This experience has led me to speculate that a similar vegetable garden would work very well on a flat roof. If my house had a flat roof, I could have a garden roof with a view that the rabbits wouldn’t eat and I could make a productive, pleasant place to read the paper and sip coffee in my PJs while watching the dewy tomatoes grow at sunrise. I’ll put this on my list of design strategies for my next green dream home (more on that in future articles). The money view from most buildings is from the roof. That's why penthouses are so expensive.
I know what you are thinking. Conventional wisdom says that flat roofs leak and putting wet soil on a roof is insane. Conventional wisdom is a good starting point for design, because it is usually informed by past failures. History has also shown repeatedly, however, that when innovators overcome conventional wisdom with new techniques, civilization makes progress. Let’s look at some unconventional wisdom for a moment.
The technology to plant stuff on the roof is now well proven and growing quickly in popularity across the country, thanks to groups like Green Roofs for Healthy Cities spreading the word about how to do it correctly. An example of the state of the art can be found at Oaklyn Public Library in Evansville, Indiana, which supports an 18-inch deep native mesic (wet) meadow with ponding elements and a subterranean trickle irrigation system for dry spells. That must have been one dimwitted architect!
Uh, as the architect for this project (when I worked for another firm, Veazey Parrot & Shoulders, in Evansville), I can attest we did learn how to put a wet natural meadow on a library roof in 2002 and it is still dry. The meadow extends over much of the seven-acre site in which this earth-sheltered 18,500 square foot building is integrated, such that it is difficult to tell where the building ends and the site begins. In case you were wondering, it cost no more to build this earth-sheltered library. The roof is expensive but three of the walls are just buried cast concrete with no expensive exterior finishes, which allowed it to be constructed for less than the established budget and less than a typical gabled-roof library building. Oaklyn also saves many thousands of taxpayer dollars each year through reduced energy and maintenance costs. For a typical above-ground building, garden roofs do add costs and those costs escalate with the thickness, complexity and amenities. Garden roofs do have paybacks, however, that will eventually recover the initial costs in most cases. Those costs and paybacks vary with each site, garden roof type and building type. The biggest payback is often reclaimed usable space and design schools now refer to the roofscape design as the “fifth facade.”
Oaklyn inside lets in sunlight through a "Lightbridge" clerestory.
When approaching new technology, it is important to examine precedents, research best practices and find the best experts. It didn’t take long for us to find the complex garden roof of Chicago City Hall and the designer of its complex garden roof, Charlie Miller, president of Roofscapes, who is one of the world’s premier experts on garden roofs (also known as green roofs or vegetated roofs). Working with Charlie and the rest of the design team, we figured out a way to pull that off. This “particularly robust” national-award-winning garden roof is designed to last for 50-75 years before replacement. It also makes the building more energy efficient, helps with stormwater mitigation, and the earth-sheltered strategy makes good use of an otherwise almost unbuildable hillside site. I would consider a similar, but less complex system for my dream home edible roof garden design. But Oaklyn is a good example of some of the technology available for less ambitious projects. Let’s go through some of the basics.
A growing medium that will support vegetables will need to be at least five inches deep to emulate the Square Foot Garden that occupies my side yard. The weight of five inches of wet soil needs to be added to the normal structural calculations. This would be known in garden roof language as an intensive system, as opposed to an extensive system, which typically are 3 to 4 inches thick and can go where you may already have a gravel ballasted roof. Ford Motor Company has a very large example of an extensive system at their River Rouge Plant. The Indianapolis Museum of Art (by BDMD Architects) has a new parking garage covered by a very intensive system that supports an allee with large trees. Other commercial garden roofs feature pools and fountains or even a full golf course hole. It is a way to reclaim expensive urban space. Chicago is bullish on green roofs because they cool the city and reduce combined sewer overflow issues. They now have 300 garden roofs throughout the city.
Eleven-foot tall windows help daylight earth-sheltered Oaklyn Library.
Oaklyn Library used a composite concrete deck (where the poured concrete locks into a specially-designed corrugated metal deck and both are strengthened) to support a very deep garden roof. A concrete deck would probably not be necessary on a residential garden roof, but it may be a good choice if you are using cast concrete walls, which would be a great way to go on a home for other reasons (more on this in a future article). Larry Crane at Crane Builders uses this type of system to build floors in his multi-level insulated concrete form (ICF) walled homes. With typical wood-framed construction, garden roof capacity could be achieved with deeper joists, parallel chord trusses or engineered wood microlam or glulam beams or with I-joists. On existing homes, it would probably mean sister joists, but be sure to check with a licensed architect or structural engineer before doing anything on your own. There are other considerations, like lateral loads in earthquakes, to consider, depending on your location and loading capacity of your walls and foundation.
Corporate roof golfing!
Many existing buildings, including the building I work in, already have the structural capacity for garden roofs as they were constructed in such a way as to be added to later and the roof was actually designed to be a heavily-loaded industrial floor. Reclaiming such space for use by people, instead of leaving it to the pigeons and the rooftop compressors, has become a hot niche market for many firms.
When you have the structure confirmed, the key decision is what type of membrane to use to keep the elements out of the building. For Oaklyn, the choice came down to two systems; a liquid-applied membrane by American Hydrotech and a heat-welded reinforced PVC sheet membrane system by Sarnafil. Both systems had been proven in very high profile projects around the world and either probably would have worked. Sarnafil allowed for the incorporation of ponding elements (plastic pipe dams heat-welded into a secondary protective cover sheet), which was also heat welded. Oaklyn’s roof system is “robust” in that is has two layers of heat-welded, reinforced PVC membranes, the primary layer and a protective layer. This type of membrane has been used in underwater tunnels and has a thirty-year history in Germany. This membrane is also impervious to root growth, while most organic materials require some type of additional root growth barrier. To prevent excessive ponding of water that would kill the meadow, Oaklyn’s roof has a slight slope and a subterranean drainage system consisting of low-profile rectangular perforated drain pipe. The overburden of soil protects these premium membranes from ultraviolet light, extreme temperatures and physical damage. Welds are tested prior to adding soil mix by flooding the roof for 24 hours. Another advantage of a heat-welded sheet membrane is that the same system can be used to flash penetrations, parapet walls, and other interruptions where leaks are most likely to occur in any roof system. Sarnafil also has certified installers, which means you get experienced crews who know what they are doing. It is also a good idea to compartmentalize sections of the roof and embed electronic leak detection so that any eventual problems can be discovered before any damage is done and repairs can be made without removing large sections of overburden. Consider this cheap insurance.
Garden roof technology continues to improve rapidly, with new developments like pre-planted trays for rapid starting of the living part of the system. Live Roof seems to be leading this trend.
Sixteen species made up the initial meadow planting at Oaklyn Library
Oaklyn used plugs of living meadow plants in 16 species. The key consideration for roof gardens is whether they will be accessible and frequently maintained, like a vegetable roof garden, or if they will have to fend for themselves without much maintenance and without irrigation. In the latter category are plantings of sedums and mosses that can survive extreme conditions normally seen on rooftops without irrigation.
Getting back to a garden on your home roof; some of the advantages, other than more usable outdoor space, include: increased energy efficiency, sound isolation, disaster resistance (fire, hail, high winds), significantly longer roof life, less stormwater runoff and a supply of very fresh food. Aesthetically speaking, this will work best when the building is already designed in an architectural style that features a low-slope roof. My guess is your subdivision's covenant would prevent you from lopping off your massive neo-French hyper-hip roof that matches those of all your neighbors. This would be great in an urban setting, however, when you don't have much of a yard or garden space on the ground plane or on a contemporary-style custom home. For a design from scratch, you would have the option of raising the garden to the roof, or, if you have a nice south-facing slope with a view, put the house under the garden. Since you have all that new flat landscape, you may also want to put some solar hot water panels and solar photovoltaic panels up there to wipe out your electric bill and charge your electric car. More on that later as we continue to explore the green dream home.