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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"
Example of a slate roof system
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
Caution should be exercised when roof decks are constructed out of the following
- 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
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 oneven 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
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 30Âº 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.
An ice dam protection membrane should be applied starting at a roof's eaves and
extending upslope a minimum of 24 inches from the exterior wall line of a building.
For slopes less than 4:12 (18 degrees), a minimum of 36 inches is recommended. See
Figure 1 - Example of ice damming
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 Â¾ 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
Flashings for slate roofs fall into four categories: perimeter edge metal, penetrations,
valleys and vertical surfaces. See Figure 2.
Figure 2 - Basic sheet metal flashing components
- 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.
Figure 3 - Example of through-wall metal counterflashing inset in masonry mortar
Figure 4 - Example of metal counterflashing embedded in masonry mortar joint
Figure 5 - Example of surface-mount metal counterflashing
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 covering or in the case of slate, failure in the
slate itself. Please read NRCA's consumer
advisory bulletin addressing roofing warranties
for more information. Once
the project is complete, be sure the contractor provides you with a certificate
for your records.
Second, the roofing contractor will provide you with a warranty covering his workmanship.
Typically, this will cover installation and related issues. The warranty should
contain what items are covered and what will void them. Many contractors offer one
year or two years of coverage; however, there is no industry standard.