Decking and Timbers

In some cases, Western Red Cedar decking is specified and designed by the architectural team, especially when part of a custom or commercial project. Two grades apply to decking, and a third knotty grade is specified for timbers:

▶ Architect Knotty. As with siding assemblies, this grade is specified for the best-quality, most visually consistent knotty timber, boards, or trims. The Architect Knotty standard, defined by WRCLA, requires sound and tight knots. Architect Knotty may not have any holes or wane on the graded face, which describes the presence of bark or lack of wood from any cause on edges or corners. Kiln-dried materials are well-suited to factory finishing.

▶ Custom Knotty. Defined to combine good looks and budgetary value, the Custom Knotty standard is applied only to decking. Unlike the Architect Knotty grade, the Custom Knotty grade limits holes and restricts wane and unsound knots to a lesser degree.

▶ Appearance Knotty. Used for timbers only and for both structural and nonstructural uses, Appearance Knotty grades of Western Red Cedar are manufactured to meet the visual requirements of quality projects, thus explaining its name. However it is known for value, not just appearance. No holes or other inappropriate characteristics are allowed, and wane allowances are highly restricted to ensure well-defined corners.

There are other standards for Western Red Cedar that architects may employ in project specifications, including the lower-cost Standard and Better category, used for general construction and other applications where structural benefit should outweigh the need for uniform, quality appearance. This contrasts with Select Knotty, the grade of product for trim boards with sound, tight knots and other natural features known for strong visual character and performance traits ideal for exterior and interior applications. In some cases, this grade may include a percentage of Quality Knotty grade.

For decking and similar applications, the knotty varieties contrast with the clear grades of Western Red Cedar, including Architect Clear, a WRCLA standard that requires the greatest possible, strictly controlled visual appearance. Whether seasoned or unseasoned, the products' fine appearance can be costly, and many projects require a custom order for the lot of specified material, rather than finding off-the-shelf availability. Alternatively, some decking and other applications may benefit from the Custom Clear grade of Western Red Cedar, which delivers the wood's stability and durability in a sophisticated clear appearance ideal for the most discriminating designers. Custom Clear requires a finely machined surface that shows limited characteristics that could detract from its visual attractiveness, making it a frequent specification for custom homes and nonresidential projects requiring fine woodwork.

Western Red Cedar is also widely used for large buildings in which “both the strength and appearance of exposed wood members are of equal importance; and second, in landscape, park, and garden structures where appearance is paramount,” according to WRCLA, which may include outbuildings, gazebos, and pergolas as well as classic exposed heavy-timber lodge buildings, residences, and commercial structures. In these cases, the naturally resilient qualities of the wood provide a lasting and weather-resistant construction of exposed timbers. For timber construction, the offered grades of clear Western Red Cedar include:

▶ No. 2 and Better Clear, for the finest-quality rough lumber.

▶ C and Better Clear and D and Better Clear, which offers good appearance in high-quality lumber suitable for architectural landscape structures and as exposed posts and beams in heavy timber construction. The grades may be mixed, and D Clear will have large and more numerous natural traits that are acceptable for outdoor structures. Cross-cutting may be necessary with lower grades of Clear to achieve the required results.

Appearance Knotty grades are also offered as a nonstructural component of timber and outdoor structures. In addition, there are three structural grades: Select structural, No. 1 Structural, and No. 2 Structural, which are evaluated for strength and stability and employed in engineered applications where appearance is not an important specification criterion. (Timbers may be ordered that meet both appearance and structural standards.) Similarly, Standard beams posts and timbers are like utility lumber or rough lumber, used for “general construction purposes where serviceability is more important than appearance.”

Benefits of the Wood Species

Yet it is often the appearance of fine grades of Western Red Cedar that lead to its wide use in innovative architectural statements. And with the wide range of grades available, it's no surprise that some architectural design teams have found ways to blend two or more grades for varied visual effect, special application needs and for sensible, cost-conscious product selection. In all the grades, however, there are performance traits endemic to Western Red Cedar that should be understood and exploited in building design.

Versatility and Workability. For both architecture/engineering firms and contractors, Western Red Cedar is easy to use. Compared to many wood species, both the clear and knotty grades are relatively low in density and high in material consistency, improving the ability of trades to cut, nail, and glue effectively—even with fine woodworking tools. The resulting pieces are highly durable and dimensionally stable: For example, the size differential between dry and unseasoned Western Red Cedar is about 1/16 inch for a nominal dimension of between 2 inches and 4 inches; the percent shrinkage is also very low, varying from 0.3 percent to 3.8 percent, depending on whether the wood is green or kiln-dried. It is one of the most stable species in North America.

Architects find they can apply a wider range of finishes and colors to Western Red Cedar because of the ability of this pitch-free, resin-free species to accept and maintain stains and finishes. Standard colors are just a start: effective applications range from lightly toned, clear solutions to two-coat solid colors.

Resistance to Decay and Insects. Though it is free of pitch and resins, Western Red Cedar is rich in natural extractives that boost durability. One of North America's more resilient wood species, Thuja Plicata cedar fibers contain thujaplicins, compounds that have natural preservative qualities, making the wood resistant to mold, insects, and decay and helping it last for decades when properly detailed and finished. (Some Pacific Northwest totem poles last for centuries, typically finished with natural pigments.² See “Western Red Cedar: Material Attributes,” page 4.) While protecting buildings against deterioration even in coastal areas and harsh environments, the anti-fungal, anti-bacterial properties also give the wood its characteristic, memorable aroma.

Environmental Benefits. Maintenance requirements are also comparatively low for cedar building elements, which along with their durability helps to improve the green qualities of the design. There are other green considerations: According to the Minnesota Sustainable Housing Initiative (MSHI), common wood species used for siding include Western Red Cedar, redwood, cypress, Northern White Cedar, and Eastern white pine,³ which offer numerous environmental benefits. Starting with their renewable traits and also their forests, which support diverse ecological habitats, there are multiple green benefits. Western Red Cedar also “is energy-efficient to produce, very little waste is produced in its manufacture (since waste wood is often used for other manufactured wood products), and it is biodegradable,” adds the MSHI.

Yet the group also raises a sticky point: The benefits for the environment of specifying wood depend heavily on forestry management practices. Sustainable practices help limit soil erosion, maintain species diversity, and protect waterways, and provide habitat for wildlife. For this reason, architects with green building practices can specify from a broad array of forests independently certified to various third-party management standards, which in British Columbia account for about 85 percent of the material's supply.

There are other qualities inherent to the wood species that merit attention for green building. For example, Western Red Cedar may contribute to a lighter carbon footprint than alternatives because its low density can increase the R-value of cladding assemblies. Compared to masonry, vinyl siding, and fiber cement board, Western Red Cedar has been tested to demonstrate lower U-values, meaning it can insulate better and retain more heat or cold. Similarly, its acoustic performance compares well with other materials, providing a good sound transmission class (STC) because of its low density and good damping of vibrations. These are benefits both in envelope applications and as interior paneling. In addition, architects can design fire-resistant construction by specifying and detailing suitable Western Red Cedar shingle envelopes, according to Mark Rutledge, vice president of sales and marketing for producer Shakertown.

Last, Western Red Cedar can reduce the need for toxic chemical treatments. Alternatives such as pressure-treated woods have preservatives that are unnecessary for the thujaplicin-bearing species, making the latter ideal for schools and playground equipment.

Reduced Weight and High Strength. The characteristically large, open cell structure of Western Red Cedar gives it a distinct appearance but also makes it less dense than most other softwoods. Lighter in weight, the timbers and panels require less structural support and less energy for transport from the mill yard to the fabricator to the job site. Yet in spite of the low density, Western Red Cedar is naturally strong, durable and dimensionally stable.

Western Red Cedar timbers make for excellent structural members. Depending on their size, they may be classified as posts and timbers or beams and stringers. The latter are rectangular in section, either surfaced or just rough sawn, in pieces measuring 5 inches and thicker with widths of at least 2 inches greater the width. Posts and timbers, on the other hand, are generally square members of a minimum 5 inches by 5 inches and larger, though widths that may be up to 2 inches greater than the thickness, though not more.

Aesthetic Range. Among the favored attributes of high-quality finish wood is its ability to lay flat and remain straight. This stability and resistance to warp ensures both a crisp, finished look in the wood assemblies as well as a long-term durability for the building. According to experts in wood species, Western Red Cedar has an even grain and relatively consistent density—two qualities that reduce the undesirable tendencies to swell, cup or twist, which are common issues for many softwoods and hardwoods. Sources including The Wood Database provide specifics on such variables as Janka hardness, elastic modulus, and shrinkage; the latter is given as radial (2.4 percent), tangential (5.0 percent), and volumetric (6.8 percent) for Western Red Cedar.⁴

In addition, the availability described above of both the knotty and clear standards of good architectural performance allow architects to mix and match the grades to achieve varied aesthetics while holding the cost-benefit ratio within necessary bounds. For varied budgets, the Western Red Cedar palette provides a range of expression without the pitch and resin found in many softwoods. And though the wood is rich in extractives that enhance durability, the lack of pitch and resin improve the ability of Western Red Cedar to maintain a wide range of finishes, from lightly toned clear solutions to two-coat solid colors.

Like any wood, the natural luster, graining and knot patterns provide a warm and often comforting aspect to a building or interior. The grain and knots can enhance the benefit to building occupants of biophilia, a notion advanced by Harvard biologist E. O. Wilson in his 1984 book about the “instinctive bond” between human beings and other living systems that has been shown to have various positive effects on personal experience and health. As decking, wood provides natural spring and bounce that some synthetics and composite boards may lack. Western Red Cedar also has a subtle, natural aroma that is “another hallmark of its universal appeal,” according to WRCLA.

For many centuries, wood carvers and artists—as well as finish carpentry trades—have gravitated to Thuja Plicata for its positive qualities. With the benefits listed above, it's not surprising that some architects include the Latin name for Western Red Cedar in specifications to ensure the species is not conflated with other wood products, some bearing the name “cedar.”