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How Long Do Cedar Roofs Last?

September 29, 2021 by ROTban Services Corp. How Long Do Cedar Roofs Last

There are five important variables that influence the lifetime of a cedar roof.  The age of the tree when it’s harvested, the splitting or sawing system, the installer, the climate of the location, and the maintenance it receives. A story I’ve heard many times in this industry is of value as an illustration.  The person will say variations of the following. “My uncle’s farm in Saskatchewan has a barn which has had the original cedar roof on it for 50 years and it’s still working great.”  This will usually come up when I tell of my study of the life of a cedar roof in southern BC.  Here, they last 19 years to 25 years, and are often replaced at the age of 15.  The average over all the roofs studied was 20.  Why such a difference?

The age of the tree is significant. Old growth, which I’ll define for this purpose, as being well over a couple of hundred years of age, have greater innate preservation because of the resin content.  Resin in the wood is its preservation, and cedar is well known to have lots of it.

The cedar trees we harvest today is usually, I believe, under 100 years old and could be as young as 30.   The thirty-year-old tree will tend to split easier and when used as a roofing product, and rot faster because of lower resin content. Let me state here that pine is much different that cedar and has far less usefulness for roofing, because of less resin and a tougher cell wall.  It splits easily in unfortunate ways.

The millwork into roofing product is at least as important.  Traditional shakes were made by hand splitting.  This is a skill much like plowing a field with a team of horses.  You learn it on the heritage farm.  But today we have mills along the rivers, where logs can be delivered cheaply, and they are processed with the use of machinery.  Manufacturing always leans toward producing what’s popular.  The builders and consumers will dictate that.  Builders want to invest in products that consumers want, but at a good price.  So, although the mills do produce split shakes, they also produce sawn shakes, which are more economical to buy because they’re easier to mass produce and utilize more of the wood.  These sawn shakes are quicker and easier to install on a house, too, so they’ve become the norm. But they don’t last as long.  Here’s why.  Think of a piece of celery.  You can cut it up with a knife, which cuts across the cell walls, exposing the contents of each cut cell, or you can tear it lengthways, leaving cell walls exposed.  The cell walls are strong.  The contents are weak.  So, the cut pieces will rot much faster than the split ones.  It’s the same with wood. In addition to that, cutting pieces can be done in any directions, and cross grain is unacceptable, but still happens.  Honest, certified grading guards against that, but the rule is that 10% or so of the certified product must still be found and discarded by the installer!

Because the skilled cedar roofer must throw out some below standard pieces out of every bundle, even in certified bundles, I categorize the roofer as being part of the process.  The only way to be sure on the application process is by having a qualified inspector.  Much skill and integrity are needed in a cedar roofer.  The opportunities to short cut abound, mostly because of the difficulty in getting up on a roof to observe the worker and finished product.  Skill is required in the ventilation provided in the roofing project.  There are rules and principles to be adhered to there as well.  I’ll explain more about attic temperature.

The climate is important.  Between southern BC and its interior there is a significant difference in climate, and this affects how long wood lasts before it rots.  In the Lower Mainland, the Fraser Valley and Vancouver Island here, we have high moisture levels, warmer weather and shelter from the sun provided by clouds.  These are conditions much loved by rot producing organisms.  Moss, mold, mildew, fungus, algae and lichen proliferate.  They are nature’s first responders to biodegrade dead materials, of which exposed wood is highly vulnerable.  Ventilation is most easily categorized under climate.  Ventilation must be good in any roof, because the temperature in an attic should be kept as close to the outside temperature as possible. Heat helps rot grow, when there is moisture and shade present. Cedar lignin and cell walls aren’t strong, so when the resin is low, or depletes over time, nature’s little rotters have a party.  The party’s side effects, (actually, its purpose) is rot, and rot is bad for your pocketbook.  Your roof protects everything you own and your loved ones, so this is about your personal economics.

Conclusion.  The Saskatchewan barn was likely made from old growth wood, split by hand from a caring farmer, installed carefully, and has enjoyed weather that is hard for the rot producers to grow in.  We can add to that another interesting factor.  That barn was not very warm. The wood has enjoyed a great environment and lasted 50 plus years.

The “rainforest climate” situation is quite different.  Thousands upon thousands of roofs have been quickly installed with cedar shakes that are not taking full advantage of the wood’s benefits. Cut cell walls are exposed to the elements as a policy.  Roofers worked fast for financial benefit.  The climate was perfect for biodegrading activities. Because of these problems, the lifetime was cut in half.

Maintenance.  Another oddity to cedar roofing is the ridge caps on the tops, which statistically rot about 25% faster than the shakes used on the sloped roofing. Also, some areas rot faster than others.  On the dark side of a roof, the rot producers, algae and moss, grow readily, and on the sunny side the UV rays make microbe growth harder.

Some competent maintenance inspection, planning, and physical repairs will be a big asset to cedar roof life.

Whatever quality of cedar shakes you have, it will work better and last longer if it has excellent ventilation and is preserved.  Preservation can be done before the shakes are installed, by pressure treatment.  It can be done afterwards by topical application. This is to fight against moss, algae and fungus.  That’s where ROTban™ fits, and it would be of maximum benefit if done by the third year after installation, and every four or five years from then on.