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INFORMATION SERIES: Wood Roofing

Shingles are sawn from wood blocks; they are tapered and generally have a relatively smooth surface. Shakes are split from wood blocks; they are less uniform in thickness than shingles, are sometimes grooved, and frequently have little taper. Shakes may be split and then sawn (from top edge corner to opposite bottom-edge corner) to provide taper as well as a relatively flat side, which is turned away from the weather during installation. Shakes have a rustic appearance. Shingles and shakes can be used on sidewalls as well as roofs, but only roofs are addressed here. However, much of this information is applicable to sidewall installations, especially concerning sealing the back-side of shingles prior to installation.

The most important wood property to consider when buying shingles is durability, its resistance to decay (rot). Some woods, such as western red cedar, have natural durability. The heartwood of old growth western red cedar is rated as extremely durable because of its extractives. However, the generally small amount of sapwood associated with this species is not durable. There is general consensus that some second growth timber, even from a decay-resistant species, is not as durable as the old growth timber. Nevertheless, the durability of any wood decreases as rain or other sources of moisture leach extractives from the wood. For these reasons, the use of a “durable and treated” wood for shingles is increasing. The most commonly used wood for shingles is western red cedar. Treated southern yellow pine taper-sawn shingles are also available. If the shingles are properly treated, other species can also be used. Using preservative-treated or naturally durable wood for shingles should result in a roof free of decay for 25 to 30 years.

Weathering is erosion from sun, wind, debris, and precipitation. Even wood that does not decay is still subject to weathering. The weathering process removes about 1/ 4 inch (6 mm) of unprotected wood per century for softwoods (e.g. cedar) on vertical exposures, but more wood is removed from roofs. Shingles are often left to weather naturally and, depending on exposure and climatic conditions, the wood will turn silver, dark gray, or dark brown. Weathering can be reduced by applying finishes, especially those with pigment. Some treatments retard both decay (preservatives) and weathering (finishes). Proper selection of materials will appreciably influence the service life of wood shingles. Use only the top grade of shingles manufactured with edge- grained heartwood (or treated sapwood). A lower grade of shingles can be used on sidewalls or areas that require an undercourse. Roofs, unlike walls, have the most direct and extreme exposure to rain and sunlight.  Shingles rapidly absorb moisture because their lower edges are end grain, where wood is similar to a bundle of straws. The swelling and shrinking of the wood results in cracks, which can increase the entry of moisture. Decay occurs when nondurable, untreated wood remains wet for sufficiently long periods. Under high moisture conditions, naturally durable and treated shingles last longer than untreated shingles. In warm, humid climates and on heavily shaded roofs, mildew, moss, algae, and lichens can grow; because these organisms retain moisture, the wood will decay with time. In some cases, particularly where warm, humid conditions persist for substantial parts of the year; it is desirable to extend the life of wood shingles with special preservative treatments. For maximum effectiveness and long life, purchase shingles pressure treated in the factory. Chromated copper arsenate (CCA, Type C), copper– 8– quinolinolate, and copper naphthenate are effective wood preservatives when applied at the recommended levels. A CCA treatment on shingles will also decrease weathering.  (Pigments may be required if the green stain from the copper is objectionable.)Wood roofs are more flammable than composition roofs! Wood roofs may be unsuitable in arid regions, in high density neighborhoods, or where appreciable amounts of flammable vegetation grow close to the structure. Application of fire retardants may be required by local ordinances.

Leaves and other debris that accumulate on roofs, particularly in the valleys and gutters, trap moisture in shingles, increasing the likelihood of decay. Therefore, clean loose debris from roofs and gutters routinely. Overhanging limbs and vines that provide excessive shade keep the shingles wet for longer periods, encourage moss growth, and may encourage decay.  Periodically check the roof for moss or lichen growth and apply a chemical treatment if necessary. A solution of 1 quart (1 liter) household bleach, 1 ounce (30 g) detergent, and 3 quarts (3 liters) warm water can be used to clean the roof.  Surface treatment of the roof with selected chemicals can also provide some protection. A solution of copper naphthenate with 3% to 4% metal content, copper octoate with 1% to 2% metal content, and copper– 8– quinolinate with about 1% active ingredient content can be used to control moss, lichens, and surface decay. You can also purchase commercial treating solutions. All solutions are best applied by brushing or dipping. Even if the roof is surface treated, serious decay problems can still occur within the shingled or unexposed parts of the roof that are not treated. Nevertheless, surface treatment helps to lengthen the life of a wood roof by preventing the growth of moss and lichens.  

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