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Roofing with cedar shingles: historically correct and durable

While still common siding material on Cape Cod and the islands, wood shingles have largely been replaced by composition or asphalt-based shingles for roofing. Some buildings – particularly those that are historic – still use wood shingles for roofing. With proper installation and maintenance, cedar shingles can provide a roof or sidewall that lasts a quarter of a century.

Shingles are sawn from wood blocks. They are tapered and generally have a relatively smooth surface. They are less rustic than shakes, which are split from wood blocks and are less uniform in thickness than shingles. Both can be used on sidewalls, as well as roofs. On the Cape, white cedar is most commonly used as siding, while red cedar is used for roofing.

Select durable or treated wood
The most important property to consider when buying shingles is durability: its resistance to decay or rot. Some woods, such as cedar, have natural durability. Using preservative-treated or naturally durable wood for shingles should result in a roof free of decay for 25 to 30 years. Use only the top grade of shingles manufactured with edge-grained heartwood (or treated sapwood).

But even wood that does not decay is subject to weathering. Weathering removes about a quarter-inch of unprotected wood per century from softwoods like cedar on vertical exposures. More wood is removed from roofs because roofs, unlike walls, have the most direct and extreme exposure to rain and sunlight. Shingles are often left to weather naturally, and depending on exposure and climatic conditions, the wood will turn silver, dark gray or dark brown. Applying a finish can reduce weathering.

Apply the appropriate finish

Various finishes and preservatives can be applied to shingles to reduce weathering and potential decay and to obtain a particular color. Some shingles come pre-finished.

Film-forming finishes, such as paint, solid-color stains or varnish should never be used on roofs. Such finishes do not tolerate shrinking and swelling and will crack, providing a place for water to enter. Semi-transparent penetrating oil-based stains are the most effective finishes for roofs. Some stains are specially formulated for use on shingles. The stain should contain a wood preservative and a water repellent. Water-repellent preservatives alone can also be used on roofs, although their life expectancy is less than that of semi-transparent stains.

The first coat of finish is best applied before shingles are installed so that the back, butt-end and face of each shingle are thoroughly coated. Dipping is the most effective method. Dip and dry vertically before installing.

Install properly

Well into the 19th century, shingles were fastened to widely spaced nailing strips without the use of tarred or asphalted felts as a secondary barrier. Today, asphalted felt is used as a secondary barrier over sheathing, so wood shingles typically dry less quickly, which is not good news for the service life of the roof.

Providing airspace between the shingles and the felt-covered sheathing vastly improves drying. The airspace can be created by attaching furring strips to the felted roof deck parallel to the trusses or rafters and then by attaching widely spaced nailers perpendicular to the furring strips. Water that gets past the shingles can drain away, and the airspace allows drying. Alternatively, a commercial thick plastic mesh can be applied over roofing felt to create airspace between the shingles and the felt-covered sheathing.

To prevent moss from developing on roofs, use zinc, galvanized or copper flashings. The normal corrosion of these metals provides some control of moss, mold and mildew for 15 feet or more down slope from the metal. So copper flashings that turn green (verdigris) in time are better than copper flashings that presumably do not change color. The metal can be used as a ridge cap, or strips can be placed under the top course with at least one inch exposed.

Perform routine maintenance

Leaves and other debris that accumulate on roofs, particularly in the valleys and gutters, trap moisture in shingles, increasing the likelihood of decay. Roof failure from decay can occur in less than 10 years. To prevent decay, routinely clean loose debris from roofs and gutters. Overhanging limbs and vines that provide excessive shade keep the shingles wet for longer periods, encouraging moss growth and decay.

Source: U.S. Department of Agriculture Forest Products Laboratory

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