Building in Versatility and Durability

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44 D+D FEBRUARY 2015 About 10,000 years ago, around the end of the last ice age, man began to make the transition from a nomadic hunter-gatherer existence to a more settled one, at least where food

1 44 D+D FEBRUARY 2015 About 10,000 years ago, around the end of the last ice age, man began to make the transition from a nomadic hunter-gatherer existence to a more settled one, at least where food was abundant. Man started to look for materials from which to build more permanent structures, and wood was a good choice it was often plentiful, easily worked, fairly durable and lightweight when compared to stone. Once seasoned to remove moisture, it is quite strong. Wood can catch fire or decay from moisture or insects, but otherwise it has great structural and decorative properties. Wood buildings can survive a long time the oldest wood building in known existence is the Horyu-ji temple in Japan. The main temple is believed to date to A.D. 711, while the wood used in the central pillar of the adjoining fivestory pagoda has been tree-ring dated to A.D Wood is both a sustainable and renewable resource with a low level of energy-based processing relative to many other building materials such as steel, aluminum, concrete or plastic. Today, modern architecture features wood in both decorative and structural forms. You By Allen Zielnik, Atlas Material Testing Technology LLC Building in Versatility and Durability Wood is one of our most used and adaptable building materials staining, coating and other treatments can enhance and protect it outdoors. see wood as a building material in log homes or as post-and-beam construction; dimensional lumber is used for structural framing (including gluelam-engineered materials) and as primary and decorative roofing and siding. For primarily aesthetic aspects, wood can be used for decorative trim such as wood moldings, wall panels and carvings. Wood also finds its way into functional-decorative usage as flooring material, stairway banisters, decking and in doors and windows. Today, wood is one of our most widely used and versatile building materials. A variety of staining, coating and other treatment techniques will enhance its appearance and protect it, but in order to use them properly, a little knowledge of wood and lumber proves useful. Although there are plenty of considerations for interior wood applications, this article mainly focuses on exterior applications. Know the Anatomy While trees are classified as either hardwoods or softwoods, this does not refer to the hardness of the wood. Rather, botanically speaking, hardwoods are broad-leafed flowering trees such as oak and cherry. Softwoods are generally cone-bearing conifers with needle-like evergreen leaves such as pine. A cut tree reveals the anatomy of the tree, including the growth rings, the outer sapwood and the darker inner heartwood. istockphoto/simonox

2 45 defects (cosmetic or mechanical) in the sawed lumber. Small-scale structures also affect the appearance and properties of cut lumber. For example, the annual tree rings often consist of two distinct parts fast growing, softer early growth and slower growing, denser late growth sections within each ring. This is one reason you should never high-pressure-wash a wood deck: the high-pressure stream literally blasts away the softer early growth wood, leaving the deck boards with a corrugated texture. Mechanical distortion often results from the orientation of the annual growth rings in relation to the position of the lumber cut from a cylindrical log. Cupping and warping during kiln drying or upon exposure to wet conditions can occur as the various wood cells change dimension with moisture content. The strength and durability of wood is exhibited in two structures in Japan. The Horyu-ji temple is the oldest known wood building, dating to A.D The central pillar in the nearby five-story pagoda has been dated to A.D Photo courtesy of 63highland via Wikimedia Commons. Softwoods have only two cell types, while hardwoods have more cell types and a greater variety within those types. These differences lead to distinct wood properties, and therefore primary uses; textural variations; and ability to accept decorative and protective treatments. This is why pine and oak stain very differently. A cross section of a tree reveals various concentric layers and structures. Once the outer and inner bark is removed, the primary tree structures are revealed as the lighter-colored outer sapwood and the darker inner heartwood. Basically, the narrower sapwood is the living biochemical factory and transport system closest to the bark. The larger inner heartwood, though dead, serves as a storage vessel for chemicals, or extractives, generated by the living sapwood. These extractives protect the heartwood from decay and produce the darker color. The heartwood also serves as the primary structural support of the tree. The extractives, particularly from exotic hardwood species, can bleed through many surface treatments and cause problems in finished wood. Other large-scale structures define the appearance and properties of wood and contribute to grain texture and pattern; these depend largely on how the log is sawed. Each board has a unique orientation to the growth rings, sapwood, and heartwood, meaning surface grain patterns can be very different, affecting its ability to accept stains and treatments uniformly. This unique patterning can be desirable or objectionable, depending on use. Similarly, structures like radial lines (which serve for nutrient transport) and knots (which often form at the location of dead branches) provide aesthetic character or are deemed Select the Right Treatment This last point moisture is one of the keys to protecting wood, especially when exposed to weather. To paraphrase R. Sam Williams, now retired from the USDA Forest Products Laboratory, from a wood coatings conference presentation he once gave, There are three keys to preserving wood: (1) keep wood dry, (2) keep wood from getting wet and (3) keep water away from wood. Though tongue-in-cheek, there is sage wisdom in this statement. As water is the primary cause of wood decay, it is not surprising that the primary treatments for exterior wood involve either providing a barrier to moisture or to the microbes and insects that it fosters. Penetrating Sealers One product class is penetrating sealers. Water repellents (WRs) and water-repellent preservatives (WRPs) are relatively simple wood treatments that slow the uptake of water and help keep wood dry.

3 46 D+D FEBRUARY 2015 Here, exterior vertical and horizontal surfaces receive different treatments based on their exposure and use. The siding received an application of an oil-penetrating stain to enhance color while providing moisture protection. The deck received a semi-solid coating, bridging semitransparent and solid-color stain, for wood with more imperfections. Photo courtesy of PPG.

47 Semitransparent woods stains do more than enhance color; they also provide protection from water damage and in some cases UV protection that inhibits color fade, mildew growth and other damage to

Research has shown that new unprotected wood becomes sunburned in as little as two weeks.

4 47 Semitransparent woods stains do more than enhance color; they also provide protection from water damage and in some cases UV protection that inhibits color fade, mildew growth and other damage to the wood surface. Photo courtesy of PPG. Weathering, particularly sunlight UV, will adversely affect paint adhesion on unpainted wood, as the exposure degrades and weakens wood cells. Research has shown that new unprotected wood becomes sunburned in as little as two weeks. Latex and other acrylic paints are more flexible and generally have better adhesion than alkyd-based paints, particularly when subjected to wide temperature ranges. However, oilbased paint is the best choice when painting over a surface that was coated with an oilbased paint. Latex should not be used over an oil-based paint because it will peel. For high-wear applications such as wood porches or stairs, a 100 percent acrylic formulation is recommended because the binder resins are harder than those of latex or latexacrylic blends. The only difference between WRs and WRPs is that the latter contains a biocide to inhibit mold and fungi. Both usually contain a binder (such as varnish resin) or drying oil (such as linseed or tung oil); a solvent; and a substance, such as a wax, that repels water. The oil or varnish resin penetrates the wood surface and cures to partially seal the wood surface. Some high-solids formulations form a thin surface film. However, some WRPs are formulated with nondrying oils that act as solvents. These oils penetrate the wood, but since they do not dry, the wood surface may remain oily until the finish absorbs, a process that takes several days. Tracking the oil indoors can be a problem if used on decks. A WR or WRP can be applied to all exterior wood that is normally finish-painted. Depending on the particular formulation, it can usually be used as a pretreatment for paint or as a standalone finish. During WRP application, particular attention should be paid to sealing board ends and lap joints. Note that the wood surface still degrades, though at a slower rate than unfinished wood, so timely refinishing is essential to avoid excessive wood degradation. In most wet climates reapplication should be done annually. Wood Stains Penetrating sealers and light film-formers can be clear or tinted as semitransparent wood stains; the better grades include UV absorbers to help protect against color fade and to protect the wood surface. Solid-color, or opaque, stains are not true penetrating stains since they form a film on the surface of the wood as paint does. In fact, they are formulated in the same way as paints, only with fewer solids, leaving a thinner, less protective film. It is recommended that solid wood sealers, which have more pigment, be used only on vertical projects such as fences and siding. If used on decking, foot traffic will wear through the pigment unevenly. Primers and Paints Priming and finish painting exterior wood is another option. Priming will improve the adhesion of the finish coat and will help hide the color when painting over a darker color with a lighter one. Unlike penetrating stains, solid-color stains form a film on the surface of the wood much like paint does. Photo courtesy of Western Red Cedar Lumber Association. Never paint wood with a moisture content over 20 percent; this can be measured easily with a wood moisture meter. Ideally, the wood should be painted at its average moisture content for the climate about 12 percent for most of the United States, 9 percent for dry southwestern states. Wood species and grain also affect paint durability. In general, less-dense woods such as

48 D+D FEBRUARY 2015 Paint is another choice for protecting exterior wood from the elements. Latex and acrylic paints exhibit better adhesion when exposed to wide temperature fluctuations.

However, wood weathered for as little as four to eight weeks before painting can reduce paint adhesion by 50 percent or more, with the less dense woods losing the most.

5 48 D+D FEBRUARY 2015 Paint is another choice for protecting exterior wood from the elements. Latex and acrylic paints exhibit better adhesion when exposed to wide temperature fluctuations. Photo courtesy of Behr Paints. western red cedar hold paint better than denser woods such as Douglas fir, at least for new wood. However, wood weathered for as little as four to eight weeks before painting can reduce paint adhesion by 50 percent or more, with the less dense woods losing the most. Also, within a single species, vertical-grain (also called edge-grain) wood holds paint much better than the more common flat-sawed lumber because flat-sawed wood shrinks and swells more from changes in relative humidity. Vertical-grain wood has narrower bands of latewood; when paint becomes brittle with age, especially oil-based, it tends to peel from the latewood. If a painted finish is desired, it is best to use kiln-dried treated lumber; then seal, prime and apply two coats of 100 percent acrylic top coat. Otherwise, it takes too long for the wood to dry, increasing the risk of sunburn. Special Considerations Extractive bleeding can occur in wood species such as cedar, redwood, Douglas fir and mahogany. Moisture can cause the natural Its versatility makes wood a popular choice in modern architecture and construction due to its many decorative, structural and aesthetic applications. Here, wood gives a rustic effect to a hotel entryway. istockphoto/ryhman007

49 The most common treatment for pressuretreated wood is an oil-based, semitransparent stain applied over a sealer or over factory-sealed lumber. Two coats of this treatment should last several years.

While the natural weathering may be desirable in woods like cedar, as seen above, stains and finishes can be used to enhance the natural beauty of the wood grain, texture and color.

6 49 The most common treatment for pressuretreated wood is an oil-based, semitransparent stain applied over a sealer or over factory-sealed lumber. Two coats of this treatment should last several years. The second coat should be applied before the first coat dries completely, or the second coat cannot penetrate the wood. Wood naturally turns gray when not protected from the elements. While the natural weathering may be desirable in woods like cedar, as seen above, stains and finishes can be used to enhance the natural beauty of the wood grain, texture and color. Photo courtesy of Western Red Cedar Lumber Association. tannins and other byproducts in the heartwood to migrate to the surface, leaving a reddishbrown stain on the finish. Sealers and stainblocking primers can help to minimize this problem but do not always eliminate it. Keeping a natural wood finish outdoors is very difficult. Wood turns gray as UV radiation degrades the outer surface and as mildew feeds on the wood surface. Clear WRPs with UV blockers can slow down this aging process but will need to be reapplied every year or two to keep the wood from turning a weathered gray. To help retain the look of new wood, the best approach is to use a WRP or penetrating oil with both UV blockers and a tint added to match the wood s actual color to help cover the natural graying. If new wood is protected during the first few years of exposure, the natural color of the wood can often be partially restored by scrubbing the surface with a wood-brightener/ water mixture, allowed to completely dry and then refinished. If the wood was already partially weathered prior to treatment, color restoration is usually limited. How long a paint or stain finish on exterior wood lasts depends on many variables, including the quality of the finish, type and texture of wood, application conditions, and exposure. South- and west-facing walls get the most sun and are, therefore, often the first to need recoating. Finishing Pressure-Treated Wood Painting or staining pressure-treated wood requires special attention. Pressure-treated lumber with very high moisture content is often shipped to lumberyards. If painted while wet, the paint film may trap moisture and cause peeling. Also, the species most commonly pressure-treated (flat-sawed southern yellow pine in the eastern United States and Douglas fir and Ponderosa pine in the west) do not hold paint well to begin with. Pressure-treated exterior trim should be sealed with a WRP as soon as the surface is sufficiently dry. This will protect cut ends and help keep the wood from checking, cupping and warping as the wood dries out. If this is the only treatment, it will need recoating every one to two years. Some newer varieties of pressure-treated product only require WRP application to cut ends. Capture the Essence of Wood Along with stone, wood is perhaps mankind s most versatile building material. Today over 60 species of wood are harvested in North America, with about a third of them used extensively for structural and decorative architectural purposes, along with about an equal number of imported species. Wood can be used au naturel; however, the inherent beauty of its grain texture and color can often be accentuated or enhanced through the use of stains or finishes. Wood can be protected from its natural enemies, inside or outside, through the use of appropriate wood coatings or treatments. Finally, as a building material, wood can be painted in a variety of colors and sheens to disguise its nature. But you must understand its inherent characteristics in order to beautify and/or protect it. About the Author Allen Zielnik is senior consultant-weathering science in Atlas Material Testing Technology LLC s global Consulting Solutions group, where he specializes in the weather durability testing of materials and products. A frequent speaker at various worldwide technical symposia, he is the author of more than 120 publications and conference presentations. He has degrees in electronics engineering and analytical chemistry, and since 1994, he has been involved with natural and accelerated weathering technology and solar simulation with Atlas Material Testing Technology. D+D

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