Cedar Shake and Shingle Roofing Philadelphia
Flat Roofing Philadelphia
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In looking for information regarding the use of Copper Flashing on Cedar Roofing, many frequent this website and post questions regarding our stand on this issue. For many years it was known that Copper was adversely affected by Cedar acid and that it would prematurely deteriorate under a Cedar run-off. This knowledge was passed down through the ranks of roofing tradesmen but has of late been lost with the passing of the lifetime artisans.

Large Photos Below

cedar tannic acids destroys copper flashings


 Similar to the fundamental ignorance that exists in the roofing trades when dissimilar metals are mixed, this fact of Cedar eroding Copper has also been lost not only to the tradesmen, but to the “experts” to whom the current crop of roofers look for direction.


“Although the acidity of rain in the eastern part of the United States is high (low pH), rain by itself isn't acidic enough to harm Copper.”

According to Donald Baerman, AIA, CSI


It is mentioned on the "www.cedarburaeu.org" website that “Copper can be used as a flashing in areas that are not accustomed to acid rain.” It is also described in several other writings found on the web that sulfur dioxide, the primary component of "acid rain," deteriorates buildings and especially Copper.

Certainly the folks over at the Cedar Bureau do recognize the problem of Copper degradation under a Cedar wash. On their question and answers section, an inquisitive visitor asked:

Can I use fire retardant treated cedar shakes or shingles as siding above a copper roof? Answer: Certi-Guard® products will not affect the copper; the treatment seals any tannic acid into the shakes or shingles.

The old-time craftsmen knew from experience that the sulfur dioxide from chimneys on coal burners within homes emitted caustic exhaust that tended to corrode Copper flashings. This fact contributed more to the use of Terne Coated Steel flashings on old Slate roofs than the added expense of the Copper metal, but the expense of the Copper is often given for the reason of the steel flashings’ use. A careful search for information regarding these facts will often yield only a modernist viewpoint that the cost was the primary factor in the use of Terne Flashings over Copper in these old roofing applications.

Similarly, one can also find information regarding Copper degradation from "acid rain," but for deterioration from the surrounding materials on a given project, rarely can one glean anything from contemporary articles on the subject of architectural Copper degradation.


"...this writing is to inform the many ill-informed in this industry that the run-off from Cedar will degrade Copper flashings."

Long before anyone heard the term "acid rain," it was well known that the naturally occurring acids from Cedar would degrade Copper flashings. It was also known that the soot generated from a coal-fired furnace would eat away at Copper flashings. A true student of old-time craftsmen and their methods can only be in awe of the respect they had for the realities that the "so-called" experts of today can only derive from the scientific community and long-distance theory. The science of yesterday's craftsmen consisted of a pragmatic view and from lasting statements in their predecessor’s workmanship. They knew how different materials functioned and a certain harmony for relationships between given materials was respected in the field. In today's market, however, tradesmen are hard to find and the workers that do visit the trades are often transients and most look to the "experts" for advice and true experts are few and far between.

For all intents and purposes, this writing is to inform the many ill-informed in this industry that the run-off from Cedar will degrade Copper flashings. Acid rain, or not, the acid contained within the Cedar Shingles rots Copper! It has been known for ages that Copper should not be used on Cedar Roofs! Having experience in the field of roofing, our craftsmen have learned from the tradesmen of yesterday. The roofers of yesteryear did not use Copper on Cedar Roofs!

There seems to be an argument from some within the industry regarding the use of Copper on Cedar Roofs. It is apparent from the modern applications where Copper is improperly used as a flashing on Cedar Roofing that a certain type of erosion occurs that is unique to the metal on Cedar Roofs flashed with Copper. Acid rain plays no role in the increased degradation of Copper flashing on Cedar roofs as compared to slate roofs. Cedar Roofing degrades Copper. The blue-green protective patina developed on the surface of Copper flashings balances and protects the metal from sulfur dioxide degradation. This patina forms on Copper installed onto Cedar Roofs, but the metal is quickly robbed of its protective patina where the Cedar bleeds acid onto the metal. Witnessing this, it is apparent that the amount of run-off and not just the contact with, or "trapping" of water between the materials, is the leading contributing factor.  

“Copper located below Cedar Shingles especially is vulnerable, probably because of acid extractives in the wood.”

The "Copper.org" website answers the question as to the compatibility of Cedar Roofing and Copper. The suggestion that the Copper and the "leading edge" of the Cedar should be isolated tells of the reaction between the two materials. Copper and Cedar should not be mixed so that the Copper accepts run-off from the Cedar. This is the fundamental truth that has been known for centuries. It has only been of late, in this past half century, where we have lost the career tradesmen that used to pass down the knowledge of their predecessors, where we rely more upon opinions based upon theory than from solid experience.

As stated by Donald Baerman, an expert in this field and one who has gone to great lengths to find the truth, “Copper located below Cedar Shingles especially is vulnerable, probably because of acid extractives in the wood.” Mr. Baerman also states, “Although the acidity of rain in the eastern part of the United States is high (low pH), rain by itself isn't acidic enough to harm Copper.” We say thank you to Donald Baerman, who has taken the time to investigate the subject in great detail.

Just do not mix Copper and Cedar and you will not have to worry about any problems down the road.

As for Cedar Sidings and Roofing, we will be writing about the New Growth Wood issues and the truth about Cedar and life cycles of exterior wood claddings.

A must read:  Environmental affects on copper roofing.

Cedar Roof with Copper Flashings in service 10 years

Cedar Roof destroys Copper Valley

Copper is struggling to maintain protective patina and Cedar Shingle Acid is washing it away.

Note: Areas of Copper that shine are degrading from the light dew that forms and releases high concentrations of tanic acid from the Cedar.

Copper installed so as to receive Cedar run-off will degrade, but fllashings above Cedar Shakes and Shingles tend to wash down onto the Cedar, which is not a problem.


Things are changing (See Update below). The “experts” used to specify copper in the the valleys of cedar roofs. There was a clear specification to keep the edges of the shingles and shakes from contact with the copper. As you can see in the photograph below, the tannic acids drip and degrade the copper, regardless of contact. Roof below: 20 years old, Cedar Shake and in very poor condition. Obviously New Growth Wood.

copper valley under cedar shake degrading

Update 2012: It seems that the “experts” over at the Cedar Shake & Shingle Bureau have changed their valley specifications now only to recommend painted, galvanized steel or aluminum on cedar roofs. This is a good thing!


Cedar Shake and Shingle Philadelphia, Pa. Roof

Website written by, Robert Wewer