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All woods are affected significantly by moisture and to a lesser degree by heat. Lumber swells and shrinks primarily in two directions: thickness and width. There is insignificant change in length. The changes in dimension due to moisture vary with different species, thus influencing the selection of lumber to use and the design elements. Prevention of dimensional problems in architectural woodwork products as a result of uncontrolled relative humidity is possible. Wood products perform, as they have for centuries, with complete satisfaction when correctly designed and used. Problems directly or indirectly attributed to dimensional change of the wood are usually, in fact, the result of faulty design or improper humidity conditions during site storage, installation, or use.

shrinkage expansion diagram

(INSERT CALCULATOR THAT CHOOSES CUT, WIDTH AND RH)

Wood is a hygroscopic material, and under normal conditions all wood products contain some moisture. Wood readily exchanges this moisture with the water vapor in the surrounding atmosphere according to the relative humidity. In high humidity, wood picks up moisture and swells. In low humidity, wood releases moisture and shrinks. As normal minor changes in humidity occur, the resulting dimensional response in properly designed construction will be insignificant. To avoid problems, it is recommended that relative humidity be maintained within the range of 25% and 55%. Uncontrolled extremes - below 20% or above 80% relative humidity - are likely to cause problems. Together with proper design, fabrication, and installation, humidity control is the important factor in preventing dimensional change problems. The book Understanding Wood by Bruce Hoadley contains excellent data of wood and moisture.

 

Wood is anisotropic in its shrinkage characteristics. It shrinks most in the direction of the annual rings when it loses moisture from the cell walls. This illustration from the USDA Wood Handbook shows the typical distortion of cuts from various parts of a log.

 

distortion log cuts

 

Moisture can also cause iron stain (oxidation) in wood, also referred to as blue/black stain. Iron stain is a natural reaction of acids with iron, oxygen, and moisture (either high relative humidity or direct moisture) in wood. Control of moisture is a simple way to protect wood products from iron stain.

Availability

The supply of lumber is in constant flux throughout the world. It is affected by many factors such as current demand, export regulations of the country of origin, natural forces of weather, fire, disease, political situations, etc. Consult an AWI woodworker before specifying uncommon species, as well as large quantities of a species, thickness, width, or long length.

Size Limitations

Certain trees (species) naturally grow larger, thus producing longer and wider lumber. Other trees are smaller and produce narrow and shorter boards. The architectural woodworker must work with the available lumber, which must be considered when selecting any species.

Cost

The cost of lumber, as with other commodities, is influenced by supply and demand, both of which are constantly changing. For current comparative costs consult an AWI woodworker.

Strength, Hardness, Density

Always a consideration is the ability of the selected lumber species to sustain stress; resist indentation, abuse, and wear;

and to carry its anticipated load in applications such as shelving and structural members. The Wood Handbook (link to exact page), published by the U.S. Forest Products Society contains comprehensive data on the mechanical properties of wood.

Comparative Table of Wood Species

Comparative Table of Wood Species

In order to simplify species selection, the Comparative Table of Wood Species has been prepared showing pertinent characteristics of some species of domestic and foreign woodlouse the architectural woodwork industry. The table can quickly confirm or deny the wisdom of a species selection byte architect or designer or conversely lead to a proper selection after studying the characteristics.“Cost" has been broken into both Lumber and Plywood headings,with data divided into Low, Moderate, High, and Very High [V. High]. (Important: Market conditions cause these relationships to vary. Current ratios are likely to be different.) The reason for cost variations in the two products is obvious when we consider the physical differences. Generally, the prices of veneered products reflect the relatively high labor and equipment cost and relatively low material cost in their manufacture. On the other hand, the price of lumber in most species reflects cost factors that are exactly the opposite. In spite of their physical differences, the two products are always compatible, and both are essential to complete design freedom in contemporary buildings.End use determines the importance of Hardness in selecting a species for each particular type of application. Counters, door
frames, wall treatments in high-traffic areas, etc., are obvious uses of wood products where hardness and resistance to abrasion must be considered. In many other applications these factors,relatively speaking, are not of great importance. Dimensional Stability — The dimensional stability table is helpful in selecting woods for use where humidity conditions may vary widely and where design or fabrication of a wood product does not allow free movement or the use of plywood. The column figures indicate extreme conditions and show the maximum amount of movement possible in a 305 mm [12"] wide piece of unfinished wood where its moisture content increases or decreases from 10% to 5%. The possible change in dimension demonstrates that unfinished interior woodwork must be carefully protected prior to finishing by keeping in rooms where relative humidity is between 25% and 55%. The column also shows the variation between species, and between flat grain and edge grain where such cuts are available commercially. Careful analysis of the table will make it possible for an architect,designer or specification writers (who may have only limited knowledge of architectural wood species) to make uninformed selection. It is our intent that this tool will enhance understanding between the manufacturer of the woodwork you have designed and your profession, thereby enabling the building industry to better service the client.

Adaptability for Exterior Use

Years of performance have shown certain species to be more durable for exterior applications. Heartwood shall be furnished when these species are designated for external use, excluding the sapwood. The following is a list of species generally considered acceptable for exterior use, from the Wood Handbook(USDA):

Eastern and Western Red Cedar Oak, white

Cherry, black Teak, old growth

Douglas-Fir Redwood, heartwood

Mahogany, Genuine Locust, black

Chestnut Spanish Cedar

Bald cypress (Taxodium distichum) has a long tradition as species resistant to decay, but beware! There are at least nine other species of four different genus which are marketed under the common name cypress. Only the heartwood of T.distichum, often marketed as Tidewater or Red Cypress, is decay resistant. Sinker Cypress, that is old trees which have been brought up from below water in which they have been submerged for some time and properly cured and dried, nonresistant. None of this Cypress will come from new cutting, but as salvaged wood.

Fire-Retardant Wood

The natural fire-retardant qualities and acceptability of treatments varies among the species. Where items of architectural woodwork are required to have a flame spread classification to meet applicable building and safety codes, the choice of lumber species must be a consideration. Most treated species are structural softwoods. Following are some references to assist in making these choices. Additional data on various species may be available from the U.S. Department of Agriculture Forest Service, Fire Safety of Wood Products Work Unit —(608) 231-9269.

Flame Spread Classification: This is the generally accepted measurement for fire rating of materials. It compares the rate of flame spread on a particular species with the rate of flame spread on untreated Oak. Most authorities accept the following classes for flame spread:Class I or A 0-25 Class II or B 26-75 Class III or C 76-200

Built-up Construction to Improve Fire Rating: In lieu of solid lumber, it is often advisable, where a fire rating is required,to build up members by using treated cores clad with untreated veneers not thicker than 1 mm [1/28"]. Some existing building codes, except where locally amended, provide that facing materials 1 mm [1/28"] or thinner finished dimension are not considered in determining the flame spread rating of the woodwork. See Section 200, Panel Products.

In localities where basic model building codes have been amended, it is the responsibility of the specifier to determine whether the application of the facing material specified will meet the code.

Fire-Retardant Treatments (FRT): Some species may be treated with chemicals to reduce flammability and retard the spread of flame over the surface. This usually involves impregnating the wood, under pressure, with salts suspended in a liquid. The treated wood must be redried prior to fabrication. FRT wood may exude chemicals in relative humidity above 85%, damaging finishes and corroding metals in contact with the FRT surface. Consult with an AWI woodworker about the resulting appearance and availability of treated woods prior to specification.Hardwoods currently being treated (Flame spread less than 25)include 4/4 Red Oak, and 4/4 to 8/4 Poplar. These woods cane machined after treatment, although machining may void the label classification. Fire retardant treatment does affect the color and finishing characteristics of the wood.According to the traditional model codes in the USA and subject to local code modifications, untreated wood and wood products can usually be used in up to 10% of the combined surface area of the walls and ceiling. Cabinetry, furniture, and fixtures are rarely fire rated, and can be built of combustible materials.

Finishing of Fire-Retardant Treated Lumber: Fire retardant treatments may affect the finishes intended to be used on the wood, particularly if transparent finishes are planned. The compatibility of any finishes should be tested before they are applied.

Intumescent Coatings for Wood: It is possible to reduce flammability by using intumescent coatings in either opaque or transparent finishes. These are formulated to expand or foam when exposed to high heat, and create an insulating effect,which reduces the speed of flame spread. Improvements are continually being made on these coatings. Consequently, the specifier must ascertain whether they will be permitted under the code governing the project. The relative durability of the finish and the effect of the coating on the desired color of the finished product vary from manufacturer to manufacturer. In general, the coatings are less durable, softer, and more hygroscopicthan standard finishes.

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