Green Roof FAQ
Why would anyone want to put plants on a roof?Green roofs offer a broad spectrum of environmental benefits, many of which are explained in our Green Roof Primer:The Big Idea. Plus, they’re just plain nicer to look at than other kinds of roofing.
What's underneath the plants?The diagram below shows the typical layers in a green roof assembly. The exact pieces used on any specific roof may vary, but this covers the basics. From the top down:
Read more about the plants in the answer to the next question.
Don’t call it dirt! It’s a highly-engineered, lightweight mix with relatively low organic content. Ordinary topsoil or potting mixes can promote weeds, clog drainage, overload the structure, or even support fire. And erosion control blanket (not shown) helps keep wind and water from carrying away the growing medium, and provides a mulch layer while the plants grow in.
This keeps small particles of growing medium from clogging the drainage system.
Drainage and Retention Layer
This is a specialized product, often consisting of a perforated and dimpled sheet of plastic. During a rainstorm, the dimples store water for the plants to use; once they’re full, excess water passes through small weep holes into the void space below.
A rigid, closed-cell foam insulation reduces the building’s heat gain and loss.
This prevents the plants’ roots from getting into the seams of the waterproofing.
A quality waterproofing membrane keeps the water out of the building. A green roof cannot be installed over shingles and some other types of roofing.
Green roofs can be installed over buildings of just about any material, including concrete, steel, and wood.
What kinds of plants are used?Sedums and other succulents are common, since they’re hardy, shallow-rooted and drought-tolerant. Natives plants can also be used, and we’re lucky here in Minnesota to have native species that are adapted to bedrock bluff prairies (or “goat prairies”). They’re used to living in shallow soils over rock ledges, with lots of sun and wind exposure, so they’re good candidates. Turf grass is rarely used: it’s too water-intensive and requires a lot of maintenance.
Aren't they heavy?They can be, yes. Some green roofs are only a few inches thick (“extensive green roofs”) while others are thick enough to support shrubs or even mature trees (“intensive green roofs”). In general, both kinds are heavier than conventional ones, so a structural engineer must be consulted for every green roof project.
Can you put one on a sloped roof?Yes, although the steeper the roof, the more complicated (and dangerous) things get. Erosion is one concern, so some means of keeping everything on the roof must be considered if the roof is steeper than about 3:12. After a few years, once the plants grow in, their roots will help hold everything together.
How much do they cost?That’s a little bit like asking what a car costs: it all depends. Simple systems can cost as little as $15 per square foot, but a typical budget would start around $25 or $30 per square foot. For very small buildings, an ambitious do-it-yourselfer could probably install a green roof for less than $10 per square foot. And keep in mind that while green roofs seem expensive up front, they also tend to last much longer than conventional roofs (see the next question).
How long do they last?Because the waterproofing membrane is protected under the plants and growing medium, it isn’t exposed to high temperatures, UV-radiation, or freeze-thaw cycles, three things that age roofs. A well-installed green roof can last 35 to 50 years, or even longer. And since the plants are perennials, regular replanting isn’t necessary.
Do they require a lot of routine maintenance?All roofs require maintenance, and green roofs are no exception. Weeding and watering are particularly important for the first two or three years, as the plants become established. After that, watering may still be necessary during extended droughts. Regular inspections are important, to make sure that the plants are thriving and the drainage systems are working properly.
How are they repaired, with all those plants everywhere?That’s a great question, but one of the first things to understand about green roofs is that they are actually LESS likely to fail then conventional roofs (see question “How long do they last?”). Also, green roofs are often constructed as “Protected Membrane Roofs” which make leaks easier to find – or an electronic leak detection system can be used. Once a leak is found, the assembly is cleared away, the membrane is repaired, and layers put back in place.
What about fire, wind and hail?A well-designed and well-maintained green roof is extremely resistant to fire (shingles and other roofing products are often made of petroleum, after all). In a bad hail storm, some plants may die, but others will bounce back and eventually fill in the gaps – or you can plant more.
How are they installed?There are three ways of getting a green roof onto a building. Integrated green roofs are built up in layers, right on the roof, and then planted in place. Vegetated mats are grown on the ground, cut into strips like sod, and rolled out onto the roof. Pre-grown trays are also grown on the ground, or in greenhouses, and are then laid out on the roof like tiles. Each method has its advantages and disadvantages, depending on the nature of the project.
Where do I get one?There are several manufacturers that work with approved installers around the State of Minnesota. We hope to have a list posted soon of companies you can contact.
Can I make my own?Under certain circumstances, yes – but they do require some professional guidance. Once a structural engineer has established that the building is strong enough to support a green roof, and a quality waterproofing membrane is installed, the rest of the assembly comes together pretty easily. Depending on the nature of your project, you might also want to consult with an architect and a landscape architect. The Minnesota Green Roofs Council has a program called RoofBloom that gives advice to homeowners interested in having their own green roofs on garages, sheds, and other small outbuildings.
Do I need a building permit to install one?In almost all cases, yes. Check with your local building department.
Is there such a thing as a 'green wall'?Yes! To put it simply, green walls are walls covered in plants, and they offer some of the same benefits as green roofs. They can be very simple, with plants growing from the ground (or a planter) onto a trellis structure (a ‘green facade’), or very complex, with plants rooted directly in a hydroponic wall assembly (a ‘living wall’). Green walls can be installed indoors or out.
Where do I go for more information?