How to build a green roof
Plants can help us eliminate our use of fossil fuels in ways beyond providing us with local food. By incorporating vegetation into our landscape through the use of green roofs, we can greatly diminish the supplemental energy our homes require by reducing the amount of excess heat and cold that enters in the first place. Just as monocultures that are isolated from other living things require fossil-fueled technofixes when problems arise, air conditioners, heat pumps and dehumidifiers, and air filters are needed when our homes remain segregated from the living world.
To the traditional home-owning American, the garden stops where the home begins. But by incorporating living things into and onto our home’s roofs, we can provide great benefit to ourselves and our surrounding community. The benefits break down as follows:
Aesthetic. Plants and flowers are beautiful to behold and attract beautiful creatures like butterflies and hummingbirds. By combining the random and organic growth of plants with the rectilinear and staid architecture of most existing homes, we can soften hard edges and incorporate our home into its surrounding environment. Doing so communicates the ecological concern of the home’s inhabitants with the surrounding communities, both human and otherwise.
Environmental. Additional green space provides forage and housing to a wide variety of insects, birds, and smaller animals. Indoor and outdoor plants cleanse the air of pollutants created by furnaces and passing cars and trucks and take carbon dioxide out of the air. Storm water that can overflow sewage systems is instead retained and released back into the environment, reducing flood potential. If done on a large enough scale, green roofs can reduce the ambient temperature by several degrees in the summertime.
Economic. Greening of roofs can reduce roof temperatures by up to 70 to 80 degrees F. Substantial reductions in interior temperatures result, often by as much as 6 to 8 degrees F. Considering that a 1- degree F decrease in interior temperatures can reduce air conditioning use by 8 percent, this can lead to huge energy savings. By opening up additional areas for gardening and food production of high-value items such as fruits and herbs, families can save on their grocery bill in terms of both money and energy, especially in space-constrained urban areas.
Building a Green Roof. The term green roof is a catchall for any roof with vegetation on it and is also sometimes referred to as an ecoroof. The modern green-roof movement began in Germany in the mid-1970s. Its proponents were interested in creating green space in otherwise barren urban areas. As the number of green roofs increased and their environmental and energy benefits became better studied and understood, the movement spread to other parts of Europe and then to North America. Green roofs on large civic buildings are now being built in cities such as Chicago, Illinois, and Asheville, North Carolina. Some new, architect-designed residences are also incorporating aspects of green roofs into their design.
For the owner of an existing home, the ability to create a green roof may be limited, due to concerns about weight, although not as much as might be initially supposed. Lightweight green roofs are possible on existing roofs with a pitch of up to 30 degrees. If the homeowner has access to the framing members of the roof via an attic, then reinforcement of load-bearing members is possible, and a heavier, more diverse green roof can be achieved. As a retrofit, the effect of going from a dead zone of tar or asphalt shingles to a thriving garden of flowers and birds can be very dramatic and is a joy to witness.
Green roofs are differentiated by the depth of the growing medium, as this directly relates to weight—the primary structural concern.
Intensive roofs are those greater than 4 inches in thickness, with some as deep as 2 feet or more, making a wide variety of plants, including bushes and even small trees, possible. Retrofitting a home for an intensive green roof requires structural analysis and almost certainly involves beefing up support members, not only of the horizontal span of the roof rafters but also the vertical support of the walls. Since access to wall members in an existing home is likely to be limited, creating an intensive green roof is more than likely cost- and time-prohibitive for most do-it-yourselfers. If, however, you are considering building some type of passive solar addition onto your home, then an intensive roof may be relatively simple to incorporate into your designs.
Extensive roofs are thin, from 1 to 4 inches thick, and can support less diverse plant communities. They are relatively easy to install and often-times require no additional structural support. Nevertheless, their environmental and energy benefits are still quite high.
Construction of your green roof will likely vary depending on your circumstances, making a general how-to difficult. For homes, we recommend only expert carpenters undertake such an endeavor, although less experienced carpenters may want to see what they can do with other outbuildings where the consequences of failure (i.e., collapse) won’t be so catastrophic.
In general, all green roofs have five basic components, although sometimes one layer achieves two or more functions:
- Weatherproof membrane
- Root-protection barrier
- Drainage layer
- Growing medium
Weatherproof membrane. Generally, there are three options available to create a waterproof seal as the base of any green roof project: a built-up roof, which is composed of layers of asphalt roofing felt; a single-ply rubber membrane, also called a pond liner; or a fluid-applied membrane, which can be applied hot or cold and forms a continuous seal.
The first thing you may note is that a green roof is made from some not very “green” materials. While constructing our green roof, we took to referring to it as the “black roof,” since this was its color for several months as the project was completed. While this is somewhat of a contradiction, it’s important to keep in mind that most of these materials are no worse than the average asphalt shingle roof but will have much more positive environmental and energy effects during their lifetime. Also, a properly constructed green roof will substantially improve the roofing material’s life by protecting it from direct sunlight as well as extremes of hot and cold.
Applying any of these roofing materials is fairly straightforward. Before they can be applied, a relatively flat continuous roofing deck free of all protrusions must exist, preferably with at least a few inches of fall for every 10 feet of deck. If the slope is greater than 15 degrees (about 2/12), then a latticework frame that creates 2 × 2-foot or smaller boxes will be necessary to ensure there is not slippage of the substrate material. Roofs with slope greater than 30 degrees (or 4/12) should not be attempted unless a green roof professional is consulted.
Root-protection barrier. A root-protection barrier is necessary if the weather-protection barrier contains chemically organic materials such as bitumen or asphalt (which contains bitumen), which is the case with built-up roofs and some fluid-applied membranes. Single-ply membranes (EPDM or butyl rubber) are inorganic and do not need a root-protection barrier. Occasionally, single-ply membranes made of recycled material can be found. Hopefully this will be the case more often in the future.
Chemically organic materials in the weather-protection barrier are susceptible to rot and infiltration from plant roots. Ultimately, in order for a green roof to perform the two functions of water protection and plant container, some inorganic material is required. Rolls of PVC are the most common material used to create this impermeable layer (if needed), which should be at least 1 mm in thickness. Note that PVC as well as the single-ply membranes consisting of EPDM will degrade over time if exposed to UV sunlight. Plan to have all parts of these materials out of sunlight either via green roof cover or some kind of trim.
Drainage layer. For roofs with slope greater than 7 degrees (1/12) a drainage layer will most likely not be necessary. In fact, a drainage layer could lead to the growing medium drying out too quickly. For flatter roofs, a drainage layer prevents soggy, anaerobic conditions that could lead to plant death and roof leaks.
There are two methods for creating a drainage layer: channels or a bulky substrate. Channeled flat plastic material elevates the planting medium off the roof. Excess water can then percolate down into the channels and run off the edge of the roof. A wide variety of materials can be used for this, some specifically made for green roofs; others, such as wavy plastic roofing materials, can be modified with drain holes to achieve this purpose.
The second and much more common option is to have a granular substrate material that allows excess water to flow through. A wide variety of options are available for this layer, some much heavier than others. Gravel, brick rubble, lava stone, sand, pumice, pebbles, vermiculite, and light expanded clay granules are common. The cost as well as the weight varies greatly. Brick rubble can often be had for free (okay, and a few hours with a sledgehammer) and makes an excellent intermediate choice between weight, moisture retention, and drainage.
Thinner extensive green roofs can dispense with any additional growing medium and plant directly into the drainage layer. Keep in mind that in order to do this your drainage layer material needs to be heavy enough not to blow away and to hold enough moisture for the xerophytes (dry-climate plants) to survive. Brick rubble makes an excellent compromise between these two needed attributes. Free colonization of such simple extensive roofs by local mosses and grasses will ensure hardy species are planted and money is not wasted purchasing lots of expensive plants that could very well perish in the extreme conditions.
One exciting recent development are pre-seeded extensive mats that simply roll out on top of the membrane layer. The sedums then sprout and keep the roof cool and green with little maintenance. As usual, convenience comes with added cost.
Growing medium. Where deeper roofs of 4-6 inches, sometimes referred to as a semi-extensive green roofs, can be supported, a drainage layer is covered with a separate growth layer. Landscaping cloth that allows water and some root penetration but retains soil is spread over the drainage layer. A few inches of growing medium such as loamy soil is spread on top. The growth layer retains most of the moisture and nutrients for the garden, but the drainage layer also retains air and moisture for the plants. Semi-extensive roofs can support a broader range of herbs and prairie flowers.
Where access is possible, the roof becomes a potential gardening spot that would otherwise be barren. Soils that contain large quantities of organic matter should be avoided or diluted with more inert sand, as these soils can promote growth that is too vigorous and severely stress the plants during extended dry spells. If there’s water access, although this is often rare or difficult, then a more dense organic soil is possible. Generally, green roofs behave similarly to any container planting, meaning they are more prone to drying out and experience wider fluctuations in temperature, increasing the stress level on plants with higher moisture requirements.
Beyond sedums, a wide variety of bulbs, grasses, and prairie flowers do especially well in extensive roofs and make dramatic mass plantings. Local wildflowers also have the potential to do quite well, especially those that thrive in full sun. Herbs like sage, thyme, rosemary, and lavender are usually quite drought hardy. You won’t know what’ll grow until you scatter a few seeds on your green roof and find out!
Structural considerations. Of course, a dramatic mass planting isn’t very worthwhile if it falls down on your head, although it would still be very dramatic. Obviously, many green roofs involve placing a large amount of weight on your roof. Understanding structural issues is of paramount importance to creating a stable, functioning green roof.
To determine if your existing roof has the capacity to carry the weight of any type of green roof you must have a firm understanding of live and dead loads, lumber sizes and spans, and wood species and their related compression capabilities. All this is in addition to general framing and carpentry expertise. Failure in proper planning can have catastrophic consequences, including death. If you have any doubts about the potential loading of your green roof or your home’s ability to support it, consultation with a structural engineer and/or building inspector is a must. This doesn’t necessarily have to cost any money. Some counties’ building inspectors are structural engineers by trade, and if you go in with some reasonably drawn-up plans and a thorough understanding of the concepts listed above, there’s a good chance they will help you out. The American Wood Council (www.awc.org) has a wide variety of information on required spans and wood types available for free from its Web site. Much of this information is also available in local building code books or the Universal Building Code book.
Generally speaking, an extensive green roof (2-6 inches in depth) will increase the load on your roof rafters by between 14 and 35 pounds per square foot. The lower end of this is easily accommodated by any solidly built residence (please don’t put a green roof on top of your trailer!). Obviously, the material used as a substrate has much to do with the final weight. A 1- inch layer of pebbles can increase the load on your roof by about 10 pounds/square foot when saturated, while saturated light expanded clay granules will increase the load by only 2 pounds/square foot.
Regardless, checking the spacing, span, and wood type of your rafters and checking to make sure they are free from rot and capable of accommodating this extra weight, in addition to the maximum potential snow load or other stresses of your particular area, is required. The information is available for free at the above Web site and there is no excuse for not checking to make sure your home can support your green roof.
Intensive green roofs, depending on the depths of the soil and the substrate used, can increase loads by as much as 200 pounds/square foot. Intensive green roofs require plans approved by a structural engineer.
This is an excerpt from The Carbon-Free Home: 36 Remodeling Projects to Help Kick the Fossil-Fuel Habit (Chelsea Green, 2008) by Stephen and Rebekah Hren. It has been adapted for the web.