Authentic Roof slates are made from a proprietary thermopolymer olefin compound that consists mostly of recycled postindustrial waste from auto-industry products. The shingles taper from about 1/4 inch thick at the butt end to 1/8 inch thick at the top, and measure 12 inches wide by 18 inches high. They’re available in three standard shapes and six standard “color-through” hues and come packaged in 25-slate bundles that weigh about 42 pounds apiece. It takes roughly 185 slates to cover 100 square feet when the exposure is 6.5 inches (recommended for roof slopes between 7/12 and 9/12).
A square of Authentic Roof shingles weighs between 280 and 330 pounds, depending on the exposure; by comparison, 1/4-inch-thick natural slate roofing weighs at least 800 pounds per square. Roof framing for synthetic slate shingles is the same as for asphalt and metal roofs.
Because Authentic Roof slates are made from plastic, they can be cut, hammered, and walked on with little risk of damage. They have a Class 4 impact resistance (the highest rating), Class A “stand-alone” fire rating (not dependent on the underlayment), and a 110-mph wind resistance rating. They’re also UV-protected and come with a 50-year warranty. The company has a 20-year track record.
Cuts can be made with a utility knife by first scoring the shingle and then snapping it on the score line. For repetitive cuts, we used a Shingle Shark asphalt-shingle cutter (800/231-5647, qualcraft.com). Both methods work fine for straight cuts, but a jigsaw worked better for curved cuts around the tubular skylights and L-shaped and rectangular cuts around the photovoltaic mounting blocks. We even used a portable table saw to cut some slates.
Authentic Roof slates can be scored with a utility knife and snapped.
A Shingle Shark makes quick work of repetitive cuts.
Layout and Starters
When we arrived on site, the roof deck was already sheathed with Zip System roof sheathing (800/933-9220, huberwood.com). This 5/8-inch-thick sheathing has an integral waterproof top layer, and the seams are sealed with a special tape. The product is meant to be watertight without an underlayment, but for added protection and to satisfy Authentic Roof’s installation instructions, we opted for a layer of Grace Ice & Water Shield (866/ 333-3726, graceconstruction.com) at eaves and valleys and around penetrations.
Everywhere else, I used Grace’s Tri-Flex 30, a synthetic roof underlayment. I prefer it to felt because it resists tears better and doesn’t wrinkle when it gets wet. I also like that it’s packaged in lightweight 10-square rolls. We used painted 26-gauge galvanized drip edge and flashing per Authentic Roof’s recommendations.
The company warns installers to mix slates together from the 25-piece bundles and from every pallet they’re shipped in, as inherent color variation could show up as unintended patterns in the finished roof. Our pallets arrived on a tractor trailer without a boom, so we had to hump all the material onto the roof ourselves. The way the shingles are packaged, I can’t see how you could store them on a steep roof anyway, so be prepared for the extra time that unpacking, mixing, and stocking the slates will require. I used a rope and a milk crate to get them onto the roof.
The slates weigh about the same per square as asphalt shingles, but they’re much bulkier. Here, the author uses a milk crate to stock the roof a dozen pieces at a time.
Starter shingles are made by removing 6 inches from the top of the company’s standard 12-inch-by-18-inch slates. The manufacturer suggests several ways to do this, but the fastest way I found was to use a portable table saw equipped with a rip fence. Before installing the starters, I snapped a chalk line so that they would overhang the drip edge by one inch. Once I’d checked that my planned layout would have reasonably sized pieces of shingle at the rakes, I snapped a pair of vertical layout lines as well.
The first shingle course is placed over the starters with the joints lapped halfway. Unfortunately, Authentic Roof slates can’t be installed with a roofing nailer, since the nail has to be placed in the exact center of the molded nail hole, with the nail head flush with the surface of the slate. This holds the relatively temperature-stable slates in place while allowing the roof deck underneath to expand and contract with changes in temperature. We used 1 1/2-inch hot-dipped galvanized nails; copper and stainless roofing nails can also be used.
Authentic Roof slates also have spacing tabs that project from the edges and automatically gap the shingles 3/16 inch apart. Each slate has an exposure guide on the face. Exposures can be adjusted from 6 to 7 inches, although the company recommends a 6-inch exposure for high-wind areas. Using blue chalk to prevent staining, we snapped a line for every course.
The slates have tabs that keep them spaced 3/16 inch apart and a built-in exposure index. During installation, the author snaps a chalk line for each course.
Authentic Roof recommends using W-shaped valley flashings, which have a rib down the center. I use W-shaped — rather than V-shaped — valley flashings on most roofs anyway, because they’re better at preventing water and melted snow from getting under the shingles that line the valley. That’s an important benefit during our harsh North Country winters.
Valleys are lined with peel-and-stick underlayment before the 26-gauge galvanized metal W-flashing is installed; the slates butt directly against the rib in the flashing.
Since they can’t be installed with a pneumatic nailer, the slates are hand-nailed.
I nailed each valley flashing over a layer of Ice & Water Shield, then sealed the edges to the roof deck with additional strips of the membrane. A layer of synthetic underlayment laps over everything.
At the valleys, I cut and installed the slates so that they butt up against the rib. This detail — which minimizes the appearance of the valley — is one that I’ve used successfully with real slate roofs.