Learning Objective: Upon completing this section, you should be able to identify the types, sources, uses, and characteristics of the common woods used on various construction projects.
Of all the different construction materials, wood is probably the most often used and perhaps the most important. The variety of uses of wood is practically unlimited. Few Seabee construction projects are accomplished without using some type of wood. It is used for permanent structures as well as concrete forms, scaffolding, shoring, and bracing, which may be used again and again. The types, sources, uses, and characteristics of common woods are given in table 3-1. The types of classifications of wood for a large project are usual] y designated in the project specifications and included in the project drawings.
Table 3-1.-Common Woods
The terms "wood" "lumber," and "timber" are often spoken of or written in ways to suggest that their meanings are alike or nearly so. But in the builder’s language, the terms have distinct, separate meanings. Wood is the hard, fibrous substance that forms the major part of the trunk and branches of a tree. Lumber is wood that has been cut and surfaced for use in construction work. Timber is lumber that is 5 inches or more in both thickness and width.
SEASONING OF LUMBER
Seasoning of lumber is the result of removing moisture from the small and large cells of wood— drying. The advantages of seasoning lumber are to reduce its weight; increase its strength and resistance to decay; and decrease shrinkage, which tends to avoid checking and warping after lumber is placed. A seldom used and rather slow method of seasoning lumber is air-drying in a shed or stacking in the open
until dry. A faster method, known as kiln drying, has lumber placed in a large oven or kiln and dried with heat, supplied by gas- or oil-fired burners. Lumber is considered dry enough for most uses when its moisture content has been reduced to about 12 or 15 percent. As a builder, you will learn to judge the dryness of lumber by its color, weight, smell, and feel. Also, after the lumber is cut, you will be able to judge the moisture content by looking at the shavings and chips.
DEFECTS AND BLEMISHES
A defect in lumber is any flaw that tends to affect the strength, durability, or utility value of the lumber. A blemish is a flaw that mars only the appearance of lumber. However, a blemish that affects the utility value of lumber is also considered to be a defect; for example, a tight knot that mars the appearance of lumber intended for fine cabinet work.
Various flaws apparent in lumber are listed in table 3-2.
Table 3-2.—Wood Defects and Blemishes
|COMMON NAME ||DESCRIPTION |
|Bark Pocket ||Patch of bark over which the tree has grown, and has entirely or almost entirely enclosed |
|Check ||Separation along the lengthwise grain, caused by too rapid or non-uniform drying |
|Cross Grain ||Grain does not run parallel to or spiral around the lengthwise axis |
|Decay ||Deterioration caused by various fungi |
|Knot ||Root section of a branch that may appear on a surface in cross section or lengthwise. A cross-sectional knot maybe loose or tight. A lengthwise knot is called a spike knot |
|Pitch Pocket ||Deposit of solid or liquid pitch enclosed in the wood |
|Shake ||Separation along the lengthwise grain that exists before the tree is cut. A heart shake moves outward from the center of the tree and is caused by decay at the center of the trunk. A wind shake follows the circular lines of the annual rings; its cause is not definitely known |
|Wane ||Flaw in an edge or corner of a board or timber. It is caused by the presence of bark or lack of wood in that part |
|Warp ||Twist or curve caused by shrinkage that develops in a once flat or straight board |
|Blue Stain ||A blemish caused by a mold fungus; it does not weaken the wood |
CLASSIFICATION OF LUMBER
Trees are classified as either softwood or hardwood (table 3-3). Therefore, all lumber is referred to as either “softwood” or “hardwood.” The terms “softwood” and “hardwood” can be confusing since some softwood lumber is harder than some hardwood lumber. Generally, however, hardwoods are more dense and harder than softwoods. In addition, lumber can be further classified by the name of the tree from which it comes. For example, Douglas fir lumber comes from a Douglas fir tree; walnut lumber comes from a walnut tree, and so forth.
The quality of softwood lumber is classified according to its intended use as being yard, structural, factory, or shop lumber. Yard lumber consists of those grades, sizes, and patterns generally intended for ordinary building purposes. Structural lumber is 2 or more inches in nominal thickness and width and is used where strength is required. Factory and shop lumber are used primarily for building cabinets and interior finish work.
Lumber manufacturing classifications consist of rough dressed (surfaced) and worked lumber. Rough lumber has not been dressed but has been sawed, edged, and trimmed. Dressed lumber is rough lumberthat has been planed on one or more sides to attain smoothness and uniformity. Worked lumber, in addition to being dressed, has also been matched, shiplapped, or patterned. Matched lumber is tongue and groove, either sides or ends or both. Shiplapped lumber has been rabbeted on both edges to provide a close-lapped joint. Patterned lumber is designed to a pattern or molded form.
The grade of a piece of lumber is based on its strength, stiffness, and appearance. A high grade of lumber has very few knots or other blemishes. A low grade of lumber may have knotholes and many loose knots. The lowest grades are apt to have splits, checks, honeycombs, and some warpage. The grade of lumber to be used on any construction job is usually stated in the specifications for a set of blueprints. Basic classifications of softwood grading include boards, dimension, and timbers. The grades within these classifications are shown in table 3-4.
Lumber is graded for quality in accordance with American Lumber Standards set by the National Bureau of Standards for the U.S. Department of Commerce. The major quality grades, in descending order of quality, are select lumber and common lumber. Table 3-5 lists the subdivisions for each grade in descending order of quality.
Table 3-3.-Different Types of Softwoods and Hardwoods
|SOFTWOODS ||HARDWOODS |
|Douglas fir |
Sweet gum White ash*
|Ponderosa pine |
Western red cedar
Table 3-4.-Softwood Lumber Grades
Grades of hardwood lumber are established by the National Hardwood Lumber Association. FAS (firsts and seconds) is the best grade. It specifies that pieces be no less than 6-inches wide by 8-feet long and yield at least 83 1/3 percent clear cuttings. The next lower grade is selects, which permits pieces 4-inches wide by 6-feet long. A still lower grade is No. 1 common.
Lumber in this group is expected to yield 6623 percent clear cuttings.
Standard lumber sizes have been established in the United States for uniformity in planning structures and in ordering materials. Lumber is identified by nominal sizes. The nominal size of a piece of lumber is larger than the actual dressed dimensions. Referring to table 3-6, you can determine the common widths and thicknesses of lumber in their nominal and dressed dimensions.
Table 3-5.-Grades and Subdivisions of Lumber
|SELECT LUMBER |
|Grade A ||This lumber is practically free of defects and blemishes |
|Grade B ||This lumber contains a few minor blemishes |
|Grade C ||This lumber contains more numerous and more significant blemishes than grade B. It must be capable of being easily and thoroughly concealed with paint |
|Grade ||This lumber contains more numerous and more significant blemishes than grade C, but it is still capable of presenting a satisfactory appearance when painted |
|COMMON LUMBER |
|No. 1 ||Sound, tight-knotted stock containing only a few minor defects. Must be suitable for use as watertight lumber |
|No. ||Contains a limited number of significant defects but no knotholes or other serious defects. Must be suitable for use as grain-tight lumber |
|No. 3 ||Contains a few defects that are larger and coarser than those in No. 2 common; for example, occasional knotholes |
|No. 4 ||Low-quality material containing serious defects like knotholes, checks, shakes, and decay |
|No. 5 ||Capable only of holding together under ordinary handling |
Table 3-6.-Nominal and Dressed Sizes of Lumber
Figure 3-29.-Laminated lumber.
Laminated lumber (figure 3-29) is made of several pieces of lumber held together as a single unit, a process called lamination. Usually 1 1/2-inches thick, the pieces are nailed, bolted, or glued together with the grain of all pieces running parallel. Laminating greatly increases the load-carrying capacity and rigidity of the weed. When extra length is needed, the pieces are spliced—with the splices staggered so that no two adjacent laminations are spliced at the same point. Built-up beams and girders are examples. They are built as shown in figure 3-30, usually nailed or bolted together, and spliced.
Figure 3-29.-Laminated lumber.
Figure 3-30.-Built-up beam.
Lamination can be used independently or with other materials in the construction of a structural unit. Trusses can be made with lamination for the chords and sawed lumber, or for the web members (figure 3-31). Special beams can be constructed with lamination for the flanges and plywood or sawed lumber, for the web, as shown in figure 3-32. Units, such as plywood box beams and stressed skin panels, can contain both plywood and lamination (figure 3-33).
Figure 3-31.-Truss using laminated and sawed lumber.
Figure 3-32.-Laminated and sawed lumber or plywood beam.
Figure 3-33.-Stressed skin panel.
Probably the greatest use of lamination is in the fabrication of large beams and arches. Beams with spans in excess of 100 feet and depths of 8 1/2 feet have been constructed using 2-inch boards. Laminations this large are factory produced. They are glued together under pressure. Most laminations are spliced using scarf joints (figure 3-34), and the entire piece is dressed to ensure uniform thickness and width. The depth of the lamination is placed in a horizontal position and is usually the full width of the beam (figure 3-35).
Figure 3-34.-Scarf joints.
Figure 3-35.-Laminated beam.
Plywood is constructed by gluing together a number of layers (plies) of wood with the grain direction turned at right angles in each successive layer. This design feature makes plywood highly resistant to splitting. It is one of the strongest building materials available to Seabees. An odd number (3, 5, 7) of plies is used so that they will be balanced on either side of a center core and so that the grain of the outside layers runs in the same direction. The outer plies are called faces or face and back. The next layers under these are called crossbands, and the other inside layer or layers are called the core (figure 3-36). A plywood panel made of three layers would consist of two faces and a core.
Figure 3-36.-Grain direction in a sheet of plywood.
There are two basic types of plywood: exterior and interior. Exterior plywood is bonded with waterproof glues. It can be used for siding, concrete forms, and other constructions where it will be exposed to the weather or excessive moisture. Interior plywood is bonded with glues that are not waterproof. It is used for cabinets and other inside construction where the moisture content of the panels will not exceed 20 percent.
Plywood is made in thicknesses of 1/8 inch to more than 1 inch, with the common sizes being 1/4, 3/8, 1/2, 5/8, and 3/4 inch. A standard panel size is 4-feet wide by 8-feet long. Smaller size panels are available in the hardwoods.
Plywood can be worked quickly and easily with common carpentry tools. It holds nails well and normally does not split when nails are driven close to the edges. Finishing plywood presents no unusual problems; it can be sanded or texture coated with a permanent finish or left to weather naturally.
There is probably no other building material as versatile as plywood. It is used for concrete forms, wall and roof sheathing, flooring, box beams, soffits, stressed-skin panels, paneling, shelving, doors, furniture, cabinets, crates, signs, and many other items.
Softwood Plywood Grades
All plywood panels are quality graded based on products standards (currently PS 1/74). The grade of each type of plywood is determined by the kind of veneer (N, A, B, C, or D) used for the face and back of the panel and by the type of glue used in construction. The plywood veneer grades are shown in table 3-7.
Table 3-7.-Plywood Veneer Grades
Many species of softwood are used in making plywood. There are five separate plywood groups based on stiffness and strength. Group 1 includes the stiffest and strongest; group 5 includes the weakest woods. A listing of groupings and associated woods is shown in table 3-8.
Table 3-8.-Classification of Softwood Plywood Rates Species for Strength and Stiffness
GRADE/TRADEMARK STAMP.— Construction and industrial plywood panels are marked with different stamps.
Construction Panels.— Grading identification stamps (such as those shown in figure 3-37) indicate the kind and type of plywood. The stamps are placed on the back and sometimes on the edges of each sheet of plywood.
Figure 3-37.-Standard plywood identification symbols.
For example, a sheet of plywood having the designation “A-C” would have A-grade veneer on the face and C-grade veneer on the back. Grading is also based on the number of defects, such as knotholes, pitch pockets, splits, discolorations, and patches in the face of each panel. Each panel or sheet of plywood has a stamp on the back that gives all the information you need. Table 3-9 lists some uses for construction-grade plywood.
Industrial Panels.— Structural and sheeting panels have a stamp found on the back. A typical example for an industrial panel grade of plywood is shown in figure 3-38.
Figure 3-38.-Structural stamp.
The span rating shows a pair of numbers separated by a slash mark (/). The number on the left indicates the maximum recommended span in inches when the plywood is used as roof decking (sheeting). The right-hand number applies to span when the plywood is used as subflooring. The rating applies only when the sheet is placed the long dimension across three or more supports. Generally, the larger the span rating, the greater the stiffness of the panel.
Figure 3-39 lists some typical engineered grades of plywood. Included are descriptions and most common uses.
Table 3-9.-Plywood Uses
SOFTWOOD PLYWOOD GRADES FOR EXTERIOR USES
|Face ||Back || |
|A ||A ||C ||Outdoor where appearance of both sides is important |
|A-B ||A ||B ||C ||Alternate for A-A where appearance of one side is less important |
|A-C ||A ||C ||C ||Siding, soffits, fences. Face is finish grade |
|B-C ||B ||C ||C ||For utility uses, such as farm buildings, some kinds of fences, etc. |
|C ||C ||Excellent base for tile and linoleum, backing for wall coverings |
|C-C ||C ||C ||C ||Unsanded, for backing and rough construction exposed to weather |
|B ||B ||C ||Concrete forms. Reuse until wood literally wears out |
|B ||B or C ||C or |
|Medium density overlay. Ideal base for paint; for siding, built-ins, signs, displays |
|A or B ||A or B || |
|High density overlay. Hard surface; no paint needed. For concrete forms, cabinets, counter tops, tanks |
SOFTWOOD PLYWOOD GRADES FOR INTERIOR USES
|Face ||Back || |
|A ||A ||D ||Cabinet doors, built-ins, furniture where both sides will show |
|A ||B ||D ||Alternate of A-A. Face is finish grade, back is solid and smooth |
|A ||D ||D ||Finish grade face for paneling, built-ins, backing |
|B ||D ||D ||Utility grade. One paintable side. For backing, cabinet sides, etc |
|C ||D ||D ||Sheathing and structural uses such as temporary enclosures, subfloor. Unsanded |
Figure 3-39.-List of engineered grade of softwood plywood.
Exposure Ratings.— The grade/trademark stamp lists the exposure durability classification for plywood. There are two basic types or ratings: exterior and interim. The exterior type has a 100-percent waterproof glue line, and the interior type has a highly moisture-resistant glue line. However, panels can be manufactured in three exposure durability classifications: Exterior, Exposure 1, and Exposure 2.
Panels marked “Exterior” can be used where there is continual exposure to weather and moisture. Panels marked “Exposure 1“ can withstand moisture during extended periods, but they should be used only indoors. Panels marked “Exposure 2“ can be used in protected locations. They may be subjected to some water leakage or high humidity but generally should be protected from weather.
Most plywood is made with waterproof exterior glue. However, interior panels may be made with intermediate or interior glue.
Hardwood Plywood Grades
Hardwood plywood panels are primarily used for door skins, cabinets, and wall paneling. The Hardwood Plywood Manufacturers’ Association has established a grading system with the following grades: premium (A), good grade (1), sound grade (2), utility grade (3), and backing grade (4). For example, an A-3 grade hardwood plywood would have a premium face and a utility back. A 1-1 grade would have a good face and a good back.