Types of Roofs and How They Can Be Made Cool
There are many types of roof systems available, but the surface exposed to the sun is the one that determines if a roof is cool or not. You can usually make a new or existing roof cool by selecting the appropriate surface.
Cool roof coatings are white or special reflective pigments that reflect sunlight. Coatings are like very thick paints that can protect the roof surface from ultra-violet (UV) light and chemical damage, and some offer water protection and restorative features. Products are available for most roof types.
Low Sloped Roofs
Single-ply membranes are pre-fabricated sheets rolled onto the roof and attached with mechanical fasteners, adhered with chemical adhesives, or held in place with ballast (gravel, stones, or pavers).
How they can be made cool: Reformulate or coat black membranes to make them reflective.
Built-up roofs consist of a base sheet, fabric reinforcement layers, and (usually) a dark protective surface layer.
How they can be made cool: The surface layer can be made different ways, and each has cool options:
· Substitute reflective marble chips or gray slag for dark gravel in a flood coat of asphalt
· Use reflective mineral granules or a factory-applied coating rather than a dark coating on a mineral surfaced sheet
· Apply a cool coating directly on top of a dark asphaltic emulsion coating.
Modified bitumen sheet membranes have one or more layers of plastic or rubber material with reinforcing fabrics, and are surfaced with mineral granules or a smooth finish. These can also be used to surface a built-up roof—known as a "hybrid" roof.
How they can be made cool: Pre-coat with a cool roof coating at the factory.
Spray polyurethane foam roofs are constructed by mixing two liquid chemicals together that react and expand to form one solid piece that adheres to the roof. Foams are highly susceptible to mechanical, moisture, and UV damage, and rely on a protective coating.
How they can be made cool: The protective coatings are usually already reflective, and offer cool roof performance.
Steep Sloped Roofs
Shingle roofs consist of overlapping panels made from a variety of materials such as fiberglass asphalt, wood, polymers, or metals.
How they can be made cool: Buy cool asphalt shingles, which use specially coated granules that provide better solar reflectance. (Coating existing asphalt shingles to make them cool, however, is not normally recommended or approved by shingle manufacturers.) Other roof shingles can be coated at the factory or in the field to make them more reflective.
Tile roofs can be made of clay, slate, or concrete. Tiles can be glazed to provide waterproofing or coated to provide customized colors and surface properties.
How they can be made cool: Some are naturally reflective enough to achieve cool roof standards, and surface treatments can transform tiles with low solar reflectance into cool roof tiles.
Low and Steep Sloped Roofs
Metal roofs are available with natural metallic finishes, oven-baked paint finishes, or granular coated surfaces.
How they can be made cool: Unpainted metals are typically good solar reflectors but poor thermal emitters, so they rarely satisfy low slope cool roof requirements. Painting a metal roof can increase its solar reflectance and thermal emittance, allowing it to achieve cool roof status. Alternatively, you can apply cool reflective coatings.
Green Roofs
You may also consider installing a green roof. Green roofs are ideal for urban buildings with flat or shallow-pit roofs, and can include anything from basic plant cover to a garden. The primary reasons for using this type of roof include managing storm water and enjoying a rooftop open space.
Green roofs also provide insulation, lower the need for heating and cooling, and can reduce the urban heat island effect. This roof type can be much more expensive to implement than other efficient roof options, so you should carefully assess your property and consult a professional before deciding to install a green roof.
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