A Roof That Grows on You
A Roof That Grows on You
December 6, 2013 by Taylor Studios
While the dealt with roofing materials that require no maintenance, a planted (or vegetative) roof involves a bit of work. This kind of truly “green” roof has big advantages that reach beyond the structure, but it is not an approach taken lightly. Fortunately, many people, businesses, and institutions have invested in vegetative roofs and all that experience and know-how is available for you to use.
The majority of vegetative roofs are installed on flat rooftops, or those with a slope with less than 15 degrees. Steeper slopes are possible, but steps need to be taken to keep the substrate from slipping, which increases costs. Vegetative roofs are often described in terms of the depth of the growing substrate.
How thick is your roof?
An “intensive” roof has a substrate depth of six or more inches, but can be as deep as two feet. The depth of the substrate directly affects the weight of the roof and the amount of structural support required. Obviously, an intensive roof weighs a lot and would be difficult to put onto an existing building without significant structural strengthening. Why choose an intensive roof? They can support a wide variety of plants, from grasses and flowers up to bushes and small trees. Aesthetically, they look more naturalistic, and the large mass acts as excellent insulation for the building beneath. An “extensive” roof has a substrate depth of one to six inches, so it weighs much less than intensive roof. They can often be installed without additional structural support, but they support fewer plant species.
Part of Chicago City Hall’s intensive roof
Layer it on
No matter the thickness of the roof, the layers are similar. The bottom layer is the weatherproof membrane, which I described in Flat Roof Options of my last post. The next layer is a root-protection barrier, which is needed if the weatherproof membrane contains organic materials (like the asphalt in a tar and gravel roof). The plant roots will penetrate and break down the organics in the membrane, which will cause your roof to leak and/or fail. Some installers add a layer of foam sheeting as insulation above the root-protection barrier, especially when the bottom layer is a metal roof.
The drainage layer is next, and it uses either pre-made materials with channels or a bulky (and loose) substrate. Channel materials come in many forms, with lots of options . Some channel drainage products include water storage, or are made from recycled materials. Loose substrates are materials like gravel, brick rubble, lava stone, and vermiculite. These are laid down in a layer through which the water flows to the edge of the roof.
The top layer is the growing medium. Some extensive roofs have drought resistant species planted directly into the drainage layer, but most roofs have at least some growing medium. Landscaping fabric often underlies the growing medium to discourage deep root growth and keep the soil intact. Since this layer can get rather thick, how much growing medium you use is determined by whether you want an intensive or extensive roof. In a downpour, six inches of soil can hold a lot of water, so this is where the strength of your structure comes into play.
Extensive roofs with little or no growing medium can support grasses and sedums without much additional watering. Slow growing, drought resistant plants are preferred on extensive roofs, since they can handle the large temperature and moisture variations on a roof. Intensive roofs require more careful monitoring of moisture levels. The thicker growing medium can hold more water, but the wider variety of plants used means less hardy species may need help during dry spells.
A sample layer system
So why go to all the trouble of planting a vegetative roof? Through insulation, it can reduce the interior temperature of the building by 6-8 degrees Fahrenheit, which reduces cooling load. A vegetative roof removes heat from the air through evapotranspiration, which reduces the air temperature above the roof and reduces the . A vegetative roof reduces and slows stormwater runoff and can filter pollutants from rainfall. Most obvious of all, it looks great.
My only problem with vegetative roofs is that they are hard to find, since I am rarely above roof level. If you know of a vegetative roof accessible to the public, give it a shout out here!