Wood shakes and wood shingles are manufactured from western red cedar, cypress, pine and redwood trees. Shakes are split from logs and reshaped by manufacturers for commercial use. They are thicker at the butt end than shingles; generally one or both surfaces are split to obtain a textured effect. A split and resawn shake has a split face and sawn back. A taper sawn shake has a natural taper and is sawn on both sides. Wood shingles are sawn on both sides and have an even taper and uniform thickness. When applied to shingles, the industry terms "Perfection" and "Royal" mean 18 inch and 24 inch lengths, respectively.
Cedar shakes and cedar shingles are available pressure treated with fire retardants and chemical preservatives for increased fire resistance and to prevent premature rot and decay in some climates.
Pine shakes are made from southern yellow pine and are taper sawn. They also are available pressure treated with preservatives to protect against decay and insects. Interlayment felts are required for pine shakes.
Wood roof systems may be applied over continuously or closely spaced wood decking or over a spaced, sometimes referred to as "skipped," sheathing. Solid roof decking or sheathing should be used in areas of the roof deck where an ice dam protection membrane is required.
The most common materials used for roof decks are plywood or oriented strand board (OSB). When plywood is used, NRCA recommends the use of a minimum 15/32 thick or Â½ inch nominal exterior-grade plywood for 16-inch rafter spacings and 5/8 inch nominal thickness for 24-inch rafter spacings. For OSB, NRCA recommends a minimum 15/32 inch thick or Â½ inch nominal exterior-grade OSB for 16-inch rafter spacings.
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 and interlayment
Asphalt saturated, nonperforated organic felts are among the most common underlayments used for wood shakes and wood shingles; 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.
When underlayment (or "felt paper" as it is frequently called) is specified, No. 15 or No. 30 asphalt-saturated, nonperforated felt should be applied shingle fashion on roof decks having a slope of 4:12 (18 degrees) or more. NRCA does not recommend using wood shakes and wood shingles on slopes less than 4:12 (18 degrees).
In the case of wood shakes, these sheets are produced in 18 inch (450 mm) widths as "interlayment" felts; that is, they are applied between courses of wood shakes rather than directly over a substrate. See Figure 1.
Figure 1 - Example of a wood shake roof system with underlayment and interlayment
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), NRCA recommends a minimum of 36 inches. See Figure 2.
Figure 2 - Example of ice damming
Wood roofing may be attached to a roof deck with noncorroding, galvanized steel or stainless steel nails or noncorroding metal staples. A minimum of two fasteners should be used to attach each shake or shingle. 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.
Flashings for wood roof systems fall into four categories: perimeter edge metal, penetrations, valleys and vertical surfaces.
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 wood 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 joint
Figure 4 - Example of through-wall metal counterflashing embedded in masonry mortar joint
Figure 5 - Example of surface-mount metal counterflashing
NRCA does not make any recommendations about which shake or shingle products or manufacturers to use. There are no ASTM standards for wood roof coverings; however there are standards for grading. Cedar Shake and Shingle Bureau (CSSB) Standard CSSB-97 contains grading rules for wood shakes and wood shingles. NRCA recommends that cedar shakes and cedar shingles be a minimum No. 1 grade, which requires 100 percent edge grain, clear heartwood and no face defects.
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 failures in the roof covering product. 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.