Converting a Cedar Roof to Composition - By: Grew Simson

Over an extended period of time, favored building materials come and go. Sometimes the popularity stems for the availability of a local resource. Other times, certain aesthetic styles come into vogue that drives the usage. Yet on other occasions, improvements in technology lead to the usage switch.

During the timber harvest booms of the mid and late 20th century, cedar was a cheap natural resource that both worked very well for the purpose and was very beautiful. That is a combination that is hard to beat and accordingly, millions of homes, particularly those in areas close to cedar producing zones, had cedar roofs.

Since then, timber over-harvesting has been a commonly acknowledged problem. The resultant increase in wood and lumber prices have affected many areas of our economy over the past 25 years, most notably in the building materials sector. One of the more common results of these price increases has been the conversion of formerly cedar roofed homes to composition roofs.

Composition roofs, as the name implies, are made of several material but primarily asphalt, mineral rock and often fiberglass. While they lack a natural beauty, they are common and thus well accepted. With the addition over the years of additional styles of composition roofing, their aesthetic appeal has improved.

However, homeowners need to realize that both the cedar shakes and the composition shingles are just the exterior components of an entire roofing system. This is critical because there is more to converting a cedar roof to composition than just ripping up the cedar and putting down shingles. The roofing system itself needs to be changed.

One of the primary components of a roofing system is the sheathing. The sheathing, or sometimes called the decking, is what sits on top of the rafters and provides a smooth surface for the installation of the underlayment (felt) and exterior roofing material. There is a big difference between the sheathing for a cedar and a composition roof.

A cedar roof needs to breath to keep the wood from rotting form the bottom up. Because of this, sheathing for a cedar rooting has horizontal gaps between the boards which allow ventilation. This is called skip sheathing. The gaps are small and the overall surface is still flat and smooth, similar to residential deck with slightly larger gaps. Composition roofs need no such ventilation. Additionally, compositions shingles are flexible and not rigid like a cedar shake. Therefore, they require a solid sheathing. Installing comp shingles on top of skip sheathing would results in dips in the material which would result in eventual leaks and a shortened life. Plus, it would look hideous.

Therefore, part of the conversion process it to install solid sheathing, usually plywood or OSB (oriented strand board) on top of the existing skip sheathing. Note that the skip sheathing does not need to be removed. This would be a waste of labor and even with the added weight the roof is still well within the structural weight limitations.
While replacing a composition roof with another composition roof can be a relatively inexpensive roof project, the need to install solid sheathing for the first time negates much of the cost savings of switching to composition that is the majority of the impetus for the conversion in the first place. Savings is still there but it is much less than what many homeowners expected. Because of this, it is generally a wise idea to obtain the cost of what a new cedar roof just so you can have that perspective.

Additional changes need to be made when converting away from cedar although they involve materials that would have to be replaced anyway so the change doesn't involve any wasted components. Some of these changes involve different felt type, different flashing sizes in roof to wall areas (in case you where hoping to reuse old flashing) and different methods of felting around valley flashing sections.

Finally, before you make the jump, consider what the intended original style of the house. By this I mean, make sure that your home is not a style where a cedar roof was designed to be as much a part of the house as the kitchen or bathroom. Imagine sleek sports car needing new tires and being refitted with some designed for a pickup. It wouldn't look quite right. So take the overall exterior look of your home into consideration. Also realize that a composition roof brings a little less resale value than a cedar roof.

Once these issues have been considered, you should be able to proceed with full confidence on your roof replacement decision.Find more information on Portland Roof Repair.

About the Author

All Surface Roofing and Construction is an award winning, Portland Oregon family owned business. All Surface Cleaning Company works primarily on roof cleaning, restoring roofs, decks, gutters, and All Surface Roofing and Construction is involved in slightly larger projects, such as roof installation and replacement. Call Portland Roof Repair at 503-598-7404 for a reliable Portland Roof Contractor.

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