Wood has been used as a roofing material for centuries, especially where other traditional materials such as slate were in short supply. The two most common forms of wood roofing are shakes and shingles. Shakes are typically thicker than shingles and were prepared by splitting off sections of material with a maul. As a result, they tended to be rougher and required some type of building paper to limit the risk of water intrusion. Many currently produced shakes have at least one smooth side so that they can be more easily laid on a roof. Shingles are sawn from wood blocks and tend to be thinner and more uniform. Both materials can provide long, reliable service when properly installed and maintained. Shake or shingle roofs also make use of wood, a renewable material, while alternative roofing materials, such as asphalt singles or steel, require large inputs of non-renewable energy for their production.
Almost any wood can be used for roofing, but the preferred woods are more durable and dimensionally stable. The most common wood species used for roofing is western redcedar (Thuja plicata), but coastal redwood (Sequioa sempervirens), bald cypress (Taxodium distichum), and eastern redcedar (Thuja occidentalis) are all locally important. These species are characterized by having a core of wood (called the heartwood) that contains compounds that tend to resist moisture and are toxic to fungi and insects that could colonize the wood. These compounds, called extractives, also make the wood more dimensionally stable. As a result, shingles of these tree species can resist deterioration far longer than woods of other species. Alternatively, more perishable woods can be artificially impregnated with preservatives such as copper-based biocides to prolong their useful life.
All materials eventually degrade, and wood roofing is no exception. Over time, chemicals present in the wood can be depleted and sunlight can degrade the wood surface. However, these effects can be partially offset by periodic maintenance to maintain appearance and prolong the useful life of the roof. First, it is helpful to understand what can damage a shake or shingle roof.
Weathering is a combination of damage from sunlight, rain, and/or wind. As sunlight strikes the surface, its low energy is released into the wood, creating compounds that begin the degradation process. This process is typically slow (a few mm per century) but causes the wood surface to become slightly weaker. Cedar roofs will turn light gray within a year after installation, but the wood beneath retains its original color.
Rain or wind (which can carry tiny sand particles) can wear away the shallow weathered wood on the surface, exposing new wood to sunlight. This process results in a gradual wearing away of the surface. This process typically occurs more rapidly in the softer parts of the wood. In cedar, these parts are at the beginning of each growth ring (called earlywood). Over time, the shake or shingle can take on a washboard appearance.
Repeated wetting and drying can also lead to physical changes in the shake or shingle. Wood swells as it wets and shrinks as it dries. Repeated wetting and drying can induce stresses in the wood that eventually lead to the development of cracks or splits as well as cupping or warping. The wood species used for roofing tend to have lower degrees of shrinkage than other species, but even these materials can eventually develop physical damage.
Wood can be attacked by a variety of organisms, but fungi are the most common. Fungal spores that float through the air or move with moisture settle on a roof. They germinate to produce strand-like cells that grow through the wood cells. Some fungi live only on compounds stored in the wood, while others attack the structural materials that give wood its exceptional strength properties.
The fungi that live on the stored sugars present in the wood and/or on compounds that fall on the shingle surface do not cause structural damage. However, they can be colored marring the wood appearance. For example, cedar roofs are often colonized by dark-pigmented fungi that disfigure the wood surface. These fungi tend to be more prevalent on the exposed surfaces of the shingle where moisture conditions are more suitable for growth. Some of these fungi can detoxify the heartwood extractives, allowing other fungi to attack the wood.
Decay fungi also begin life as small spores and grow into the wood, but unlike other fungi, they produce enzymes that begin to degrade the wood structure. Eventually, these fungi can degrade the wood to the point where it is no longer serviceable. Decay fungi are typically found under the shingle or shake overlap where moisture conditions are more suitable for steady growth. Most species used for roofing are resistant to these fungi, but even these materials can eventually succumb to fungal attack.
Mosses and Lichens
Mosses are primitive plants while lichens are a symbiotic relationship between an alga and a fungus. Both groups of organisms can readily grow on roofs especially in the Pacific Northwest. The most common mosses found on roofs in the Willamette Valley are Dicranoweisia cirrata and Bryum capillare. The growth of these organisms by themselves is not detrimental to the wood; however, lichens and especially mosses tend to accumulate soil and other debris on the roof. Thus, the roof wets quickly and remains wet for longer periods of time. This encourages the growth of fungi and accelerates the decomposition process.
How Long Does this Process Take?
There is no single answer to this question. Durability is closely related to the temperature and amount of rainfall to which the wood is exposed. Wood degrades more rapidly when exposed to higher amounts of rainfall and warmer temperatures. Thus, a roof exposed on the wet, warm side of Hawaii may only last a quarter as long as one exposed in cooler, wet Western Oregon and only a tenth of the time it might last in the dry high desert of eastern Oregon. In addition, the slope of the roof will play a factor since steeper roofs will tend to shed water more quickly. Overhanging vegetation will tend to shade the roof and drop debris on the roof that can accumulate and retain moisture. Wetter conditions encourage the growth of fungi, mosses, and lichens—all of these shorten roof life.
Care and Maintenance
Maintenance is essential for the performance of any material including a wood roof. Roofs need to be regularly cleaned of all leaf litter, conifer needles, and debris to limit the growth of organisms such as mosses and lichens. Debris can be removed by sweeping, leaf blowing, and/or washing. It is best to do this before the fall rains in the PNW.
Vegetation Removal: To the best extent possible, avoiding and removing overhanging branches will improve the life of a roof. Trees can shade the roof, slowing drying between rainfalls. Debris falling from these branches can accumulate on the surface, acting as a water trap. Removing overhanging vegetation will also reduce the fire risk.
Cleaning Gutters: Gutters are designed to move water away from a structure and failure to maintain clean gutters can result in water backups that extend to the lowest course of the roof.
Surface cleaning: Many roof maintenance companies offer a power washing service, although this is more of a cosmetic treatment. This process strips away weaker wood that has weathered and can improve overall roof appearance. Improper power washing can damage the wood surface and fracture shingles. Typically, improper washing involves using excessive water pressure or washing against the downslope of the roof. Power washing will not alter the progression of weathering on the wood surface; the clean roof will begin to weather soon after it is washed. Power washing can be useful for removing moss, lichens, and accumulated debris.
Many professional roof maintenance companies recommend that oil or other surface treatments be applied after power washing. These will be discussed later.
Roof Treatments: Do it yourself vs Professional Care
Roof maintenance is relatively simple, but it may be more practical to hire a professional (licensed, bonded, and insured) depending on the roof, its condition, and your physical condition. There are few precautions that should be taken:
- Use footwear that provides good traction and will grip the roof surface (avoid leather-soled shoes).
- Use a ladder that meets all safety requirements. Best sure to set the ladder on stable ground.
- The ladder should extend at least 3 feet above the roof line and be secured to the roof at the top.
- Use a bucket and rope to transport tools and other items to the roof (do not carry them up the ladder).
- If possible, walk across the roof rather than up and down to reduce the chance of slipping.
- Use safety lines and belts, especially where:
- the roof pitch is 5-12 or greater (>23°)
- the eves are more than 16 feet from the ground
- you are working closer than 6 feet to the roof edge
- Never work alone.
Scrubbing with water or using mild pressure can help remove large pieces of debris from shingles, but cleaning solutions may be needed to remove more deep-seated stains. For all cleaning solutions, wear eye protection, rubber gloves, and some type of coverall to protect clothing. It is best to test any cleaning solution on a small, inconspicuous spot on the roof to make sure that it does not adversely discolor the wood.
One cleaning solution that is often used includes the following:
- 3 ounces of trisodium phosphate (available in paint or hardware stores)
- 1 ounce of laundry detergent
- 1 quart of 5% sodium hypochlorite (bleach)
- 3 quarts of warm water
Mix thoroughly and apply with a soft brush or broom. Scrub lightly, allow to stand for 10 to 30 minutes, then rinse away with fresh water. Be sure to rinse accidental splashes off plants.
Do not include the phosphate if your gutters lead directly into a storm drain as phosphates can foster algal blooms in nearby lakes and streams.
For more persistent stains, repeated applications or the use of stronger bleaching solutions can help. Bleach can be used full strength but be sure to thoroughly rinse the wood surface to remove excess bleach.
Western redcedar shingles are inherently resistant to moisture uptake, which is why they make such a good roofing material. Thus any surface treatment will only penetrate a short distance into the wood. Roofs also experience severe sun exposure and rainfall that result in rapid degradation and loss of any treatment chemicals.
A good material to use should contain a water repellent and an ultraviolet (UV) light inhibitor. Water repellents are usually waxes that resist moisture uptake but they can also include resins that fix to the wood surface. UV inhibitors can include pigments, iron particles, hindered amines, and a host of other compounds that slow, but do not completely prevent the process of weathering. Some treatments also contain fungicides and algaecides (that kill mosses and lichens) but be sure to make sure that these treatments are properly used to avoid damage to plants. Any product that claims to control moss, lichens, or fungi must be registered with the U.S. Environmental Protection Agency or exempt from registration under Section 25(b) of FIFRA (state registration is still required).
While some roof treatments make claims for protective periods up to 10 years, the reality is that most treatments last no more than 1 to 3 years. Some products also make fire retardant claims; be very cautious with these claims as very few fire retardants resist rainfall, and these are typically applied at the factory prior to installation.
The protective period afforded by a surface treatment will depend on the condition of the roof (i.e. heavily worn vs new), the roof pitch, the presence of overhanging vegetation and the climate (rainfall, temperature).
Oil Treatments: Many roof treaters claim that oils applied after cleaning can extend the life of the roof. Cedar and other species contain oils that do impart some moisture resistance and these oils may have a similar effect. The depth of oil penetration is generally shallow, and the materials are exposed to UV degradation. Thus, the protective period applied by these treatments, if any, is limited. They do, however, improve roof appearance for a short time after cleaning.
Moss Removal and Control
When there is so much moss on the roof or the roof condition is too fragile to allow the growth to be removed mechanically, application of a pesticide may be necessary (these treatments can also be used before moss growth is heavy). These products tend to be most effective when applied to actively growing moss. Wetting dried moss prior to chemical application can improve effectiveness. These products can be toxic if mishandled and typically carry warnings about what to do if accidental exposure or ingestion occurs. Call the manufacturer, the Country Extension Agent or the closest poison control center (1-800-222-1222) if there any doubts about safe handling. Pesticides used to limit moss growth on roofs are recommended in the following order of effectiveness:
Zinc sulfate (monohydrate): Mix 3 pounds of powder in 9 gallons of water. This volume will treat 600 to 1000 square feet of roof using a pump sprayer, depending on the wood condition. In areas of heavy growth or where conditions favor moss growth, use a solution of 1-pound powder mixed in 1.6 to 3-gal water to treat 200 sq. ft. Do not use this treatment if you have copper gutters, downspouts, or flashing since zinc sulfate corrodes this material.
Potassium salts of fatty acids: These soap-based biodegradable products kill moss by penetrating the cell walls and changing the membranes so that the cells leak and the moss dies. These products are non-corrosive and have minimal risk to humans and animals. Avoid over-spraying on adjacent plants and rinse any plants that are accidentally treated. Thoroughly rinse any tools or equipment used to apply these chemicals.
Both products will wash off the roof with continued rainfall and thus do not provide long term protection against moss growth. Check the label for appropriate protective gear to wear such as goggles or face shield.
Fungi tend to be more deeply established in the shingle or shake, so it is generally not possible to completely eliminate them; however, there are several surface treatments than can arrest wood decay. These chemicals are collectively termed wood preservatives and can help extend roof life. The effectiveness of a treatment depends on roof condition. Preservative performance will be reduced on roofs with existing decay and heavy weathering compared to more recently installed roofs.
Treatments can be applied by brushing, rolling, or spraying. Read the product label carefully to determine the best application method. Spraying, when allowed by the label, generally works better than rolling or brushing because of the uneven nature of the surface, but care must be taken to avoid spraying on windy days (wind >4 mph). The coating should be as uniform as possible, making sure to coat the exposed end as well as the surface as thoroughly as possible. Cedar is somewhat resistant to fluid uptake so it may be better to apply 2 coats with a drying period in between.
There are a limited number of preservatives that can be applied to cedar shingles or shakes. All are registered with the U.S. EPA for this application and this must be listed on the label for the product. Some are general-use preservatives that be purchased by anyone at the local hardware or building supply store, while others are restricted use and can only be applied by applicators who have passed specific tests administered by their state. Regardless of their classification, all are pesticides and should be handled carefully according to the label. These products are available in water or organic (oil) solvent-based formulations. Oil-based systems can penetrate more deeply, but there have been substantial improvements in the effectiveness of water-based systems.
The available products include:
Copper naphthenate: This is an effective preservative that is ideally suited for roof applications. The system is available for residential use as a ready-to-use solution containing the equivalent of 1 or 2 % copper metal and can be pigmented brown (it is naturally green) to blend in with the natural cedar color. Commercial products may contain up to 8% copper metal equivalent and must be diluted prior to use. One gallon will typically cover 100 to 300 sq ft, depending on wood condition. For shakes and shingles, a rate of 1 gallon per 100 sq ft has been reported to provide up to 5 years of protection in the Pacific Northwest.
Copper-8-Quinolinolate (oxine copper): This is a less toxic alternative to copper naphthenate with many of the same attributes but is much more expensive and not as widely available.
3-iodo-propynyul butyl carbamate (IPBC): is a common additive to paints and coatings. It is effective against a variety of fungi but is sensitive to UV degradation. It is likely to provide only a year or two of protection and is only available in commercially premixed wood preservative coatings.
Tributyltin oxide (TBTO): This chemical is sometimes seen in roof treatments, but it is susceptible to UV degradation and is not likely to provide more than 1 or 2 years of protection.
Zinc strips: Placing strips of copper or galvanized steel (which is protected with zinc) on the roof ridge can help protect portions of the roof. The metal is slowly released from the strip and flows down the roof, inhibiting fungal growth. Multiple strips would need to be used for larger roofs since the metal concentration would dilute as it moved down the roof to the point where it was no longer at an effective level. This protection only functions on the exposed surfaces, but it can help keep the roof clean and bright.
For Further Information
For design criteria contact an architect, building or roofing contractor or the Cedar Shake and Shingle Bureau, PO Box 1178, Sumas, WA 98295-1178.