal the top of the ICF walls and to provide an edge to hold the soil on the roof. Since our house is built into a hill, part of the roof is at grade level, so we could back the cement truck close and wheel barrow the concrete into the forms.
As you can see from the photo, there will be about 45 cm of soil on top of the sand that is already there on this part of the roof.
Once the caps were poured and finished, we were ready to withstand the Canadian winter and plan our next steps.
Our living roof may be somewhat unique because it has different levels of soil on different parts. The deeper, 60-cm layer could be called intensive. Intensive roofs tend to be more than 15 cm deep to allow for the growth of larger plants such as some small trees and shrubs. With that soil thickness and height of the plants, there is more structural loading imposed by an intensive roof, and it may require more maintenance such as watering and feeding. This extra weight and maintenance is what makes these roofs more costly. The plants chosen for an intensive roof tend to be chosen based on visual appeal.
The living roof over our kitchen area is more on the extensive side because it is about 30 cm deep. Extensive roofs contain smaller plants, including shrubs, sedums (low-lying ground covers), and herbs. They are self-sustaining except for bi-yearly maintenance and tend to have lower construction costs. Due to the low level of all the plants, they are often subjected to more weather such as wind and frost. As such, when choosing the plants used for an extensive roof, plants that are common to the area and can withstand the harsh conditions are desired.
The chart below shows our differing soil depths. However, this is all somewhat academic since we are going for a low-maintenance woodland look on the whole roof anyway. The reason we have more soil on the red-outlined part is for greater insulation and because we could hide the steel beams necessary to hold the weight. Since we couldn’t hide I-beams in the kitchen, we decided to go for less soil there.
As you can probably tell, there is a fair amount of flexibility around green roofs as long as you choose your plants carefully for the depth of soil that you have. For example, we will be encouraging native sedums because of their ability to store water in their leaves for extended periods of time in preparation for drought conditions. As well, native dry land and tundra grasses are suitable for use because they can survive the harsh climate conditions that can be experienced on a rooftop. Any garden centre can provide advice on drought-resistant, hardy native species but don’t bother buying plants because they will cost you a fortune. Look for a seeding mixture you can sow instead. It may take an extra year but your pocketbook will thank you.
A living roof with 15 cm or less of soil and proper plants will do a fine job of reducing your maintenance, heating, and cooling costs. Plus it is cool to have a picnic on your roof.
A green roof may make sense for you and your roof doesn’t have to be flat to have one. There are modular green roofs in plastic trays and also green carpets that you can just roll out over a traditional roof. Be sure to consult a structural engineer to see what weight your roof can hold.
It makes sense to have the best roof you can and a green roof will perform much better in our climate than the traditional Canadian dark shingles.