A very common misconception is that it is not necessary to preserve cedar shingles. This is a myth that has some historical truth to it.
Up until about 60 or so years ago, cedar shingles were made from old growth trees, two to three hundred years old. The lignans in the wood, the glue that holds the cellulose materials together, developed over hundreds of years, was much stronger than anything that you will find today. Old growth wood has better stability, durability, and longevity. These trees in vast virgin forests grew slowly due to limited light and competition from the other trees. Because of this slow growth rate, the growth rings on the trees were packed very tightly together. We began seriously depleting these virgin forests during the industrial revolution, and by the 1940s, most of them were gone. For several decades now there has not been very much old growth wood available.
Enter second-growth and new-growth wood, grown on tree farms. The trees grow in open areas with little to no competition for sun, which causes them to grow very quickly so they can be harvested in 10-20 years as opposed to old-growth wood, from trees as old as 200-300 years before being harvested.
This is why houses in San Francisco, built over a hundred years ago, can still be in such fine condition, while more recent homes need to protect the wood to keep it from deteriorating. Without preservation, our shingles dehydrate and begin to cup, curl, and disintegrate. Unprotected shingles with a south and west exposure will only last 20 years.
The product we use, TWP 200 series (AKA Radcon), is a penetrating paraffinic oil that rehydrates and beautifies the wood with transoxide pigments
If you have further questions or would like to discuss your home, please feel free to call me, Paul Burrous, on my cell phone 415-717-3721.