About Exposed Wood
Woods such as cedar and some hardwood species are naturally durable and are therefore commonly used for wood exposed outdoors, such as weathersiding, shakes and shingles, decks, furniture, and fences. Durability is imparted by natural chemicals, which are contained in extractives in the heartwood of these species. Heartwood is the center part of the wood; the outer part is sapwood (Figure 1).
Since only the heartwood contains extractives, timber that contains a high proportion of sapwood does not have the natural durability of timber that contains a high proportion of heartwood.
Wood is the material of choice for many structures. As with any building material, how wood is used depends on its properties, such as strength and stiffness, as well as its finishing characteristics and maintenance requirements. Problems such as poor finish performance, mildew, checking and splitting, and wood decay can be controlled with proper care and maintenance. Such problems can be avoided or attenuated through knowledge about the factors that affect wood, particularly wood exposed outdoors. If wood structures are given proper care initially and are maintained periodically, they can be functional and structurally sound, as well as aesthetically pleasing, for decades.
Properly seasoned wood that stays dry is not subject to decay, premature failure of paints and finishes, and problems associated with weathering, such as excessive splitting and checking, raised grain, extractive bleed, and discoloration.
Effects of Outdoor Exposure
Moisture Effects
Water is one of wood"s worst enemies. Whether in the form of vapor or liquid, water can cause shrinking and swelling, which can lead to dimensional changes of the wood and degradation of the finish. Water causes decay or rot of the wood and early failure of paint, and it accelerates the weathering of wood exposed outdoors.
Shrinking and Swelling
In general, wood shrinks as it loses moisture and swells as it gains moisture. More precisely, wood only changes dimension between an absolutely dry state (completely free of moisture) and its fiber saturation point (the point at which the wood fibers are completely saturated with moisture). This fiber saturation point typically occurs at about 30% moisture content for most species. At this point, all the water in the wood is bound within the cell wall. As moisture content changes above fiber saturation, the cell cavities take on or lose unbound water but the wood cell walls do not change dimensionally. Below the fiber saturation point, however, the wood changes dimension with changing moisture content. The magnitude of this change is dependent on species and is always different for the three axes: radial, tangential, and longitudinal (Figure 3).
A large percentage of wood finish degradation (e.g., paint defects, peeling, and cracking) results from moisture changes in the wood and subsequent dimensional instability.
Water Vapor and Water Effects
Shrinking and swelling of wood occur whether the water is in the form of vapor or liquid. For example, wood swells during periods of high humidity and shrinks during periods of low humidity; it also swells and shrinks as it gets wet from liquid water and then dries. As discussed, wood can swell until it reaches fiber saturation. If wood is exposed to water vapor, such as occurs indoors, the moisture content can only reach the fiber saturation point. This requires exposure to 100% relative humidity for an extended period. Since wood is seldom exposed to this level of relative humidity for long periods, it seldom reaches fiber saturation because of high humidity. However, if the wood gets wet from liquid water, it can quickly reach, or even go beyond, fiber saturation. Problems with poor performance of wood occur when the moisture content of wood reaches or goes beyond fiber saturation — this is almost always caused by liquid water. Throughout the remainder of this document, the term water refers only to liquid water, the term water vapor to humidity, and the term moisture to both water and water vapor.
Weathering
Regardless of the care taken in building a structure, wood ages when exposed outdoors. This aging process is called weathering. Weathering is a degradation of the wood"s surface caused by the combined effects of the ultraviolet (UV) radiation in sunlight, water, and abrasion by wind-blown sand or other particulates. This degradation should not be confused with decay.
Weathering is first manifested by a change in the color of the wood. The color of most preservative-treated timber is either light green (from copper and chromium salts in the preservative) or brown (from added dye). Cedar and redwood have the freshly sawn natural color of these species. With weathering, dark wood, such as redwood and cedar, tends to get lighter, whereas light wood, such as pine and fir, tends to get darker. In some climates, such as along the seashore, wood tends to weather to a silvery grey. This color is a combination of mildew growth and cellulose enrichment of the surface. The weathering process removes the colored extractives and lignin, leaving cellulose. If protected from excessive moisture, redwood and cedar are prone to weather to silver&mdashgrey. Wood that has been pressure-treated with CCA but not treated with a WRP will initially turn to dull grey. Eventually, this wood will also become silver&mdashgrey. The change in color is followed by a loosening of wood fibers and gradual erosion of the wood surface (Figure 4).
Rain washes the degraded wood materials from the surface. Rain and changes in humidity also cause dimensional changes in the wood that accelerate this erosion process. Erosion is more rapid in the less dense earlywood than in the latewood, which leads to an uneven surface (Figure 4).
Surface erosion, however, proceeds slowly. The erosion rate for solid softwoods in temperate zones is on the order of 6 to 12 mm per century and depends mainly on the intensity of UV radiation and on the wood species (Figure 5).
For hardwoods, the erosion rate is 3 to 6 mm per century. The erosion rate depends on the exposure of the wood to sun and rain and the care the wood receives. Control of water absorption by the wood retards weathering and decay.
Extractive Bleed
A common cause of discoloration is extractive bleed. All species contain extractives, but extractive bleed is most prevalent on highly colored woods. The discoloration often occurs around fasteners (Figure 6) because the hole in the wood caused by the fastener cuts many wood cells. These cut cells increase water absorption. Water dissolves the extractives, and when the wood dries, the extractives accumulate at the surface and sunlight causes them to polymerize. Although extractive bleed can be a problem on wood siding, it is seldom a problem on horizontal wood surfaces such as decks because the extractives are usually washed from the deck by rain before they polymerize. If extractive bleed is a problem, the extractives can be removed by scrubbing the wood with soap and water. Do not use a wire brush because the brush will contaminate the surface with iron, which will cause iron stain.
Iron Stain
A common form of staining on wood surfaces results from contamination with iron. A portion of the extractives in wood includes a group of chemicals collectively called tannins. The amount of tannins depends on species; oak, redwood, and cedar are rich in tannins. Tannins react with iron to form a blue&mdashblack stain on wood. Iron stain and extractive bleed are compared in Figure 6.
Note the darker color of the iron stain. Common causes of iron stain include use of ungalvanized or poorly galvanized fasteners, cleaning with steel wool or a wire brush, and contact of the wood with any iron or steel.
Raised Grain
The wetting and drying cycle of wood exposed outdoors can
raise the grain of the wood, resulting in a rough surface. On
flat-grained timber, the raised grain may appear as thin knifelike
feathers along the earlywood&mdashlatewood interface
(Figure 7).
This degradation leads to a splintered wood surface and eventually to checking and cracking. Checking may increase the uptake of water, thus accelerating the degradation process.
Decay
Whereas weathering is degradation of the wood surface, decay (also called rot) affects the full volume of wood. Decay is degradation caused by a variety of decay fungi that are capable of breaking down the structural components of wood for food. The fungi tunnel throughout the full volume of the wood, degrading the polymers that form the wood cells through a complicated biochemical process. Since these polymers give wood its strength, considerable loss of strength occurs long before visible damage is apparent. Wood decay fungi must have adequate moisture to grow. Although the amount necessary for growth varies depending on the species of fungi, in general, the wood must be near fiber saturation for fungal growth.
Mildew
Mildew is caused by a type of stain fungi, which differ from decay fungi. Mildew is not capable of degrading the structural components of wood; therefore, it does not cause a decrease in wood strength. Unlike decay fungi, mildew fungi do not tunnel through the wood but live only on the surface. Like decay fungi, mildew fungi often flourish when excessive water is present. Moisture also encourages the growth of lichens and other microorganisms that discolor the wood surface (Figure 8).
Wood can develop mildew growth rapidly, particularly if treated with linseed oil, eucalyptus oil, tung oil or other natural oils, which form a food source for the fungi. Because the conditions that favor the growth of mildew fungi also favor the growth of wood-degrading fungi, be suspicious if wood has mildew or other discoloration. Mildew can affect all species of wood, including naturally decay resistant species. Some extractives are food for mildew. Thus, species with high extractives content are slightly more susceptible to discoloration by mildew.
Mildew fungi are objectionable because they discolor the wood. The most common discoloration is an overall grey. Mildew can also appear as black blotchy stains (Figure 9).