Sustainable Roofing Strategies

Lowering Energy Costs, Advancing Environmental Goals
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Advertorial course provided by: American Hydrotech, Inc., Cool Roof Rating Council, Evergreen Slate, NEOGARD®, division of Jones-Blair Company, and Petersen-Dean Roofing Systems
Barbara A. Nadel, FAIA

SLATE ROOFING

Slate is a dense, natural stone that is ideally suited for roofing. Known for its strength, durability, fire resistance, water resistance, and minimum maintenance, slate is practically non-absorbent, with porosity between 0.15 to 0.4 percent. Additionally, slate is impervious to fungus and mold, dimensionally stable, and resistant to climate change inherent in the freeze-thaw cycle. Slate roofs are chemical-resistant, easy to repair, and are classified as  cool roofs. Three common types of slate roofs depend on the type and thickness of slate used:

 

Semi-weathering slate roof Varied slate colored roofing gives architectural effects
Semi-weathering slates complement a stone façade, with random widths for a traditional application.
Photo courtesy of Evergreen Slate.
Varied slate colors, sizes and textures allow many architectural effects, and contribute to the overall building appearance.
Photo courtesy of Evergreen Slate.

 

• Standard Slate Roofsare sloping roofs utilizing commercial slate with one uniform length and width, generally rectangular in shape. Standard roofs differ from other types in texture and appearance. The corners of each slate may be trimmed to give a diamond, hexagonal or Gothic pattern. Patterns can be achieved by laying two or more standard commercial slate over the same area. The appearance of this roof is flat, even, and uniform.

• Textural Slate Roofs are sloping roofs that utilize slate of various sizes, thicknesses, textures, and colors. They are composed of textural slate, which is rougher than standard slate. Textural slates are produced in different sizes and thicknesses. Various shades of textural slate are often used to enhance roof colors. Compared to standard roofs, textural roofs display more character, due to pronounced shadow lines and color variations.

• Graduated Slate Roofs combine the artistic features of the textural slate roof with variations in thickness, size, and exposure. The thickest and longest slates are placed at the eaves. As the slate courses progress to the ridge, slates of gradually diminishing size and thickness are used, creating a graduated effect. Graduated slate roofs are often customized in various patterns.

Slate Colors

Slate roofing tile is available in many rich and varied colors, sizes, and textures that enhance design flexibility and aesthetics. Slate roofs are commonly used for residences, churches, libraries, universities, and government buildings. Colors are determined by the slate's chemical and mineralogical composition.

Exposure to weather causes all slate to change slightly in color. Slates with minimal color change are classified as permanent or unfading. Those which show a more obvious color change are called weathering slates, which offer designers another variation in roof color. When color is important, designers and owners should consider the slate source, its capacity for color change, weathering effects, and specify unfading or weathering preferences. Slate color nomenclature includes black, blue black, gray, blue gray, purple, mottled purple and green, green, purple variegated, red, and weathering green (changes to buff or brown). 
 
Slate Texture

Slate has a natural cleavage, which allows it to be split in one direction. Grain is the second direction of fracture, occurring at right angles to the cleavage. Roofing slates are commonly split so that the length of slate runs in the direction of the grain. The texture of slate after it is split for commercial applications depends on the nature of the rock from which it is quarried. Some slates split to a smooth, even, uniform surface, while others are rough and uneven, providing a range of available roofing surfaces.

Some slate contains narrow ribbons of rock that are different in chemical composition and color from the rest of the slate. If chemical composition of the ribbons doesn't weaken the slate and the color is not objectionable, ribbon slates are acceptable. Clear slate is trimmed to eliminate ribbons. Slate with acceptable ribbons is called ribbon stock.

Slate Standards and Testing

There is no single slate classification in the U.S. because slates vary widely, from ribbons and clear, to smooth and rough surfaces. The American Society for Testing and Materials (ASTM) has developed tests for material characteristics and physical requirements, to measure slate quality used in roofing, including ASTM Designation: C 406-00, the Standard Specification for Slate Roofing.

ASTM classifies slate by the expected service life of three grades, based on geographic location and environmental exposure. Service life refers to the period of time over which the slate material may be expected to require no repair or replacement due to weathering. Grade S1 has a service life of over 75 years, and is highly recommended for roofing. Grade S2 is rated for 40 to 75 years; Grade S3 is rated for 20- to 40-year service life. Three ASTM tests measure qualities of slate:

• ASTM Designation:C 120-90 Standard Test Method of Flexure Testing of Slate, addresses determination of the modulus of rupture and modulus of elasticity of slate by means of flexure testing. S1 minimum: 9,000 pounds per square inch (PSI).

• ASTM Designation: C 121-90 Standard Test Method for Water Absorption of Slate, addresses determination of the water absorption of slate. S1 minimum: 0.25 percent.

• ASTM Designation:C 217-85 Standard Test Method for Weather Resistance of Slate, addresses two procedures for weather resistance of slate in all outdoor installations by determining the depth of softening by an abrasive or by hand scraping. S1 maximum: 0.002 inches.

Roof Maintenance Tips

Maintenance is an essential part of ensuring long-term roof performance. Basic maintenance for all roofs should include inspections at least twice yearly, depending on climate and location. White roofs may need seasonal cleaning because they will show soot and dirt, which causes a loss of emissivity properties. Inspections should occur at least once in the spring and once in the fall, after major storms, to clean out leaves, clear drains, and cut nearby overhanging trees. Contractors, owners, or consultants can perform roof maintenance. During inspections, workers should be careful about walking on roof surfaces and avoid dropping tools and equipment, which can puncture materials and cause leaks.

1997 energy prices
Nationwide implementation of cool roofs could mean an annual savings of $1 billion in cooling costs. Values in this diagram reflect 1997 energy prices; current values are higher.
Courtesy of Lawrence Berkeley National Laboratory.

 

At Arizona State University, the Performance Based Studies Research Group investigates and tests roofing systems, including fluid-applied coatings and foam roofing systems. This non-profit research group has identified effective roofing techniques and ways to reduce risk, along with other studies that benefit the building industry. They have partnered with many public and private organizations to advance knowledge and educate building owners on the long range value of high-performance roofing systems.

 

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Originally published in GreenSource Magazine.
Originally published in July 2007

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