We have discussed the reasons you may want to apply a chemical preservative treatment to your roof. Some homeowners may not feel comfortable with the use of these chemicals, and want an alternative treatment. While non-chemical treatments are possible, their effectiveness is not known, and may not provide adequate protection. However, these kinds of treatments are worth exploring.
For those who prefer not to use chemicals, zinc and copper strips could be a safe and relatively inexpensive treatment to control moss and fungi, although it is not a guarantee. Zinc and copper can effectively kill and/or retard the growth of moss and fungi. It could be possible to fasten a 2 inch wide copper or galvanized metal (containing zinc) strip to either side of the ridges and run it the entire length of the roof, as well as across sections of the surface, and below the flashings (unless using copper flashings). It is believed that the rain will leach the zinc and copper from the stips, allowing them to wash across the the cedar, preventing organic growth.
I can not find any scientific study to verify the effectiveness of this technique, and it is unclear as to how much zinc and copper would be required, but I have seen some examples of its effectiveness. In one example, galvanized metal strips placed below chimney flashings appear to be effective for shingles and shakes in that area for the life of the flashings. In another case, on a roof considered favorable for moss growth, two strands of 10-12 gauge copper wire was placed on the roof (one near the ridge, and one about 15 ft. below at the halfway point on the slope), and no moss was seen to grow on the cedar. Again, while I have been unable to find any scientific data on this technique, it looks to be a promising solution for those unwilling to use chemical treatments.
You may see oils that are advertised to replenish the natural oils in the wood. In red cedar, these natural oils account for less than 3% of the total mass. While they do contribute to dimensional stability and decay resistance, applying so-called replenishing oils to weathered shingles may be of little value, especially if they contain no preservative or water-repellent chemicals. However, you might see some temporary benefit, with regard to controlling the cupping and warping of the shakes and shingles.
In my opinion, oil treatments are very limited. Without including an effective mildewcide or other preservative, these oils, such as linseed, may actually encourage mold, mildew, and fungi growth. Furthermore, these petroleum based products oxidize in the sunlight, so they have a very short-lived benefit, and will only increase the flammability of your cedar roof. It is also worth noting that you should never stain, varnish, or seal your cedar roof. The cedar needs to breathe, and it can not do this if a stain or seal has been applied.
One Final Note:
There has been a debate among roofing contractors about the benefits of new cedar over old cedar. In my opinion, cedar is a natural wood and any natural material will be as good as the care it’s given. Your cedar roof is organic and needs to be given special care to reach its natural lifespan before being completely replaced. Just because your cedar roof is old, does not mean it needs a total replacement; most of the time, it just needs a professional cleaning and treatment.
All the information in this series of blog posts came from research from a variety of sources (most notably S.S. Niemiec, former Reseach Assistant, College of Forestry, OSU and T.D. Brown, Professor Emeritus, Wood Science, OSU), and in part from my own professional experiences. These posts are for informational and educational purposes only. While you may prefer to maintain and clean your cedar shake roof yourself, I strongly recommend you hire a professional roof cleaner.