Wood is a natural, sustainable material. As such, when it’s continually exposed to environmental elements such as moisture, sunlight, and wind the physical attributes of the material changes and aesthetically evolves over time.
WHAT IS WEATHERING?
Weathering is defined as the chemical and physical changes that take place when timber is exposed to weather. Weathering factors that are responsible for the changes are: moisture (rain, snow, dew, and humidity), oxygen, solar radiation (ultra-violet (UV), visible, and infrared light), and temperature.1William C. Feist: Weathering and the Protection of Wood
It’s important to note that weathering should not be confused with rotted or decayed wood. Wood rot is a form of decay that results from a combination of fungi (microscopic organisms) and moisture. Wood must be consistently damp to foster fungi growth since fungi will not grow on dry wood. On the other hand, weathering is a surface phenomenon that typically only effects a small portion of the surface area. In the absence of decay, wood exposed to the weather can last many years since the performance and durability of the materials are not affected. As such, weathering should not be regarded as a problem, but rather a natural process that needs to be taken into account when planning for the preservation and protection of outdoor wood.
According to the Forest Products Laboratory, Wood Finishing: Weathering of Wood, “Weathering of wood involves changes in color, roughening and checking of the surfaces, loss of surface fibers, and warping. Without paint or treatment, wood exposed outdoors, such as siding and trim on buildings, materially will change in appearance in a few months or years. The material will then stay almost unaltered for a long time.
The color of wood is rapidly affected by exposure to weather, mainly UV. Generally dark-colored woods become lighter and light-colored woods become a little darker. As the weathering continues, all woods become a gray color. Only the wood close to the surface is noticeably affected. Initially, the affected zone, usually brown in color, appears to penetrate the surface 0.02 to 0.10 of an inch. As weathering progresses, a gray layer, 0.003 to 0.01 inch thick, develops on the surface. This layer is composed chiefly of cellulose, the most leach-resistant part of the wood. Lignin, the cementing material of wood cells, is degraded by light, rain and dew, which will extract almost all of the lignin fragments, sugars, and extractives.
The defiberization, or loss of surface wood cells, occurs most rapidly in the thin-walled fibers of the springwood and at a much lower rate in the more dense summerwood. As fibers are lost from the surface, the brown and gray layers move more deeply into the wood. The process is quite slow and only about 1/4 inch of wood thickness is lost in a century.
Weathering produces physical as well as chemical changes in wood. Exposure of wood to light and to natural weather conditions produces changes in the anatomy of the surface cells and also induces small microscale checks. As weathering progresses and extreme cycles of wetting and drying continue, most woods develop larger and deeper checks or cracks that are easily visible. Woods of a moderate-to-low density acquire fewer checks than do woods of high density. Edge-grain boards also check less than flat-grain boards.
Moisture — one of the principal causes of weathering is frequent exposure of the wood surface to rapid changes in moisture content.
Light — the photochemical degradation of wood due to sunlight occurs fairly rapidly on the exposed wood surface. The initial color change of wood exposed to sunlight is a yellowing or browning which proceeds to an eventual graying. These color changes can be related to the decomposition of lignin in the surface wood cells and are strictly a surface phenomenon.
Temperature — may not be as critical as UV irradiation or water, but as the temperature increases, the rate of photochemical and oxidative reactions increases.
Freezing and thawing — of absorbed water can contribute to wood checking. Abrasion or mechanical action, such as wind, sand, and dirt, can be an important factor in the rate of surface degradation and removal of wood.
Extractives — wood contains a diverse range of naturally occurring chemicals that are known as extractives. They’re present in relatively small quantities, and they play a key roll in color, odor, degradation, flammability and other nonmechanical properties of a wood species.
Extractives include tannins (oak) and thujaplicin and polyphenols (Western red cedar) and other polyphenolics, resins, color oils, and gums. Some extratives, like tannins, are water soluble.
When tannins interact with moisture and rain, they are leached to the surface of the timbers by rainwater. This results in color change and discoloration stains on the surface of the timbers. Leaching of extractives is typically uneven and may result in ‘tide mark’ effects. The tide marks on the weathered timber show up where these extractives are concentrated (see image).
The effects of extractives on surface performance of exterior timbers vary by species, finishes, construction practices, and the degree of protection from prolonged wetting.
Due to this visual change, unfinished exterior wood materials, such as wall cladding, roof cladding and decking, will typically look very different over time, in comparision to when the products were first installed.
Weathering is something that architects and interior designers should keep in mind when specifying standard wood materials for exterior applications. Most of the time, the exterior wood materials that are depicted in project renderings represent day one color. If specifiers set color change expectations with the end-user during the project, then the shift from day-one color to soft greys & light brown tones is nothing more than an aesthetic byproduct to be savored and celebrated. However, if day-one color is desired throughout the lifetime of the material, a UV finish needs to be applied every year or so. Alternatively, home-owners and specifiers can take advantage of advances in exterior wood manufacturing by choosing prefinished, pre-weathered exterior wood materials from the start.
PRE-WEATHERED EXTERIOR WOOD CLADDING & DECKING
There are some company’s that will “pre-weather” by putting wood outside to weather often by laying boards horizontally on the ground surface, similar to a decking application, and leave it to weather in place for a year or so, then sell it. This solution is not recommended. The integrity of the wood is often compromised. Boards can twist, cup, and bow during this time as they are not properly installed. Mold and Mildew can also harbor on the boards during this time of “weathering”.
reSAWN TIMBER co. developed our exterior pre-finished, pre-weathered product line based on a need in the building materials market, the observations of experienced industry leaders and a first-hand understanding of customer desires and expectations.
Our pre-weathered designs mimic the look of textured wood that has been exposed to years of exterior elements and left to weather with grace. This design solution solves for wood’s natural propensity to gray out by resembling a weathered look at the time of installation, with little aesthetic change as it ages in place. For interior applications, expect little to no change from the day of installation.
We work with a number of reactive weathered wood finishes to create the look of weathered wood. For example, Sioo:x is premium wood and surface protector, that results in durable, low maintenance, beautiful wood surfaces. It uses nature to form a hard silicon based shell on exterior timber. In addition to the protective benefits gained, as the mineral silicate cures over time, it will start to turn the timber a beautiful even silver/greyish tone starting from 10-16 weeks (depending on the timber species the weathering period may vary).
Benefits of specifying reSAWN’s pre-weathered exterior wood cladding, roofing, and decking:
- Color/weathering expectations are set at the time of material specification since there will be little to no change in the color of the wood material
- The ability to achieve consistent color on all differing exterior exposures, regardless of the weather conditions
- In most structures that utilize interior and exterior cladding, there’s a visual division between the exterior wall cladding and interior wall cladding. With our pre-weathered wood designs, there can be a seamless interior/exterior aesthetic from day one when exterior cladding is continued to the interior of the structure
- No additional treatment beyond normal cleaning. It’s a low maintenance material – this leaves more time to enjoy the product, less time taking care of it
- From day one, our pre-weathered designs pay homage to woods natural warm and cool tones without the need to wait years for it to patina
- Pre-weathered wood materials blend in well with nature and exterior surroundings
- Our designs display woods sought-after texture and character in its rawest form
- All finishes are ultra-low/zero VOC and can contribute to LEED projects
- reSAWN’s pre-weathered offerings are sustainably harvested in the United States
- Most designs can be fire treated to Class A for interior applications
At reSAWN TIMBER co. we celebrate and visually leverage wood’s inherent tendency to weather over time by designing modern, high-performance products that are meant to age in place with grace. We work with wood’s natural tendency to grey, not against it. Our prefinished products look beautiful on day one and continue to as nature takes over – with little to no aesthetic maintenance needed.
Each board works cohesively together, yet each board has its own visual story. The result is modern textural beauty & inherent originality with each and every project. From the start, expect a soft, consistent time-worn look with color and texture that looks natural, one-of-a-kind and incredibly interesting.
reSAWN TIMBER co.’s PRE-WEATHERED PRODUCT OFFERING:
Samples of our pre-weathered wood are available for architects and designers at no cost and shipped via UPS Ground. Click on the button below to explore reSAWN’s complete product offering – sort by species, color or application.