The Rot Resistance of Cedar and Redwood

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Both redwood and Western red cedar are rot- and insect-resistant woods used for building. Coastal redwoods, also known as giant redwoods, can grow more than 320 feet high with a trunk diameter of more than 24 feet. They are the tallest trees in the world and can live more than 2,000 years. Western red cedar trees only grow to their tallest heights of 120 to 180 feet close to sea level and are often found alongside redwoods.

Rot-Resistant Wood Grades

  • Neither the sapwood of redwood nor that of Western red cedar is rot-resistant, but they are often sold as lower-grade lumber. These lower grades are usually light-colored or streaked and should not be used for buildings that must sustain high levels of moisture. Higher-grade redwood or Western red cedar lumber uses the heartwood, which is virtually impervious to rot and insects. These woods must be treated with a clear wood finish or stain to maintain their coloring.

Why They Are Rot-Resistant

  • Both redwoods and tall Western red cedars grow in areas of high humidity, high precipitation and cool summers. Their natural habitat is extremely limited in terms of geography and climate. Redwoods grow exclusively in a narrow, 450-mile-long band along the Pacific Coast from central California to southern Oregon; their moisture is gathered from heavy winter rains and dense summer fog. Western red cedars grow only in Alaska and the Pacific Northwest. Because redwoods and Western red cedars have evolved in perpetually wet, cool environments, their woods have grown resistant to the damaging effects of long-term moisture exposure.

Expense and Controversy

  • While the rot-resistant properties of redwood and Western red cedar are well-known, they are both expensive. Redwood is the most costly wood used in construction, and Western red cedar is not only expensive, it also can only be used for decorative purposes. It is too weak to uphold structures. Additionally, the use of these trees -- particularly redwoods -- is controversial. The coastal redwoods prized in construction are rare; also, the old-growth trees are among the oldest living things on earth and store massive amounts of carbon, which is believed to help offset the greenhouse effect. Western red cedars grow to heights useful for lumber only at lower elevations, limiting their availability.

Alternatives

  • Pressure-treated wood offers a less expensive, equally rot- and insect-resistant alternative to Western red cedar and redwood. Pressure-treated wood goes through a series of vacuum and pressure cycles, which forces preservative deep into the wood pores. It is up to four times less expensive than redwood and half the cost of Western red cedar. Pressure-treated lumber also lasts longer than naturally rot-resistant woods.

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