There can be no denying that green roofing is becoming more commonplace and it’s a good way of increasing the environmental credentials of a building.
For the architect or specifier though, there is an overwhelming choice of green roof build-ups out there and it’s not always easy to decide which build-up is best suited to the project you are working on.
Here are a few tips;
Your first thought should be to the load bearing capacity of the roof. The stronger the building the deeper the growing medium you can use in your green roof. The deeper the growing medium, the better the insulation properties. Plus you’ll have a wider range of plants to choose from.
With deeper growing medium also comes the possibility of more maintenance. You’re more likely to see tree seedlings establishing themselves in the future if you don’t keep abreast of the maintenance regime.
A lightweight green roof is normally sedum-based. They’re just about the only plants that will thrive in a shallow layer of growing medium.
If the roof is going to be shaded by trees or by other buildings you will be limiting your planting choices. Talk to green roof suppliers to find out what systems are suitable for shade.
The more difficult the access to a roof is – the simpler a green roof system should be. A deep substrate roof needs to have a lot of materials taken up to the roof. A lightweight roof created with just rolls of vegetation is so much simpler in terms of logistics.
Long term maintenance
A lightweight green roof created using sedum matting is probably the simplest type of green roof to maintain. It will need minimal irrigation, an annual feed and occasional weeding. In addition, drainage outlets must be kept clear.
As substrate depth increases, so does the sophistication of the plant pallet and the need for irrigation during dry weather. Think of your green roof as a giant sized plant pot. If it is filled with sedum plants, it will rarely need attention. If it contains herbaceous perennials or hardy annuals it needs daily watering and regular weeding.
If you are decided on a lightweight green roof – what needs to be specified?
I’m the kind of girl who likes everything to be sorted out quickly and easily. I’m not one for mixing and matching unless I’m really confident in what I’m doing. In other words I want to follow a proven recipe that tells me how much of each ingredient I need and how to combine them to make the perfect dish. The more expensive the ingredients, the more careful I am not to deviate from the recipe.
That’s the approach I tend to recommend for green roofing too – unless you are very experienced, it’s best to choose one supplier and listen to their advice on which materials are best for your project.
What you will need
a green roof build up won’t make a building watertight and it won’t repair a dodgy roof. Waterproofing needs to be sound before you begin the build-up.
Ideally the roof will have a gentle fall, perhaps 1 or 2 degrees. Steeper slopes may need extra thought when it comes to anchorage and irrigation. Drainage outlets should be easily accessible and easy to keep clear.
A layer of drainage mat or drainage board is essential if the slope is 3 degrees or less.
When there is not much substrate to hold water, a water retention matting will help plants survive in dry weather. Some types of drainage mat double up as water retention – I’m thinking about the egg cup type drainage mat.
Some vegetation mats have a layer of water retention blanket stitched to the back of them. These are the easiest and quickest to lay.
If you are able to support extra growing medium, always use a light weight green roof substrate. Never opt for topsoil. Topsoil is heavy, it tends to oxidise over time and smaller particles can be washed into the drainage system.
Sedum matting usually has a 20mm layer of growing medium as part of the product. This is enough to support sedum plants provided the green roof receives a feed once a year. (On sloping or very exposed roofs, the plants may also need irrigation from time to time and possibly an extra feed.)
The plant layer
Sedum matting usually has growing medium incorporated (see above) and is definitely easy to lift onto a roof and simple to install.
Where to buy green roof systems
Enviromat offers green roof kits for pitched or flat roofs. Technical information such as dimensions and weights are readily available in our brochure and it’s easy to make a purchase when the time is right.