Scientifically Proven – On the House

Scientifically Proven

By on April 17, 2014
cleaning a cedar roof


I enjoy your column very much, and your information has been good until your recent comments on wood shake roofs. You stated that power spraying will remove dry rot from wood shakes. But in fact, if the shingles have dry rot, they need replacement. If you try to power spray a roof that has dry rot, where the dry rot is visible to the eye, the shakes are weakened at least 70 percent or more, and they will loosen or fly off, causing leakage.

The fungus that causes dry rot is in all wood. Having certain moisture conditions over a long period of time will lead to dry rot. Spraying the roof covering with a chemical can help preserve the roof covering, but the homeowner should be concerned with other factor of spraying roofs: Many chemicals make the roof covering more flammable, especially oil-base chemicals.

The chemicals will only penetrate the surface of the wood and will not kill the fungus deep in the wood or under the wood shakes. And, chemicals are toxic. Unless applied by a professional who knows how to safely apply the chemicals, they can lead to serious problems, including poisoning of humans, animals and plant life.

With all the materials on the market for roof coverings, and with the high cost of installing a wood shake roof, I actually think wood shakes should not be used at all for roof coverings.



Readers: The preceding correspondence refers to an column we did on roof care (a preventive maintenance process for wood shingle roof covers that we highly recommend). Jerry’s letter will let us dispel several common misunderstandings about roof care. Our apology for misusing the term “dry rot”. It is exceedingly common to misuse it industry-wide, and, if anything, we should be attempting to clear the air rather than cloud it.The term “dry rot” actually refers to wood damage caused by a very, very rare fungus (poria incrassata) that happens to have the extraordinary ability to transport its own moisture. Hence, it is a fungus that can flourish (and cause damage) in dry areas. What we generally (and incorrectly) refer to as dry rot is actually wood decay caused by any one of a countless number of deferent types of fungi that become active when moisture levels reach 20 to 30 percent within the wood in which they reside. We are told that what we usually see is “soft rot”, not “dry rot”.Since wood decay resulting from fungus generally occurs at wet areas, and because the saw-cut butt-end of a cedar shake is 12 times more absorbent than its’ hand-split face, simple logic tells us that the chance for decay at the butt is 12 times greater than at the rest of the shake. We’ve seen this partial decay, and we’ve watched it whisked away with a power washing. If decay exists (within reasonable limits) it can be removed without damage to the roof cover. Wood decay is removable, dry rot isn’t. And power washing does work!According to John Boyette, a wood technologist with the Texas Forest Service, oiling a deteriorated cedar roof cover does not make it more flammable. And, in fact, “extends the life of the shingles by helping the wood to resist damage by wind, rain, and to some extent, fungus as well”.

We found out that the flash-point of Chevron Shingle and Floor oil (a commonly used roof care product) is 330 degrees Fahrenheit. We asked Boyette to explain why new shingles that burn at 500 degrees Fahrenheit would not be more flammable when oiled.

He told us that although individual tests show that oil ignites at a lower temperature than shingles, the comparison is an unfair one. He advises us that the natural wood oils evaporate from shingles as they age, them (if you’ll allow our pun) “high and dry”, and more flammable. He informs us that the most volatile vapors in freshly applied shingle oil evaporate quite rapidly, decreasing its flammability. Boyette recommends the use of oil on older shingles, especially in arid climates, and, for the reasons stated, says not to worry about an increased fire hazard.

Treating shingles after installation is not nearly as effective as the factory-applied equivalent, but once they are installed there is really no practical alternative. This is why we suggest that you insist on the five year warranty available from many roof care companies that perform this process.

We don’t agree that a professional must be hired to apply roof care chemicals. Yes, pesticides are dangerous. But, so are paint and paint thinner. We suggest copper napthanate for roof care, because it is among the safest (considering effectiveness) of the chemicals available. It can be purchased in a water-disbursal form called Canapsol 5, which is excellent for use in less arid areas where oiling is not necessary to combat shingle drying and splitting. Copper napthanate can be found on the shelf of many hardware stores under the brand name Copper Green. We do, however, agree with you about the need for safety, and we feel all construction products should be used with extreme caution.

Finally, we completely agree that cedar shakes are neither the safest nor the most long-lasting type of roof cover available. And, when asked, we recommend alternatives. But, we can’t forget the millions of folks who have cedar roof covers who want to keep them in good condition for as long as possible.

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