Care and Maintenance of Wood Shingle and Shake Roofs

This article reviewed and updated 2014. Adapted from Niemiec. S.S. and Brown, T. D. 1993. Care and Maintenance of Wood Shingle and Shake Roofs. EC 1271. Oregon State University. Originally published in 1988 and revised in 1993. Removed from the catalog in 2009.

A wooden shingle or shake roof once was one of the few types available. Today it is a premium product that costs the homeowner more to purchase and install than common asphalt shingle or rolled roofing product. Architects and building contractors choose wood roofs for their beauty and natural appearance that blends well with the surrounding environment and enhances the structure’s landscaping.

Additionally, wood shingles and shakes can provide superior performance in areas that experience high winds or damage from hail. Although they do have certain limitations (susceptibility to rotting, splitting, and warping), proper maintenance and care can minimize these limitations.

A standard asphalt three-tab roof generally requires little maintenance during the time it is on the roof except for moss control. The most common mosses found on any roof type in the Willamette Valley were Dicranoweisia cirrata and Bryum capillare. Asphalt-based roof coverings also gradually degrade from the effects of sunlight and from the erosion of winds, rain, and snow. Assuming that moss is controlled, asphalt roofs tend to have a fixed term of usefulness that rarely can be extended.

Most wood shingle and shake roofs are made from western redcedar (Thuja plicata) because of its exceptional properties, including defect-free straight grain, dimensional stability, low weight (low density), impenetrability to fluids, and—probably most important—decay resistance, derived from natural substances found within the wood. In this discussion, when we mention shingles and shakes, western redcedar is the species we mean.

Other wood species, notably redwood (Sequoia sempervirens) and cypress (Taxodium distichum), have similar properties and are sometimes used; but decreasing supplies significantly restrict production and distribution. In addition, there is limited distribution of other woods that have been impregnated with preservative chemicals either by pressure-treating (best) or dipping.

Weathering

When wood is exposed to the effects of sunlight and precipitation, its surface begins to change. This change is partly physical and partly chemical, and it is cumulatively referred to as weathering.

The first noticeable change is the color. The initial red-brown color tends to fade, and a graying process begins. This change to a silvery gray is the result of ultraviolet radiation from the sun stripping the surface layer (less than 0.01 inch deep) of certain cell-wall materials.

This first change in color is rather rapid, occurring within the first year of exposure (under more severe conditions, within several months). Gradually, the silver gray will change to a darker, more graphite gray, indicating the colonization of the surface by microfungi that complete the initial phase of the weathering process.

By nature, wood rapidly attracts water to its surface. When this happens, the wood swells. As it dries and the moisture content decreases, it shrinks. This repeated wet–dry cycling causes the development of compression and tension stresses; these, in turn, cause microscopic cracks to develop.

Over time, these cracks grow larger and larger, caused by additional wettings or by freezing and thawing. The cracks also trap water and allow wood-rotting organisms to penetrate deeper into the wood. As the water penetrates deeper, the wood becomes harder to dry. With increased moisture, the wood-rotting organisms prolong their destructive activity and damage more wood material than would occur during normal weathering.

As the roof continues to age, the surface and sharp edges from manufacturing are eroded by abrasive particles carried by wind and water and by the action of sunlight. Wood-destroying organisms also continue to slowly degrade the surface and the thicker butt edge. The roof loses its new appearance. Debris from trees accumulates in the valleys and between the shingles.

After each rain, some areas stay wetter longer. Soon, other plants are beginning to grow, primarily lichens and mosses. More debris collects, and the roof appears to be natural and rustic. There are splits developing, and in certain areas cupping and curling begin to become more apparent. Then the leaks begin. Sometimes, this takes 30 years—sometimes, much less.

Care and Maintenance

Good housekeeping for a wood roof requires removing all leaf litter, conifer needles, and debris that accumulate between the shingles and shakes and in the valleys of the actual roof structure. It is best to do this before the fall wet season begins.

The debris that accumulates has two deteriorating effects on the roof. First and foremost, debris tends to retard the shedding of water. Secondly—and more destructively—debris retains moisture that allows wood-destroying fungi to grow.

Wood-rotting fungi require water as part of the life cycle. If you can reduce the amount of water so that the minimum level is never reached, the organisms cannot grow.

To promote drying, it is important to remove overhanging branches that excessively shade sections and retard the drying of the roof surface. Never allow tree branches to touch and rub against the surface of the roof. The mechanical action of this rubbing can literally wear sweeping grooves into the surface of the shingles or can loosen the fasteners that hold them.

Finally, in very woody areas, branches may retard air flow so that surfaces dry very slowly. In all cases, you will need to make sound judgments about pruning, trimming, or removing problem-causing trees. Excessive growth of lichens and mosses is a sure indicator that your roof is not drying properly—and that enough moisture is available to support wood-rotting organisms.

Removing the debris accumulation is relatively simple. In most cases, anyone who is willing to climb on the roof can wash off most of the material with a garden hose. A stiff broom is handy for removing all the leaf and conifer needle litter.

It is easy to remove litter from around chimneys and in valleys; you also need to clean all the keyways (those areas between the individual shingles and shakes). The wet sweeping of the surface in a careful manner also will remove the initial moss and lichen growth.

Professional services sometimes use high-pressure washers (available through rental firms). These are effective but must be used with extreme care because cedar is a soft, low-density wood. Excessive or imprudent use of high-pressure systems can detach shingles or erode many years of wear in moments—even though the roof will “look like new.”

If you do not choose to climb on your roof, you can find professional services in your phone book, listed either under “Moss control” or “Roofing contractors.” As you would on any service contract, choose a reputable firm that can provide proof of insurance, references if you ask for them, and written descriptions of the work to be done and the estimated amount you’ll be charged (licensed, bonded and insured).

Roof Safety Precautions

  • Use footwear that will provide good traction and grip the roof surface (avoid leather-soled shoes).
  • Use a ladder that meets all safety requirements. Be sure to set it on stable ground.
  • Your ladder should extend at least 3 feet above the roof line. Tie it off at the top.
  • Use a bucket and rope to transport tools, etc., to the roof. Don’t carry them up the ladder.
  • If possible, walk across the roof rather than up and down.
  • On slopes with a 5–12 pitch (about 23°) or greater, use safety lines and belts. On roofs higher than 16 feet from eaves to ground, use safety lines and belts when you work within 6 feet of the edge.

Cleaning Treatments

There are several types.

  1. You can easily prepare cleaning solutions to help remove dirt and mild stains. The scrubbing action may also dislodge light growths of molds and mildew.
  2. For more persistent stains, bleaching formulations will be recommended.
  3. In some cases of excessive moss growth, you can apply pesticides to manage the mosses and fungi that may be present.
  4. You may sometimes apply preservative solutions that will retard the growth of fungi and mosses.
  5. Additives such as pigments and ultraviolet absorbers can slow deterioration caused by sunlight.

Note that the following cleaning solutions and pesticides will perform differently under various environmental conditions. As a result, the frequency of application required for long-term protection depends on the prouduct you use, the amount of precipitation, average temperatures, and the roof exposure conditions.

The ingredients we recommend are available in products found in various retail outlets.

Cleaning Solutions

You have removed the debris from your roof, and you can see light staining or irregular discoloration. Cleaning the roof with a mild detergent solution may provide an improvement.

This is the recommended solution:

  • 3 oz trisodium phosphate (available in paint or hardware stores)
  • 1 oz laundry detergent
  • 1 quart of 5% sodium hypochlorite (liquid bleach such as Clorox)
  • 3 quarts warm water

Apply the mixture and scrub the surface with a soft brush or lightly with a broom. Then rinse the surface with water to eliminate any residue. If you accidentally splash any plants, rinse them thoroughly. If your gutters exit directly into city storm drains (and ultimately to streams and rivers) you may wish to use alternative solutions without phosphate.

For more persistent stains, you can increase the sodium hypochlorite (chlorine bleach) concentration, even to the point of using it full strength from the bottle for spot applications.

Formulate stronger solutions by using granular chlorine (calcium hypochlorite), mixed at a rate of 2 oz per gal water. This chemical is used for algae control in swimming pools and is available through pool supply outlets.

For safety, wear eye protection, rubber gloves, and aprons. It is best to apply these bleaches with brushes for small spots; spray large areas (use plastic or stainless steel sprayers).

Do not let bleaching solutions stay on wood more than hour before rinsing. These solutions provide more uniform results when you use them in lower concentrations; repeat the application, rather than use a single high concentration when you start.

It is best to use these solutions out of direct sunlight. Apply bleach only to the stain. Avoid drips, runs, and splashes of bleach or excessive bleaching on the roof surface—these can mar the roof’s appearance. It is not necessary to scrub bleached areas, but rinse the wood thoroughly.

Pesticides for Moss Control

It is extremely important to be careful when you apply the chemicals listed below; otherwise, you can seriously harm surrounding vegetation and—more important—people or pets. These products often carry prominent warnings about exposure or ingestion.

The products you apply to roofs to remove growth or prevent its recurrence should be handled with care! If you have any doubts about safe handling, write the manufacturer, see your county Extension agent, or phone the closest poison control center (1-800-222-1222).

When there is so much moss that patches or mats are present, abrasive removal by sweeping may not remove it all. Scraping may not be possible because of the contours of the shakes—and it may damage the roof surface. In these cases, applying pesticides or using high-pressure washers (or both) may be a more effective method for removing the residual growth. Remember that the wood surface may not dry under a covering growth of moss, and this wet condition promotes the growth of wood-decaying fungi.

Pesticides used to manage moss are recommended in the following order of effectiveness (most effective first):

Zinc Sulfate (Monohydrate) Mix 3 lb powder in 6 to 9 gal water and apply to 200 sq ft to 1,000 sq ft of moss growth. Apply higher concentrations to exceptionally heavy growths or where conditions favor moss growth. Use a watering can or pump sprayer. Do not use this solution if you have copper gutters, downspouts, or flashings. Zinc sulfate corrodes copper.

Potassium or Ammonium Salts of Fatty Acids These soap-based, biodegradable products kill the moss by penetrating the cell walls and changing the membrane characteristics, causing the loss of cell contents. These products are noncorrosive and offer minimal hazard to people and animals. Even so, overspray on plants and foliage still requires thorough rinsing.

d-limonene Limonene is a colorless liquid classified as a cyclic terpene. The more common D-isomer possesses a strong smell of oranges. Limonene is frequently included as an ingredient in cleaning solutions, particularly those that are designed to cut grease or remove wax or oil. D-Limonene is obtained commercially from citrus fruits by centrifugal separation or steam distillation. Limonene is used in a variety of foods and beverages and is classified by the U.S. Food and Drug Administration as a Generally Recognized as Safe (GRAS) compound when used as a food additive or flavoring.

When you apply any of these products, they are more effective if the moss is actively growing than if it is dormant. Moss grows vigorously in the rainy seasons—in western Oregon and Washington, during the late fall, winter, and spring. There is very little growth during the summer.

If you plan to apply the products during dry periods, it is often recommended that you wet the moss first to promote chemical uptake. To control runoff and to increase effectiveness, apply when rain is not likely, for it may be several days before the growth is killed and you can effectively remove it by sweeping it from the surface of the shingles or shakes.

When handling and disposing of treated moss and litter, remember to use care to avoid accidental exposure.

Some products listed above provide minimal protection against future growth, depending on the concentration applied and the exposure conditions. As a whole, however, their long-term effectiveness is limited; consider them only to kill existing growth.

The corrosive nature of these chemicals means that you must prevent unnecessary damage by thoroughly rinsing metal tools, gutters, and flashings that have come in contact with the solutions.

Preservative Treatments

Products may also be applied to red cedar shingles and shakes that will retard the growth of mosses and (more importantly) wood-rotting fungi. These chemicals are most commonly referred to as wood preservatives, and they are an effective means of extending the life of your roof.

When you apply these products to your roof, the roof condition at the time of treatment will determine the actual amount of increased longevity. If a roof has aged for many years, the treatment will not be as effective—simply because the years of exposure will have taken their toll. In all instances, except when the roof is badly deteriorated, you can expect these products to extend the life of your roof.

When you apply these products to shakes and shingles, you need to remember several important considerations.

First, the coating should be as uniform as possible. Avoid drips and runs. It is better to apply several light coats and obtain good absorption than to try and force it in all at once. The important point is that you are trying to reach a certain level of preservative retention. If the retention level is too low, your treatment will be ineffective.

Second, since much of the decay occurs in the butt region of the shingle or shake, it is a good idea to be conscientious when you apply the solutions, to cover the exposed butts as well as the exposed shingle surface.

The three methods available to provide proper application are brushing, using a thick-napped roller, or spraying. Because of the irregular surface, you will probably get a more uniform coating with a multi-gallon, pump-type sprayer set to produce a coarse spray.

Spraying also requires a calm day (wind less than 4 mph) to prevent overspray from drifting over plants or into your neighbor’s yard. Brushes are also effective because they allow for thorough treatment of the keyways and ends.

The materials listed below will most likely be part of formulations in over-the-counter products or part of the formulations applied by roof servicing contractors. These materials are also available in concentrated form through specially licensed supply companies. This is because of the toxicity, possible problems in preparation, and the limited demand from homeowners. In every case, regardless of the concentration, be extremely careful when you apply, handle, expose, and store it. When you purchase over-the-counter products for long-term moss control, be sure that one of the wood-preserving chemicals listed below is on the label.

Currently, the preservatives in the following sections are available as either aqueous solutions or oiled-based products.

Copper Naphthenate This is an effective preservative well-suited for applying to roofs. Solutions should consist of at least 1 percent copper (2 percent is better); apply them at a rate of 1 gal/100 sq ft for shakes, and 1 gal/150 sq ft for shingles. Using this concentration will provide up to 5 years of protection in the Pacific Northwest. Note that copper naphthenate has a greenish color that may prove unsightly. To overcome this, choose a solution that is pigmented to simulate the red-brown cedar tones—or use another type of preservative.

Copper 8 Quinolinolate This is a safer alternative to those listed above, but some tests indicate its effectiveness is short lived—not much more than 12 months to 18 months.

Polyphase Polyphase is 3-iodo 2-propynyl butyl carbamate and can be used as an additive. In solutions where concentration levels are greater than 0.58 percent to 2 percent active, you might get 2 years of protection. The durability and long-term performance of this product is still unknown, but it is limited because of light and water sensitivity. Industrial use only.

TBTO TBTO is tri-butyltin oxide. Some literature indicates its use is questionable in treating existing roofs because of mold and mildew resistance and chemical degradation.

Zinc Strips

For those who prefer not to use toxic chemicals, there appear to be safe treatments to control moss. Galvanized metals have a surface coating of zinc. For example, if you fasten a 2-inch-wide strip to either side of the ridges and run it the entire length of the roof, rain will leach zinc from the metal’s surface and wash it down the roof.

As noted earlier, zinc can effectively kill or retard the growth of mosses and fungi. Its effect is prominent below chimney flashings and vents.

There are no scientific studies to verify how long it will remain effective or how much metal is required. Its effect below chimney flashings can be seen for at least 15 feet and appears to last for the life of the flashings.

If your roof is so large that the material at the ridge does not retard moss growth at the bottom, place more strips across the surface. After a year, you should be able to judge how well the strips are controlling the growth of moss; add more if they are needed.

In one case, on a roof considered a favorable site for moss growth, two strands of 10–12 gauge copper wire had been placed, one near the ridge and one some 15 ft below (at the halfway point on the slope). No moss was observed growing. As with the zinc strips, we couldn’t find any supporting scientific studies to confirm these methods—but those willing to try may obtain favorable results.

Oil Treatments

Oils are sometimes advertised to replenish “the natural oils in the wood.” In red cedar, these oils account for less than 3% of the total mass. While they do contribute to dimensional stability and decay resistance, applying “replenishing oils” to weathered shingles may be of questionable value, especially if they contain no preservative or water-repellent chemicals.

You might get some temporary benefit, however, with regard to controlling cupping and warping. But some information indicates that applying oils may have some limiting factors. The main limitation is that unless you include an effective mildicide or preservative, an oil like linseed may provide an additional food source for wood decay fungi.

Furthermore, petroleum-based products are likely to be oxidized (broken down) by sunlight, to have only a short-lived effect, and to increase the flammability of your roof.

For Further Information

For design criteria, contact an architect, building or roofing contractor, or The Cedar Shake and Shingle Bureau, P.O. Box 1178, Sumas, WA 98295-1178. http://www.cedarbureau.org/

References

Buchanan, B. 1981. Evaluating various preservative treatments and treating methods for western redcedar shingles. Texas Forest Service Publication 128, Texas A&M University.

Ormrod, D.J., and van Dalfsen, B. 1993. Wood preservation on the farm. British Columbia Ministry of Agriculture, Fisheries and Food.

Sherwood, G.E. 1975. New Life For Old Dwellings. USDA Agricultural Handbook 481. Washington, DC: Government Printing Office.

See also the following website: http://www.watoxics.org/files/moss.pdf