Weyerhaeuser : Exterior Finish Options for Western Red Cedar

Western red cedar's natural durability and physical properties make it highly versatile and ideal for exterior applications such as siding, trim, soffits, and decking. In fact, the wood is so highly coveted that many manmade siding and trim materials attempt to mimic the grain and color of cedar.

Factors Affecting Finish Retention on Cedar Siding

A number of factors affect the performance of finishes on Western red cedar siding:

Natural Properties

The most important natural properties of Western red cedar are its outstanding dimensional stability, fine texture, pattern of growth, and freedom from pitch and resin. These characteristics contribute to its exceptional ability to accept and retain many different types of finishes. In addition, cedar is naturally resistant to rot and insect damage.

Dimensional stability

Dimensional stability is the resistance of wood to swelling and shrinkage when it gains or loses moisture. Light, low-density woods such as Western red cedar shrink less than higher density woods. Its excellent dimensional stability is an important factor responsible for the longer life of paints on Western red cedar in comparison to other woods.

Texture

Texture refers to the smoothness of the wood surface after sawing, planing, or sanding. According to the Forest Products Testing Lab, Western red cedar is the best softwood species for primer and paint applications.

Pattern of growth

Pattern of growth refers to the alternating bands of low density, springwood and higher density, summerwood. Together, these two bands constitute one year of tree growth. Western red cedar has a much higher percentage of low-density spring wood than most other conifer species. In addition, pitch

or resin, which can interfere with the adhesion or penetration of a finish, can be found in most softwoods but are absent in Western red cedar.

Manufacturing Characteristics

Manufacturing characteristics that can affect finish performance are surface texture, moisture content, and construction practices.

Surface Texture

Western red cedar products are available with a smooth (planed) or a textured (sawn or rough sanded) surface. As a rule, textured surfaces provide the best mechanical adhesion of the finish to the wood. For smooth-surfaced Western red cedar, a two-coat finish system is preferred

Moisture Content

Western red cedar is indeed available unseasoned (or green), which means it hasn't gone through a drying process, but the moisture content of this wood can be up to 25%. Finishing wood that is wet (above 20% moisture content) will increase the risk of coating failure. It is best to finish Western red cedar when its moisture content has stabilized at the level that will prevail during the service life of the product. In North America this level is approximately 12% moisture content.

The best product choice is Western red cedar that is kiln-dried versus air-dried at the time of manufacture. Air drying is when product is placed on racks to dry naturally over time, while dry kilns can "dry the material to a targeted moisture content based on whether the product is clear or knotty." Kiln-dried clear material is typically dried to between 12 and 15 percent moisture content, while kiln-dried knotty material is typically dried to between 15 and 19 percent. Knotty material is dried to a higher moisture content than clear material because knots are denser than the surrounding material and tend to star-check. If the material dries out too much, the knots can chip as the lumber runs through the planer, resulting in lower-grade material.

Construction Practices

Construction practices also have a significant influence on the performance of cedar and finishes. Care should be taken from the time the Western red cedar is delivered to the job site through to its installation and finishing. The moisture content of the wood (both prior to and during its service life), exposure to

sunlight, and surface contaminants (dirt) are important factors affecting the performance of each finish.

Exterior Finish Options

The choice of an exterior wood finish for Western red cedar depends upon the desired appearance and the degree of protection required. Architects have hundreds of options when specifying finishes for Western red cedar siding and trim, but in general, finishes can be grouped into three categories:

1. natural finishes such as transparent stains and oils

2. semi-transparent stains

3. opaque coatings such as paints and solid-color stains.

The expected service lives of different exterior finishes for Western red cedar siding and trim are summarized in the chart below:

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In addition, the first and most simple, low-maintenance option is to leave the Western red cedar unfinished. Without treatment, the wood will naturally turn gray over time, with climate conditions such as saltwater, UV, humidity, and moisture determining how quickly the cedar weathers. Western red cedar performs well as a siding product if it is left unfinished to weather naturally like this. With natural weathering the wood itself will last a very long time, but the appearance will change as it ages. Because they are more exposed to sunlight, southern and Western exposures will typically gray more rapidly, while northern and eastern facades and more protected areas such as gables and soffits will weather more slowly.

It should be noted that uncoated, weathered Western red cedar siding or trim can often be restored to its original color by applying commercial products called cleaners, brighteners, or restorers. But this will add maintenance that homeowners who opt for the natural, no-finish look may not be up for.

It is important to understand that the choice not to apply a finish to the cedar product has long-term implications. This choice requires advance consideration before natural weathering is allowed to begin. As Western red cedar weathers, it will lose its natural color and become gray. In very dry climates, it weathers to a silvery gray color, but in most other climates, because of varying moisture and sun exposure conditions, the wood does not weather uniformly and is likely to develop a dark, blotchy, gray appearance.

Although the natural weathering effect is usually only "skin deep" (less than 1-2mm), with the cedar largely unchanged beneath, extra care must be given to the design of the project, the installation of the cedar, and routine maintenance. Without the protection provided by a coating against moisture intrusion (especially end grain sealing), steps must be taken during the installation process to allow Western red cedar to readily dry following exposure to moisture, otherwise the risk of decay increases. All cedar products used in exterior applications require a degree of maintenance to keep them looking their best. Even if the choice is made not to apply a finish, contaminants such as dirt and mildew should be removed regularly to maintain its beautiful, natural appearance

This article was excerpted from the CEU course white paper "Western Red Cedar Finish Options Maximize Versatility" from the Western Red Cedar Lumber Association. For more details, including a guide to finish types,download the full course white paper here.

Weyerhaeuser Distribution is committed to delivering the reliable, consistent products you can rely on. Connect with your Weyerhaeuser representative today about cedar siding and decking solutions available in your market.