Frequently Asked Questions about Cool/Vegetated Roofs
Cool roofs are highly reflective and emissive materials that stay 50 to 60 degrees Fahrenheit cooler in the summer sun, thereby reducing energy costs, improving occupant comfort, cutting maintenance costs, increasing the life cycle of the roof and contributing to the reduction of urban heat islands and associated smog. What kind of roofing products are available for commercial and residential applications? Products for low-slope roofs, found on commercial and industrial buildings, fall into two categories - single-ply materials and coatings. Single-ply materials are large sheets of pre-made roofing that are mechanically fastened over the existing roof and sealed at the seams. Coatings are applied using rollers, sprays, or brushes, over an existing clean, leak-free roof surface. Visit the ENERGY STAR® Web site for a list of cool roof products and manufacturers. www.energystar.gov During the summer, a typical dark roof is 150 to 190 degrees Fahrenheit at peak, while cool roofs peak at 100 to 120 degrees Fahrenheit. Do cool roofs cost more than conventional roofs? Initial material costs are comparable with traditional roofing materials – some cool products cost less than traditional materials, some cost up to 20 percent more. Cool protective coatings can be reapplied repeatedly every 10 to 15 years and reduce, if not eliminate the need for expensive roof tear-offs. Combining these maintenance savings with an average 20% savings on air conditioning costs make cool roofing a better bargain over the long term. Are rebates available for cool roofs? In addition to energy and life cycle savings, rebates are available from some local utility companies Basically, vegetated roofs are vegetated roof covers, with growing media and plants taking the place of bare membrane, gravel ballast, shingles or tiles. The number of layers and the layer placement vary from system to system and vegetated roof type, but at the very least all vegetated roofs include a single to multi-ply waterproofing layer, drainage, growing media and the plants, covering the entire roof deck surface. There are two main types of vegetated roofs – extensive or intensive – although a vegetated roof is often designed with features of both and then are referred to as either semi-extensive or semi-intensive. What's the difference between an extensive and intensive vegetated roof? Commonly, the roof function or objective of the roof space determines the design – is it just an ecological cover or is it intended for human recreation, vegetable gardening, etc.? The limiting factors for vegetated roofs include: the roof loading capacity or maximum dead and live weight loads, determined by a structural engineer; the slope of the roof and perhaps the client's budget. Extensive – Also referred to as eco-roofs, and low-profile – They have thinner and less numbers of layers, so therefore they are lighter, less expensive and very low maintenance. Extensive vegetated roofs are built when the primary desire is for an ecological roof cover with limited human access. The minimum growing media or soil substrate starts at about 2 1/2" to 6" at most (although vegetative mats can actually have even less than 1" of growth media); the engineered soil media contains 70 – 80% inorganic or mineral material (or higher) to 20 – 30% organic (or less). Low growing, horizontally spreading root ground covers with general maximum plant heights of 16 – 24" are ideal. Alpine-type plants are successful because they are high drought, wind, frost, and heat tolerant, all necessary attributes for vegetated roofs. Plants include sedums and other succulents, flowering herbs, and certain grasses and mosses. Fully saturated weights range from a low of about 10 – 50 lbs/sq. ft. Compare that to common river rock ballast which weighs about 12 lbs/sq. ft. Extensive vegetated roofs can be constructed on slopes up to 30°, and steeper ones can be installed with raised grids or laths to hold plants and soil media in place. Intensive– Also referred to as high-profile – They look like traditional roof gardens because a much wider variety of plant material can be included since growing media depths are increased. The growing media starts from about 8 - 12" and can range up to 15 feet or more, depending on the loading capacity of the roof and the architectural and plant features that the building owner desires. The engineered soil media usually contains about 45 - 50% organic material to 50 - 55% mineral, and fully saturated weights range from about 80 -120 lbs/sq. ft. and up. Architectural accents such as waterfalls, ponds, gazebos, etc. are possible and these vegetated roofs provide recreation spaces and encourage interaction between people and nature. Maintenance requirements are also more intensive, and of course, these roofs are relatively flat. What's the difference between an intensive vegetated roof and a roof garden? A roof garden usually consists of containerized plantings of various sizes placed on top of a roof. In an intensive vegetated roof system, all the various layers are applied on top of the entire roof deck surface, allowing unimpeded drainage and a more even weight distribution over the whole roof. The vegetation is planted directly into the soil, not in planters or containers. Explain inorganic and organic growth media. Inorganic material refers to a high porosity natural mineral element such as expanded slate, shale, extruded clay, rock wool, lava or pumice, etc., which provides aeration (and could also provide water retention capabilities), and prevents total compaction of organic matter through settling over time, and acts as a good drainage medium. The inorganic medium maintains void or air space necessary for the plant roots to breathe and for the excess water to drain properly. Organic means well-rotted humus material (hen manure, guano, mushroom compost, etc.) augmented with organic fibrous material and a small amount of clay particles. This mixture holds and slowly releases essential trace elements necessary for the health of the soil community. Is an extensive vegetated roof really maintenance-free? No! Every roof needs to be checked periodically, and extensive vegetated roofs are no different. It is recommended to do a semi-annual maintenance review, at which time you can look for invasive weeds, disease, stray tree seedlings, etc. Plants, no matter how low growing and drought tolerant, are still living, breathing beings and should be monitored. Do I have to water my vegetated roof? Extensive – Yes, occasionally during the first year of establishment just like any landscape. Drip irrigation is ideal for large projects plus it is inexpensive and delivers the right amount of water to the best area – the base of the plants. But then the answer should be no, if you have chosen the correct drought tolerant plants wisely for your area, except in extreme periods of drought. Then the plants would certainly benefit from occasional watering during extreme periods of duress – that's one reason a water source should be close by. Intensive – Yes, since an intensive vegetated roof can accommodate a large variety of plants, shrubs and trees, their watering requirements are higher than succulents and herbs. Treat an intensive vegetated roof like any garden or landscape at ground level, but take into account that high winds can be very drying. Usually large intensive vegetated roofs have an irrigation system installed. So what about irrigation or supplemental water – can it still be environmentally friendly? Yes. You can install a traditional active irrigation system or a solar powered system. Pair this with a recycled rainwater collection system, harvested in cisterns at roof deck or at ground level, and you've got the ideal self-sustainable answer to supplemental water and how to power it. Should I fertilize my extensive vegetated roof? There is some dissention about whether it is recommended or not to fertilize an extensive vegetated roof – We believe the answer is yes, as most German roof greening professionals recommend. Use encapsulated slow release fertilizer twice during the first year of establishment in the early spring and fall, and then yearly thereafter for the next 4 years or so. At that point, the natural cycle should take over and enough organic material should have composted itself back into the soil substrate to provide sufficient nutrients to the plants. Don't use soluble N fertilizer as it can get into the runoff. Do I have to worry about a root-resistant protective layer if I only have an extensive vegetated roof with nothing but sedums? Yes! Just because you design a vegetated roof with tiny horizontal roots, it doesn't mean you can't get a surprise plant with a huge taproot looking for water however they can get it. Seeds come in from many sources – the wind, and from those beautiful birds that you love seeing on your vegetated roof so much. We've seen oak and maple seedlings growing in ½" of rotted compost on a pitched roof, for example. And, believe me, those roots will seek water wherever possible in times of drought – way into the asphalt membrane! If your roofing membrane is organic in nature – asphalt, asphaltic bitumen, etc., you need a root barrier, it doesn't matter how low growing your extensive vegetated roof plants are. It needs to be a dense inorganic material that inhibits root penetration, like polyethylene. This protective layer can be a heavy duty pond liner (EPDM, etc.) or other non-organic element that contains an injected root repellent, such as a copper element. Also, many North American drainage products bypass a separate root barrier layer and now incorporate a root-repellent ingredient in their synthetic filter fabric, placed atop the drainage layer. Does the soil really have to be "engineered?" Why can't I just put top soil on my vegetated roof?" Yes! Regular garden soil is heavy, can contain pathogens, undesirable insects, and WEEDS! Basically, you don't know what's in it. Think potting soils in that really they are engineered soiless media. Vegetated roof "soils" need to be lightweight to conform to roof loading weight restrictions, drain properly and yet retain a certain amount of rain water. Some designers will approve a certain amount of topsoil to reduce costs, but great care needs to be taken with this option. In this case, a typical mix is to use 1/3 clean topsoil, 1/3 compost, 1/3 perlite or other inorganic material. Should I worry about flammability issues? Yes! Always have access to an adequate water supply just in case of fire, which can occur to any type of building. First of all, choose plants that are inherently non-flammable – succulents or others that store water in their stems, and stay away from ornamental grasses and certain mosses that could become kindling material in an extremely dry situation. For example, Miscanthus can be very flammable. Succulent plants and a high inorganic soil media can actually act as a fire barrier. But make sure that your roof has a 12-24" perimeter of vegetation free zone around the edges of the roof – both for a fire break and for sure-footed access of firefighters to the roof. This can be crushed gravel, pebbles or pavers. It depends on the type and size of the vegetated roof. A roofing professional should if it is too large or intensive or you can work out an arrangement with the roof maintenance staff of your building. And if you are referring to an elaborate hotel roof garden, for example, there will be a garden staff for maintenance needs already. Extensive – As low as about $9/sq .ft. for 3" of growing media and sedums but more commonly the range is between $14 - $25/sq. ft., (including roofing membranes). Intensive– $25 - $40 and up. Which is the most expensive component of a vegetated roof? Just like any roof, the waterproofing membrane (or membranes) is the single highest cost item of a new roof. Why shouldn't you plant just sod or grass on a roof? Sod roofs resulted from a lack of natural resources, so people had to use the only materials they had at hand. But a monoculture of plant species is never considered healthy nor is desired in a land or roofscape for a variety of reasons: It would be open to plant disease, or an insect infestation could wipe it out. A monoculture is simply not ecological in nature – a plant community should be ecologically diverse – with many types of vegetation to be. Grass or sod needs to be watered, fertilized, and cut regularly, so they would incur greater costs be considered high maintenance. Are vegetated roofs necessarily green in color? No! Many vegetated roofs appear red, orange, yellow and all colors in between at different times of the year because the fleshy leaves of various succulents change colors throughout the year. Also, vegetated roof plants are not all evergreen, nor should they be. The beauty and anticipation of the change of seasons add to the color palette. And of course, flowers vary, too, in colors from whites to yellow, pinks, deep reds and purples and blues. A virtual living carpet or tapestry varies from season to season as plant communities naturally migrate in their random regenerative patterns. |
Sustain the life of your roofThrough the implementation of an aggressive Roof Program, the life of a roof can be significantly extended. A Roof Program will result in roofs that last 25-50% longer making your roof more sustainable and a better investment. Simon Roofing commissioned a Fortune 500 research firm to conduct a study to research the most dangerous trends facing building owners today. These statistics and much more can be found in the Five Most Dangerous Trends Facing Building Owners Today. |
 |