Dating back to the earliest days of colonization in North America, cedar shakes and shingles have offered strength, durability, insulation, and beauty to our homes. A wood roof imparts a look of quality that few other roof coverings can match.
Only a few years of exposure to the weather, however, can drastically change this “quality” roofing. Through the interaction of sunlight and rainfall, cedar shakes and shingles can lose virtually all their natural protectants in as short a time as five years. At this age, most cedar roofs begin to show signs of cupping, curling, splitting, and decay. Unlike most roofing materials, however, cedar shingles and shakes can be restored and maintained through proper care and treatment, often doubling or tripling the remaining service life.
Why Wood Roofs Weather
Western red cedar is an extremely durable material even under adverse conditions, but its useful life depends upon the environment. Cedar roofs left unprotected suffer photodegradation 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.