Design Solutions - Spring 2008 - (Page 55) DUE TO MOISTURE IN THE AIR TO Expansion of: 1” X 8” X 10’ When relative humidity increases from 25% to 85% Figure 100-01, page 23, Quality Standards Ilustrated Page 23 of the 8th Edition QSI has several illustrations showing the effects of changing moisture content on wood. Notice in Figure 100-01 on the top that the greatest percentage of movement is across the grain in plain sawn lumber, that is to say tangential to the growth rings. The most stable woods have the smallest coefficient of expansion and contraction in the Comparative Table of Wood Species, the last column in the table on Page 17 of the Quality Standards. Footnote 3 at the bottom of the table explains how these dimensions relate to changes in moisture content. One point of interest: the least stable wood species shown on the table is often a surprise to many people. Hard Maple is often thought of as “rock solid” and impervious to the hardest use—like a basketball court, butcher block top or bowling alley. In fact, when you take a look at any of these applications you will see rather narrow boards being used and, in the case of bowling alleys and butcher block tops, the boards are used on edge. As a result the coefficient is actually the radial, or thickness, shrinkage and expansion, which is usually less than the tangential. E x t E r i o r A p p l i c At i o n s , F i r E - r E tA r d A n t W o o d s A n d p r E s E r v At i v E t r E At m E n t s It’s hard to choose wood to use for exterior applications. Section 100-G-9, Page 24, lists the most useful species for outdoor woodwork: Eastern and Western Red Cedar, Eastern white Pine, Douglas Fir, American (Honduras) Mahogany, White Oak, Teak, and Redwood. In any case, the heartwood of a species is always more resistant to decay and weathering than the sapwood. A good quality painted finish is almost always superior to a “natural” or clear finish when good protection and low maintenance are a consideration. Notice that red Oak isn’t listed, because it is very porous and degrades quickly in the weather. Spring 2008 I DESIGN SOLUTIONS 55 How to Select Solid Lumber EXPANSION Many woodworkers have had bad luck trying to use Poplar outside. This is because most Poplar available is sapwood. As a matter of fact, that’s why Cypress is not used for exterior trim and windows nearly as much as in the early parts of the 20th century. The Cypress now available is mostly second growth and largely sapwood. Complete information on fire rated issues starts on Page A-610 in the Architectural Woodwork Quality Standards, in the Fire Code Summary. A few important points to keep in mind: Fire retardant solid trims are almost always not required. Subject to local codes, if your solid trims have a surface area of less than 10% of the combined surface area of the walls and ceiling, they need not be treated for fire resistance. One of the most common mistakes in specifications is to assume the fire resistance required of the framing and of structural members applies to the trims and architectural woodwork. In most cases it doesn’t. When fire resistant wood is required, there are only three species commonly available: soft Maple, red Oak, and Basswood. Even then, machining after treatment may void the label certification. Finally, AWI is often asked whether some kind of fire rated coating can be put over existing wood. Intumescent coatings are rather soft, and are not recommended if there is any other alternative. Some lumber used in contact with the ground or in high moisture situations needs to be preservative treated. With the exception of some windows and doors, very little architectural woodwork benefits from preservative treatments.
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