Roofing slate is a dense, durable, naturally occurring material that is essentially nonabsorbent. Two properties of slate are cleavage and fracture. It has natural cleavage, which permits it to be easily split in one direction. Fracture, usually occurring at right angles to the cleavage, is called the grain. Roofing slate commonly is split so the length of the slate runs in the direction of the grain. The surface texture of slate after being split for commercial use derives from the characteristics of the rock from which it was quarried. Some slate splits to a smooth, practically even surface, while other yields a surface that is rough and uneven.
The color of slate is determined by its chemical and mineral composition. Because these factors differ in various regions, roofing slate can be obtained in a variety of colors. In addition, exposure to weather causes slate to change color. The degree of change varies depending on the slate. Slate exhibiting minimal color change is known as "permanent" or "unfading" slate. Slate that shows a more marked color change is known as "weathering" slate. Between unfading slate and weathering is "semi weathering" slate.
There are several classifications for slate roof systems. The first is standard slate, which refers to slate that generally is from 3/16 inch (5 mm) to 1/4 inch (6 mm) thick with uniform length. The category "standard smooth" refers to standard slate that has a relatively smooth surface, in comparison with "standard rough" or just "rough". Rough slate has a rougher texture and generally is available in thicker pieces. Finally, there is "graduated/textural slate," which is designed with varying lengths and thicknesses and generally is rougher than standard slates.
NRCA recommends slate be applied over continuous or closely spaced wood decking. When plywood is used, NRCA recommends the use of a minimum 5/8 thick nominal exterior-grade plywood.
Caution should be exercised when roof decks are constructed out of the following materials:
Oriented strand board (OSB): NRCA is concerned with potential fastener-holding problems and dimensional stability because of the effects of moisture where OSB and other nonveneer products are used as roof decking.
Preservative-treated wood: Many roofing material manufacturers recommend wood roof decks be constructed with wood that has been treated with a nonoil preservative pressure treatment or with nontreated air- or kiln-dried lumber.
Fire-retardant-treated wood: Because of the deterioration of some fire-retardant-treated wood panels caused by premature fire retardant activation caused by heat history in service, the use of fire-retardant-treated wood panel decks should be carefully considered.
Underlayment (or "felt paper" as it is frequently called) is installed over the roof deck before the application of slate. An underlayment performs two primary functions: it provides temporary weather protection until the slate is installed, and it provides a secondary weatherproofing barrier if moisture infiltrates the slate roof covering. Many slate roofs have outlived the underlayment felts over which they were installed. Therefore, an underlayment's service life should be comparable to the design service life of the slate roof covering.
Asphalt saturated, nonperforated organic felts are among the most common underlayments; they commonly are designated as Type 15 and Type 30 or referred to as No. 15 and No. 30, which are reflective of a once used pound per square weight designation. The terms Type I and Type II now are used within the industry in lieu of No. 15 or No. 30, respectively.
Another type of underlayment is a synthetic underlayment. It is characterized as being lightweight, water-resistant and less likely to wrinkle; having high tear strength; and being easy to walk on—even when wet. Theoretically, the product may be left exposed to the elements for extended periods of time. Although synthetic underlayments and their purported attributes seem appealing, there are significant issues to consider before using them. To date, there are no applicable ASTM standards for these products. Many synthetic underlayments do not meet current building code requirements, so manufacturers need to obtain a code evaluation report for code compliance.
NRCA recommends a minimum of one layer of No. 30 asphalt-saturated felt applied horizontally in shingle fashion on roof decks having a slope of 8:12 (34 degrees) or more. Where weather conditions are severe and hard wind-driven rains are common, NRCA recommends a minimum of two layers of No. 30 asphalt-saturated felt applied horizontally in shingle fashion. For roof slopes of 4:12 (18 degrees) to 8:12 (34 degrees), a minimum of two layers of No. 30 asphalt-saturated felt are recommended as long as standard-size slate with 3-inch minimum headlap is used. NRCA does not recommend installing slate roof systems on roof slopes less than 4:12 (18 degrees).
In locations where the average temperature for January is 30º F or less, NRCA suggests installation of an ice-dam protection membrane. An ice-dam protection membrane generally is a self-adhering polymer-modified bitumen membrane.
NRCA suggests the use of copper slating nails for slate. NRCA does not recommend unprotected black-iron and electroplated nails. NRCA recommends nails for standard-sized slate are sharp-point, 3/8 inch large flat head, copper-wire slating nails. Nails should be long enough to penetrate through all layers of roofing materials and extend through the underside of the roof deck or penetrate at least 3/4 inch into wood plank or board decks. All roofing slate should have a minimum of two nails, however, slate subject to high-wind conditions and/or thicker slate should be fastened with four nails.
Flashings for slate roofs fall into four categories: perimeter edge metal, penetrations, valleys and vertical surfaces. See Figure 2.
Perimeter edge metal: Depending on the severity of climate, anticipated rainfall and freeze-thaw cycling, the use of perimeter edge metal should be considered.
Penetrations: Plumbing soil stacks, exhaust vents and pipes are flashed into slate roof systems with some type of flat flange that extends around a penetration and is installed under shingles on the upslope of a flange.
Valleys: Valleys that are called "open valleys" are typically lined with sheet metal.
Vertical surfaces: When a roof system abuts a vertical surface, there are four types of flashing commonly used: apron, step, cricket (or backer) and counterflashing.
Apron, step and cricket flashings require some form of counterflashing to cover and protect the top edges from water intrusion. In many cases, the wall covering or cladding material acts as counterflashing. When this does not occur, a metal counterflashing mounted to the vertical surface should be installed. See Figures 3, 4 and 5 for examples.
NRCA does not make any recommendations about which tile or manufacturer to use; however, NRCA does recommend clay tile roof coverings meet standards established by ASTM International.
ASTM C406, "Standard Specification for Roofing Slate"
When purchasing a new roof system, there will be two warranties to consider. First, there will be the manufacturer's warranty. In general, these warranties cover defects in the manufacture of the roof membrane, and vary greatly depending up the specific material and products chosen. The Second is the workmanship. At DC Contractors, we provide a 3 year warranty covering all workmanship unless otherwise specified for your particular project.