By John Leeke

Photo Courtesy of Tamko Composites molded from real slates, such as Lamarite from Tamko, can replicate the actual thickness and texture of stone in lighter, man-made materials.

Composite Slates
Since the year 2000, the slate stand-in market has been joined by several products that, while quite diverse in individual make-up, can generally be grouped under the heading of composite slates. What relates these products as a category is the way each takes particles of one sort or another and binds them together with plastic polymer resins, such as polyethylene. The particles themselves come from a wide variety of sources. For example, pulverized natural stone, such as slate, shale, limestone, or clay, may be employed as a low-cost aggregate that helps block UV (ultraviolet) radiation from the sun. Some manufacturers add recycled materials, such as shredded tire rubber, to provide flexibility. Carbon black may be included to block UV rays and slow down deterioration of the resin, while cellulose or fiberglass is sometimes incorporated as reinforcement. The composite mixtures are then heated and compressed into molds that imitate the shapes and textures of traditional roof materials such as slates or even wood shingles. Some products have layered or laminated construction.

Recently, manufacturers using recycled particles seem to be moving from postconsumer materials (which can be highly variable) to postindustrial materials (which are more consistent). When the resin binder used to make the composite comes from a recycled source, the resulting shingle is sometimes considered to be environmentally friendly. However, the variety of materials in the composite may complicate the high-value recycling of installation waste—and recycling of the slates themselves when they reach the end of their service period. Though manufacturers test their products in accelerated weathering chambers that scientifically suggest they will have considerable life, the wide range of ingredients also makes their long-term performance difficult to anticipate. For example, if UV deteriorates the binders, there is the potential to release particles at the surface, so manufacturers try to formulate and design their products to prevent this scenario. When considering composite slates for your roof, listen to an experienced roofing contractor who knows the products and how they perform in your region. Remember that the only certain indication of durability is the test of time.

Polymer Slates
Substantially different but equally new is another category of slate stand-ins made almost entirely from plastics. In this technology, mixtures of polymer resins, such as polyethylene or TPO (the brand name for thermo-plastic polyolefin), are injected into molds under high pressure (sometimes after adding fillers and other modifiers), to produce a slatelike product. Since these shingles are a solid resin that has “knitted” together in long strands at the molecular level, they are not layered and cannot fail by delamination. The TPO plastic polymers have been used in the automotive industry since the early 1970s, and their performance and strength characteristics out in the weather are proven and well understood. Polymer roofing technology has a proven track record in the short- to mid-term (three to 15 years) with very limited UV breakdown taking place at the weathering surface.

Environmental ratings are high when these shingles are made from recycled TPO and other post-manufacturing materials. High-value recycling at the end of the shingles’ life is engineered-in when the resin is TPO, because it is chemically designed to be readily recycled. Like most composite slates, polymer slates are light, which can be an asset if the roof structure cannot carry the weight of natural slate.

Asphalt Shingles
When they first appeared in the mid-1990s, asphalt shingles that emulate slate stood in the shadows of the immense popularity of colored wood-shingle asphalt roofing products. In recent years, however, asphalt slate stand-ins have clearly come into their own through advances in colors and creative shingle patterns. In their simplest forms, asphalt shingles are composed of felt or fiberglass mats saturated with asphalt, cut into shingle shapes like the ubiquitous three-tab strip, then covered with granules of stone. Throughout the 20th-century manufacturers refined and improved asphalt shingles with occasional major developments, such as the introduction of fiberglass mat bases in the 1970s.

New ways of exploiting these technologies are what gives many asphalt slate stand-ins their enhanced veracity. Employing laminated shingles, where two or more asphalt-and-mat layers are bonded together, creates a thicker shingle and more dimensionality and slatelike appearance. Cutting the shingles in clever patterns adds to the effect by evoking strong shadow lines along courses. Some manufacturers have also devised new ways of subtly varying shingle colors—both from shingle-to-shingle and across individual shingles—to visually create the volume and natural color variations of stone.

Compared to other slate stand-ins, asphalt shingles are a well-known roofing type with predictable performance and a reasonable life-cycle cost. While basic asphalt shingles typically last only 15 to 25 years, the added material of laminated shingles often adds to this life expectation.

Concrete & Clay
Two of the oldest and most durable man-made roofing materials are also offered in slatelike tiles. Clay roof tiles, produced for millennia by shaping natural clay and firing it in a kiln, have been manufactured in slatelike shapes and glazes off and on since 1900 and are in production again from the tile manufacturers. Rather than the traditional half-round shapes of lapping barrel or Spanish tile, slatelike tiles are flat and interlock at their edges to create the water-repelling integrity of the roof. The relatively thick ceramic material is by nature a good visual match with natural slate and is glazed in stonelike tones to complete the masquerade.
Concrete tile, an invention of the modern era, is made from portland cement and aggregates (sometimes perlite to reduce weight) and does not have to be fired. Popular mostly in the Southeast, Southwest, and in California, concrete tile is a roofing material with butt lines at least as thick as clay that is further molded or ribbed by some manufacturers to help it convincingly clone stone. The important characteristic to account for when considering clay or concrete tile slates is weight—which can be at least equal to that of true slate and requires a roof that is framed heavily enough to carry the load. Other considerations are porosity, water absorption, and resistance to freeze-thaw cycles. When installed correctly and in the appropriate regions, clay or concrete tile slate stand-ins can offer among the best fire resistance and durability measured in decades.

John Leeke is a writer and consultant who helps homeowners and contractors maintain their early buildings (26 Higgins St. Portland, ME 04103; www.historichome works.com).