Harwood Flooring Basics

Wood flooring has been a traditional American favorite since pioneer homes were first erected from that most readily available resource—the tree. Today, however, hardwood flooring is ex pensive. The selection and installation of quality hardwood flooring often costs more than other common flooring materials. Even so, the lower maintenance costs and the aesthetic qualities of hardwood flooring, combined with reduced costs to the do-it-yourselfer, have spurred a renewed interest in practical hardwood floors. An important reason is the natural beauty of hardwood floors, which blends with all types and tastes of interior decorating from traditional to contemporary.


Wood flooring materials are available in strip, plank, block, and tile form. Wood flooring is available in both hardwood and softwood. The most common types of hardwood floors are red and white oak, beech, birch, maple, and pecan. The softwoods most commonly used for flooring are southern pine, Douglas fir, redwood, and western hemlock.

Softwood finish flooring costs less than most hardwood species and can be used where traffic is light. However, softwood flooring is less dense than the hardwoods so it’s less wear-resistant and more readily shows abrasions. Softwood finish flooring is also less expensive than hardwood flooring.

The most popular hardwood flooring is 25/32 x 2 1/4-inch strip flooring. The strips are laid lengthwise in a room and normally at right angles to the floor joists. Strip flooring is typically tongue-and-groove and end matched. Strips are of random length and may vary from 2 to 16 feet or longer.

End-matched strip flooring is usually hollow-backed. The face is wider than the back so that when it’s assembled on a floor the joints are tight. A tight fit means fewer floor squeaks.

Wood block flooring uses tongued-and-grooved blocks of wood that measure 25/32 (about 3/4) inch thick and are available in dimensions from 4 x 4 to 9 x 9 inches. Larger measurements are preferred for larger rooms to seemingly reduce their size.


To guide you to an understanding of hardwood flooring (FIG. 1-3), let’s consider the properties of wood, the types of wood joints, the common types of hardwood flooring, and how they all fit into modern home construction.

1-3. Typical hardwood flooring

Any piece of wood is made up of a number of small cells as shown in FIG. 1-4. The size and arrangement of the cells determine the grain of the wood and many of its properties. Examine a freshly cut tree stump and you’ll see that the millions of small cells are arranged in circular rings around the pith, or center, of the tree ( FIG. 1-5). These rings are caused by a difference in the rate of growth of the tree during the various seasons of the year. In spring, the tree grows rapidly and builds up a thick layer of comparatively soft, large cells that appear in the cross section of the trunk as the light-colored annual rings.

1-4. The structure of wood.

1-5. I Cross section of a tree. Summer ring; Spring ring; Pith; Wood ray; etc.

As the weather becomes warmer during the early summer, the rate of growth slows. The summer growth forms cells that are packed more closely. These pairs of concentric springwood and summerwood rows form the annual rings, which can be counted to find out the age of the tree. Because of climatic conditions, some trees, such as oak and walnut, have more distinctive rings than others, such as maple and birch. White pine is so uniform that you can hardly distinguish the rings, while many other softwoods have a very pronounced contrast between summerwood and springwood, which makes it easy to distinguish the rings.

The sapwood of a tree is the outer section of the tree between the heartwood, or the darker center wood, and the bark. The sapwood is lighter in color than the heartwood, but it gradually becomes darker as it changes to heartwood on the inside and as new layers are formed on the outside. Depending upon the type of tree, it requires from 9 to 36 years to transform sapwood into heartwood.

The cambium layer, which is the boundary between the sapwood and the bark, is the thin layer where new sapwood cells form. Medullary rays are radial lines of wood cells that consist of threads of pith and serve as the lines of communication between the central cylinder of the tree and the cambium layer. They are especially prominent in oak.

When a tree is sawed lengthwise, the annual ring forms a pattern, which is called the grain of the wood. Many terms are used to describe the various grain conditions. If the cells that form the grain are closely packed and small, the wood is said to be fine-grained or close-grained. Maple and birch are excellent examples of this type of wood. If the cells are large, open, and porous, the wood is coarse-grained or open-grained. Examples of this are oak, walnut, and mahogany. Furniture and flooring made of open-grained woods require the use of a wood filler to close the pores and provide a smooth outside finish.

When the wood cells and fibers are comparatively straight and parallel to the trunk of the tree, the wood is said to be straight-grained. If the grain is crooked, slanted, or twisted, it’s said to be cross-grained. It’s the arrangement, direction, size, and color of the wood cells that give the grain of each wood its characteristic appearance.


In large lumber mills, such as those found in the Pacific Northwest, logs are usually processed into lumber with huge band or circular saws. There are two methods of sawing up logs. Slash cutting is accomplished by a series of Cuts that are made parallel to the side of the log. If hardwoods are being cut, the process is known as plain sawing. If softwoods are being cut, the process is referred to as flat-grain sawing.

Lumber that is specifically cut to provide edge grain on both faces is said to be rift-cut. If hardwood is being cut, the lumber is said to be quartersawed (FIG. 1-6). If softwood is being sawed, it’s called edge-grain lumber. Incidentally, if an entire log is slash-cut, several boards from near the center of the log will actually be rift-cut.

1-6. Slash and rift cutting.

Slash-cut lumber is usually cheaper than rift-cut lumber because it takes less time to slash-cut a log and there is less waste. Circular or oval knots that appear in slash-cut boards affect the strength and surface appearance much less than do spike knots, which may appear in rift-cut boards. If a log is sawed to produce all slash-cut lumber, however, more boards will contain knots than if the log were sawed to produce the maxi mum amount of rift-cut material. Another advantage of slash cutting is that when shakes and pitch pockets are present, they will extend through fewer boards.

For many applications, especially flooring, rift-cut lumber is preferred because it’s more wear-resistance than slash-cut lumber. Rift-cut lumber also shrinks and swells less in width. Another advantage to rift-cut lumber over slash-cut lumber is that it twists and cups less and splits less when used. Rift-cut lumber also usually holds paint and other finishes better.

After being sawed, lumber must be thoroughly dried before it’s suit able for most uses. The old method—and one still preferred for some uses—was merely to air-dry the lumber in a shed or stack it out in the open. This method requires considerable time for the wood to dry—up to seven years for some of the hardwoods.

A faster method is known as kiln drying. The wood is placed in a tight enclosure, called the kiln, and dried with heat that is supplied by artificial means. The length of drying time required varies from two or three days to several weeks, depending on the kind of wood, its dimensions, and the method of drying.

Lumber is considered dry enough for most uses when the moisture content has been reduced to between 12 and 15 percent. If you use lumber very much, you will soon learn to judge the dryness of wood by its color, weight, smell, feel, and by a visual examination of shavings and chips. Your lumber supplier can also give you a close estimate of a wood’s dryness.

Briefly, lumber is seasoned by removing the moisture from the mil lions of small and large cells of which wood is composed. Moisture, which can be water or sap, occurs in two separate forms: free water and embedded water. Free water is the amount of moisture the individual cells contain. Embedded water is the moisture absorbed by the cell walls.

During the drying or seasoning process, the free water in the individual cells evaporates until a minimum amount of moisture is left. The point at which this minimum moisture remains is called the fiber saturation point. The moisture content of this point varies from 25 to 30 percent. Below the fiber saturation point, the embedded water is extracted from the porous cell walls. This process causes a reduction in the thickness of the walls. Wood shrinks across the grain when the moisture content is lowered below the fiber saturation point.

Wood shrinks or swells when varying amounts of moisture change the size of the cells, Therefore, the lowering or raising of the moisture content causes lumber to shrink or swell. The loss of moisture during the seasoning process causes wood to be harder, stronger, stiffer, and lighter in weight—all qualities important to the lift of hardwood flooring.


There is a wide selection of wood materials that may be used for flooring. Hardwoods and softwoods are available as strip flooring (FIG. 1-7) in a variety of widths and thicknesses, and as random-width planks, and block flooring (FIG. 1-8).

1-7. Four methods of quartersawing.

1-8. Strip flooring is available in vertical (A) and flat-grain (B) hardwood and softwood stock.

Softwood finish flooring costs less than most hardwood species and is often used to good advantage in bedroom and closet areas where traffic is light. It might also be selected to fit the interior decor. While this guide primarily covers hardwood flooring, many of the principles and instructions also apply to softwood flooring.

TABLE 1-1 lists the grades and description of softwood strip flooring. Softwood flooring has tongue-and-groove edges and may be hollow- backed or grooved. Some types are also end-matched. Vertical-grain flooring generally has better wearing qualities than flat-grain flooring. TABLE 1-1 also lists the grades, types, and sizes of hardwood strip flooring. Manufacturers supply both pre-finished and unfinished flooring.

Table 1-1. Wood Flooring Grades and Graining. (1) Special grades are available in which uniformity of color is a requirement.

Perhaps the most widely used flooring pattern is the 25/32 x 2 1/4-inch strip flooring. The strips are laid lengthwise in a room and normally at right angles to the floor joists. A subfloor of diagonal boards or plywood is normally used under the finish floor. Strip flooring of this type is tongue-and-groove and end-matched (FIG. 1-9). Strips are of random length and may vary from 2 to 16 feet or more.

1-9. Tongue-and-groove, end-matched strip flooring.

End-matched strip flooring in a 25/32-inch thickness is generally hollow-backed. The face is slightly wider than the bottom so that tight joints result when the flooring is laid. The tongue fits snugly into the groove to prevent movement and floor squeaks. All of these details are designed to provide beautiful finished floors that require a minimum of maintenance.

Another matched pattern may be obtained in 3/8 x 22-inch strips (FIG. 1-10). It’s commonly used for remodeling work or when the subfloor is edge-blocked or thick enough so that there is very little defection under heavy loads.

1-10. Matched-pattern, tongue-and-groove strip flooring.

1-11. Square-edged strip flooring with face nailing.

Square-edged strip flooring (FIG. 1-11) might also be used occasion ally. It’s usually 3 inches in size and is laid up over a substantial sub- floor. Face nailing is required for this type.

Wood-block flooring (FIG. 1-12) is made in a number of patterns. Blocks may vary in size from 4 x 4 to 9 x 9 inches and larger. The thickness varies by type from 25/32 inch for laminated blocking or plywood block tile to 1/8-inch stabilized veneer. Solid wood tile is often made up of narrow strips of wood that have been splined or keyed together in a number of ways. Edges of the thicker tile are tongue-and-groove, but thinner sections of wood are usually square-edged (FIG. 1-13). Plywood blocks may be /8 inch or thicker and are usually tongue-and-groove (FIG. 1-14). Many block floors are factory-finished and need only be waxed after installation (FIG. 1-15).

1-12 Wood-block flooring.

1-13 Square-edged and splined wood-block flooring.

1-14 Tongue-and-groove laminated block flooring.

1-15 Factory-finished wood-block flooring

Figure 1-16 illustrates a typical installation of a tongue-and-groove parquet-block floor unit. Figure 1-17 illustrates cross matching parquet tiles.

1-16 Tongue-and-groove parquet- block floor unit

1-17 Cross-matched parquet wood tiles


Another important element of hardwood flooring is the way in which the wood strips or blocks are joined. I’ve already mentioned the tongue-and- groove joint. Other types of joints that are used in woodworking include the butt, lap, miter, rabbet, dado, gain, mortise-and-tenon, slip tenon, box corner, and dovetail. Few are ever used, however, for the manufacturing and installation of hardwood floors.

Lap joints (FIG. 1-18) can be used for hardwood flooring, but are usually not. The more common types of lap joints are the plain lap, cross half-lap, end butt half-lap, and corner half-lap.

1-18 Four types of lap joints.

Figure 1-19 illustrates the simplest of all joints—the plain butt joint. It’s made by butting or placing two pieces of wood together. As mentioned earlier, butt joints must be face-nailed, or nailed through the face of the wood, in order to be fastened securely.

1-19 Plain butt joints.

Figure 1-20 shows one way of overcoming this problem by using a doweled joint. Holes are drilled and connecting dowels are installed in order to fasten the members together in this joint.

Taking the doweled joint one step further is the spline joint (FIG. 1-21). A notch is cut in both members and a smaller member, which is called a spline, is placed between them. The milling and insertion of a spline joint takes time and slows down the installation of the flooring.

The most popular joint for flooring is called the tongue-and-groove (FIG. 1-22). In this joint, the spline is actually milled from part of one of the wood members. The spline is then called a tongue and is designed to fit into a groove—hence the name. This type of joint offers a rigid connection between the flooring pieces and does not require an amount of time for assembly.

Figure 1-23 illustrates one popular type of floor joint that is actually a combination of joints. As you notice, the sides are butt-jointed. The edge is a tongue-and-groove joint. If installed tightly, this combined joint can be quicker and just as tight as having a full tongue-and-groove joint on all four sides.


There are numerous ways and patterns you can use to install hardwood flooring that will make your floor unique as well as functional. Figure 1-24 illustrates how strip flooring can be installed when the boards are of the same length rather than random lengths.

Figure 1-25 shows a diagonal cross or herringbone design that is easy to install. It can be of identical strip flooring or of patterned block flooring.

1-23. Typical manufactured hardwood flooring joint.

1-24. Identical length strip flooring.

1-25. Herringbone design hardwood floor.

Figures 1-26 through 1-34 illustrate a variety of block flooring patterns that can be found in many flooring departments or stores. They offer an infinite number of combinations that can beautify your home while saving you the labor of assembling and installing such intricate patterns.

1-26 Staggered block flooring pattern

1-27 Overlap block flooring pattern

1-28 Monticello design block flooring

1-29 One-directional block flooring design

1-30 Log cabin design block flooring

1-31 Herringbone design block flooring

1-32 Ashlar design block flooring

1-33 Brick design block flooring

1-34 Assorted parquet hardwood flooring designs


Figure 1-35 shows an exploded view of the major parts of a single story, wood-frame house. The floor system, interior and exterior walls, and the roof are the major components of such a house. Houses with flat or low sloped roofs are usually variations of these systems.

The illustration shows a floor system that is constructed over a crawl space. The supporting beams are fastened to treated posts that are embedded in soil or to masonry piers or a foundation. The floor joists are fastened to these beams and the subfloor is nailed to the joists. This creates a level, sturdy platform upon which the rest of the house is constructed. This type of construction is often called platform construction.

1-35 Exploded view of a wood-frame home.

Figure 1-36 shows the typical components of a hardwood floor. The joists support the subfloor. Building paper is laid down to prevent moisture from entering the flooring from below, which could possibly damage the finish. Finally, finish flooring is installed, often at right angles to the subfloor.

Many of these construction terms will be new to you. Our glossary defines these terms. Refer to it often; it will help you understand working with hardwood flooring.

1-36 Laying out a hardwood floor over a subfloor.

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Monday, 2009-01-12 5:15