Red cedar is an extremely durable material even under adverse conditions, but it's useful life depends upon the environment. Cedar roofs left unprotected suffer photo degradation by ultraviolet light (sunlight), leaching, hydrolysis, shrinking and swelling by water, and discoloration and degradation by decay microorganisms.
Photo degradation by sunlight. Solar radiation is the most damaging component of the outdoor environment. Photo degradation due to sunlight occurs fairly rapidly on the exposed shingle surface. The initial color change from the golden, orange-brown color to gray is related to the decomposition of lignin in the surface wood cells. (Lignin is Mother Nature's way of holding wood cells together.) The wood cells at the shingle surface lose their strength and eventually are washed away by rainwater. In addition, microscopic cracks and checks develop, allowing deeper water penetration.
Degradation by moisture. Accompanying this loss of wood fiber at the shingle surface are the shrinking and swelling stresses set up by fluctuations in moisture content. These stresses cause deeper checks and splits to develop. The result: shakes and shingles begin to cup, curt, split, and check at an accelerated rate. Degradation by wood-destroying fungi. The natural decay resistance of western red cedar is due to its heart-wood extractives, including the thujaplicins and a variety of phenolic compounds.
The Thujaplicins contribute to the decay resistance of red cedar while the phenolic compounds and resins give cedar its water repellency and lubricity (slippery surface). Since the natural preservatives in cedar are somewhat water soluble they can be depleted in service. In roof exposure, extractives may leach out in a relatively short time and allow colonization by wood-inhabiting fungi. Aided by favorable climatic conditions, these in turn allow the growth of wood destroying fungi, which ultimately cause the early failure of a roof. The wood becomes soft and spongy, stringy, pitted and cracked or crumbly. This usually occurs first at the butt region of the shingles where they overlap.
An added factor is that more shakes and shingles are being manufactured today from younger, less decay-resistant cedar since old, high-Thujaplicins-content logs are no longer readily available. This shortens the roofs life expectancy even more. How fast a wood roof weathers is a function of slope, direction, and shading. The shallower a roof is, the more likely it is to suffer decay from fungi. South-facing slopes suffer because they experience the greatest swings in temperature and humidity - leading to more splits and other degradation. Shade is also a concern. Wood roofs shaded by trees are more likely to develop mold, mildew, and decay than unshaded roofs.
The older the roof, the more difficult it is to repair satisfactorily. Although you can salvage a roof that requires as many as 30 repairs per square, it may not be cost-effective. A good rule of thumb is that shingle roofs older than 20 years, and shake roofs older than 25 years, will be extremely difficult, if not impossible, to repair satisfactorily. The final decision to repair, clean, and preservative-treat a wood roof is going to be a subjective one. There are no iron-clad rules. But don't go ahead with restoration if the benefits are questionable.
Depending on the condition of the wood roof, there are two methods of cleaning prior to treatment: chemical cleaners (bleaching) and power washing. In many areas of the country, wood roofs are discolored primarily by sunlight and surface mildews or algae. These roofs can be cleaned quite easily and effectively with bleaching agents such as sodium hypochlorite (household bleach) or calcium hypochlorite (swimming-pool chlorine) mixed in tap-water. These chlorine solutions quickly remove the oxidized wood fibers on the shingle surface as well as kill surface mildews and algae. The roof is transformed from its initial dirty gray color to a "buff" or buckskin tan color.
Stronger, more cost-effective chlorine solutions can be formulated using granular chlorine (calcium hypochlorite) mixed at a rate of 2 to 4 ounces per gallon of water. The type found at swimming pool supply companies (65 percent available chlorine) does a very good job of removing mildew and discoloration due to weathering. Bleach solutions are not effective, however, in removing dirt, rust stains, extractive stains, heavy moss, or lichen growth.
Whichever type of chlorine you use, you can apply the solution of chlorine and water to the roof using a typical garden-pump sprayer. Scrubbing or brushing is not necessary. Let the chemical do the work. The recommended coverage rate is 1 to 1 1⁄2 gallons per hundred square feet of roof area. Let the solution stay on the roof 15 to 30 minutes, then rinse thoroughly with a high- pressure power washer or a garden hose. The high-pressure rinsing gives, more dramatic results and aids in removing the oxidized wood fibers, plant organisms, and leaf debris from the roof.
Because certain forms of algae, moss, and lichen are unaffected by chlorine solutions,. they must be mechanically removed using a high-pressure washer, similar to those used to clean automobiles. Power washing essentially removes the top layer of wood fibers from the shingles much the same way sandblasters remove rust from metal. Take care not to damage the shingles by using too high a pressure. Pressures of 1, 000 to 1, 500 psi are generally adequate to clean the roof quickly and efficiently. The results are truly remarkable. With the roof returning to its original cedar-brown color.
Until recently, it made little sense to preservative-treat an existing wood roof because of the low cost to replace. But as replacement costs rapidly approach $5 to $8 per square foot, the economics of preservative treatment look much better. One roofer's advertisement reads, "Why worry about a dirty roof? Because a clean and preserved roof looks a lot better than a re-roofer's bill! "The high replacement costs of wood roofs and the development of new preservative finishes that are cheaper, less toxic, and more durable has made treating wood roofs, feasible and desirable.
Preservative treatments for wood shakes and shingles, from both accelerated and long- term outdoor exposure tests indicate that a number of both water-borne and oil-borne treatments are quite effective in controlling the effects of weathering and decay for up to 5 years.
Types of Preservatives:
- Water-Borne Preservatives
- Oil-Borne Preservatives
One of the main drawbacks of water-borne treatments is they do very little to control the drying effects of the sun and subsequent cupping, curling, splitting, and surface checking. A durable oil- borne preservative containing a suitable naphthenic or paraffin oil will replenish the wood with oils that have been lost by sun and rainfall. Both naphthenic and paraffinic oils (when applicable) are resistant to oxidation by sunlight, do not contribute to the flammability of the treated wood, and are reasonable in price.
Wood shingle and shake roofs are not eternal, but they can provide long-lasting performance if cleaned and treated periodically. There are many products and techniques to choose from which offer varying degrees of protection. The worst thing to do, however, is to do nothing at all.
