- Question: Can a green roof be retrofitted on an existing structure?
It is absolutely possible to replace a conventional roof with a green roof, but it does require some extra care. You’ll need to consult with a structural engineer and carefully consider the types of loads your roof is expected to carry, as well as the local climate. If you live in a snowy area, for example, you need to ensure that your roof can hold up under snow plus the green roof structure, plus fully saturated growing medium. Happily, retrofit installations are more and more common, and many existing structures can be retrofitted without too much inconvenience.
Green roofs are best suited to flat or slightly sloped roofs, whether residential or commercial. Check with your designer or installer to see if the slope of your existing roof is suitable - you’ll usually need to install some sort of retention system to keep the growing medium from moving downslope. In some cases, modular planting systems are an ideal solution to this problem.
- Question: What are the disadvantages of green roofs?
While green roofs may have a lifespan of 50 years or more, they do require regular maintenance, which may be unexpected for a homeowner who is accustomed only to checking their roof after a severe hailstorm. Even so, for an extensive, low profile, green roof, the maintenance is fairly minimal after the vegetation has grown in. In the first year or two, the plants may need occasional watering, some weeding, and some replanting if any of the individual plants fail to thrive. Once the plants are established, well-planned extensive green roofs probably require maintenance only a couple times a year. These chores might include removing detritus like leaves or fallen branches, inspecting the structure for any visible issues like a clogged or damaged drainage layer, and perhaps some trimming, depending on the plants you’ve chosen.
The other two main “disadvantages” to green roofs are items to deal with before it is even installed, so they won’t catch you by surprise. Green roofs have a greater up-front cost than conventional roofs, which can be largely offset by the increased lifespan of the roof and potential energy savings; particularly during hot summers. If you’re retrofitting a green roof onto an existing structure, you also need to ensure the structure can bear the weight, which some may consider a disadvantage.
- Question: Do green roofs have gutters? Need drainage?
All green roofs have a drainage layer built directly into the green roof structure. It’s placed below the plants and growing medium but above the waterproof membrane. This drainage layer captures and directs water that is not absorbed by the growing medium and directs it to one or more downspouts. Water that passes through the growing medium then encounters a filter layer. The volume of water draining from a green roof is substantially less than from a conventional roof, and building owners often choose to capture that water in a rain barrel for later use elsewhere. Because the water that drains through a green roof is filtered by both the vegetation layer and the filter membrane, the semi-annual chore of cleaning out gutters blocked by leaves, branches, and random unpleasant discoveries is eliminated.
- Question: Can you walk on a green roof?
All green roofs require at least some maintenance, so yes, the structural loads for a green roof should include at least minimum access. If you’re planning on regular foot traffic over a garden-type installation, you’ll need to install an intensive green roof and include any structural support called for based on engineering calculations.
If you have an extensive green roof and you’re concerned primarily about whether your plants will survive occasional foot traffic, that’s typically ok if kept to a minimum. You’ll need to access the plants during the grow-in period anyway, and that’s when they’re most vulnerable. Check with your installer for recommendations on how best to protect them while you’re weeding in the first weeks or months. After the plants are established, you shouldn’t need to walk around much more than during semi-annual maintenance chores. Low growing succulents and grasses can generally tolerate being trod on occasionally but try to avoid doing so after a hard freeze when succulents are less flexible and more prone to damage.
- Question: Can I grow food on my green roof? Is a green roof the same as a rooftop garden?
An extensive green roof isn’t suitable for a rooftop garden for a couple of reasons: The growing medium is considerably shallower in an extensive setup and doesn’t meet the needs for food-producing plants. Food-producing plants generally need more consistent watering and regular attention for harvesting, pruning, and replanting, and this kind of regular traffic doesn’t lend itself well to a low-profile extensive green roof.
Intensive green roofs, however, are well suited to that kind of activity and in fact the term is basically synonymous with rooftop gardens. In an intensive setup, the building structure is reinforced, if necessary, to support substantially more weight, especially since regular foot traffic is to be expected. The growing medium may be 3 or 4 times the depth of an extensive green roof for roots to spread and grow strong enough to support taller plants with hanging fruits, or root vegetables. Intensive green roofs usually have irrigation systems included in the setup, which is a necessity if you’re growing vegetables and fruit.
Intensive green roofs/rooftop gardens should only be installed on flat or very low slope roofs, and safety measures like a fence should always be included to avoid dangerous accidents.
- Question: Are all green roofs flat?
Green roofs adapt most easily to flat or gently sloping roofs, but it is possible to install an extensive green roof system on buildings with a fairly significant slope. The more extreme the slope, the more engineering support and retention devices are needed. On the bright side, a sloping roof is considerably more visible from the ground.
On a relatively flat roof, drainage is more consistent, water doesn’t pool at the bottom, and the possibility of leaks is reduced. As the slope of a roof increases, specific challenges arise. Water hitting the peak of the sloped roof will drain rapidly, leaving the plants at the highest levels with the least amount of water. Even the growth medium will dry out more quickly. Many designers plan for these issues with strategic plantings, placing low growing succulents or other drought-tolerant plants at the peak, working towards thirstier perennials as they progress toward the bottom.
The next issue comes with shear forces associated with a sloped roof. Soil retention systems are necessary to keep the growth medium (and plants) from simply sagging down the slope in an elevated mudslide. Eaves and overhangs will almost certainly need additional structural support, and the waterproofing membrane should also be root-resistant, since separate layers are slippery and difficult to install separately on a sloped roof. Modular planting trays may be an excellent solution to problems with sagging soil, since the trays can be anchored independently so they don’t slip down to the eaves during a particularly torrential rain.
Overall, sloped roofs are more complex to design, install and maintain, but don’t let that discourage you. Dozens of sloped, modern, green roofs have been installed successfully across North America and those numbers will only increase.
- Question: Does a green roof need planning permission?
As concerns with climate, energy consumption, and lack of biodiversity grow, most municipalities are starting to recognize the benefit of green roofs, even outside intense urban developments. In some cases, there are a variety of financial incentives available, and even preferential zoning and fast-track permitting. An experienced green roof installer will be familiar with any code restrictions or permitting requirements in your community, but if you live in a neighborhood with an HOA, it’s a good idea to check with their bylaws as well before getting started.
- Question: Will a green roof give me LEEDS credits?
In some cases, green roofs can contribute substantially toward a building’s overall LEED rating, depending on the design and how well it integrates with other systems in the building. In other cases, while a green roof may not directly earn credits, it can be evaluated in concert with other sustainable initiatives like reducing urban heat islands, roof stormwater management, and protecting or restoring open space. Check with local LEEDS experts to help you fine-tune your installation to ensure you’re getting the most out of your investment.
- Question: Do Green roofs do any good during the winter?
It’s true that the buzz for green roofs primarily focuses on energy savings during summer and mitigating urban heat islands, but the benefits don’t end as the weather cools. During the winter, roofs are where most building heat is lost, and layers of growing medium help keep that heat inside.
The insulating properties of a green roof’s growing medium is closely tied to its moisture content, and studies have found that green roofs typically retain 25% to 40% of precipitation that falls at that time of year. This insulating layer can be very effective at reducing heating costs.
A less obvious benefit is protection offered by the vegetation layer against frost and freezing. The thermal mass of the plants and growth medium shields the waterproof membrane from the damaging effects of cold, which can cause shrinkage and cold-cracking.
Top Questions About Green Roofs
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