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  Wood Floor Properties and Characteristics
Properties of Wood In General



































While there are other species, or types, of wood used for flooring, the vast majority in this country is oak, for reasons stated in the history section. The other species will have properties and behavior similar to oak, so for simplicity's sake we will address ourselves in this section only to oak.

The Living Tree

To begin with the basic, an oak tree grows with roots in the ground and leaves in the air. Easy. The roots collect moisture and nutrients from the soil and ship them through vessels or fibers up the trunk and branches to the leaves. These vessels are similar to the "strings" in a stalk of celery. The leaves mix the moisture, nutrients, carbon dioxide, sunlight and bingo-photosynthesis gives oxygen back into the atmosphere and food for the tree. The food is then shipped through other vessels, throughout the tree and back to the roots. Growth occurs. Basic.

The foregoing is fact and logic to help understand that an oak tree is made up of fibers aligned vertically in the standing tree. Cut the tree down and the fibers are horizontal. Saw boards and manufacture strip flooring, nail the floor down and the fibers are still horizontal and running the length of the boards.

In the standing tree the fibers are loaded with moisture. The tree, after felling, begins to dry out, just like a rose wilts after being picked. As the fibers dry, they shrink in thickness or diameter, almost none lengthwise. This shrinkage, characteristic of oak (all woods) is of vital importance in the understanding of oak flooring, so let's examine it further.

Reasons for Moisture and Shrinkage

While the figures used in this example are not accurate, they help tell the story.

  1. A standing oak tree is felled in the forest and sawed immediately into a board 1 " thick, 10" wide and 8' long. Placed on a scale, the board weighs lOO pounds.
  2. This same oak board is placed in a stack of boards separated from its neighbor by stacking strips of uniform size to keep the board straight. The stack is aimed at the prevailing breezes to accelerate drying, and again this location is in our southern states enjoying many long warm months. The board stays in the stack for four to six months and again we put the board on the scales. It now weighs-50 pounds. Fifty pounds of water has evaporated.

    There are times today a mill must reach out 200 to 300 miles for lumber. With high rail and truck freight costs, the need for air drying and not paying freight on water is obvious. Subtle, however, is the companion fact that there must be this time allowed, thereby making it difficult for the industry to immediately respond to an upsurge in demand. Also, with today's raw lumber cost, the flooring manufacturer must have enough capital to buy lumber and let several millions of dollars sit in a field doing nothing but drying.

  3. Now our 50 pound board is trucked to the flooring mill and loaded into a dry kiln.DRY KILN The kiln is a concrete building many times large enough to hold 3 or 4 railroad box carloads of lumber. Within the kiln there are fans to circulate the air, steam pipes to create heat, and live steam to induce moisture. Our board is very gently treated by a highly-trained specialist. As he gradually lowers the humidity and increases the temperature over a five to seven day period until our board reaches the optimum moisture content for flooring manufacture of about 8%. During the process the temperature in the center of the board must reach 105F to sterilize the eggs of a little fellow named Lyctus who likes to eat oak floors. The board is too hot to touch, so after a couple of hours, we again place the board on the scales-42 pounds.

Controlling Moisture Content is Most Important

If we had an autoclave, or oven, into which we could place the whole board and dry every bit of moisture out of the wood, the board would weigh 40 pounds, but this 2 pounds of moisture remaining in the wood keeps the wood "alive" and flexible.

Precision kiln-drying and the resulting moisture content (M.C.) is one of several primary responsibilities of the oak flooring manufacturer. The correct M.C. makes oak flooring predictable. Violate the M.C. and all kinds of things happen. Dry oak flooring (made from these dry boards) is like a blotter. The boards really want to regain some of their lost moisture. All of us-manufacturer, distributor' retailer' truck driver, builder (see Builder checklist), installer, housekeeper, and owner-have the responsibility to keep oak floors dry. Here's why.

When we began this story our board weighed 100 pounds and measured 1 x 10"- 8 ft. The vessels and fibers were full of moisture. Now' after drying, the board weighs only 42 pounds and measures 3 1/32 x 9 1/8 -8ft.

The board has withered just like the rose. Each vessel and fiber is smaller in diameter or width and has shriveled up. This is why the board is narrower and a little bit thinner, but not shorter.

Think of the vessels as soda straws. Introduce dampness or moisture to the floor and the moisture will be drawn into the end grain (vessel ends) quickly. The fibers will accept this moisture and start to swell or fluff up. If the moisture is of short-lived nature, the ends of its boards will swell, get wider, and display what we in the industry call "fishtail" (for obvious reasons). Long-term dampness will cause all the boards to swell, called "expansion". An expanded floor, given a chance to dry out, displays "shrinkage" and results in cracks between boards.

To summarize on this property of wood, oak floors in particular:

White and Red Oak Characteristics

Now let's go back to vessels-this time looking at the cut end of pieces of oak flooring. The entire family of oak trees (Quercus) is divided into two types-- the Albas and the Rubras-White and Red. Within each of these groups there are many cousin varieties, but generally, so far as flooring (and we use them all) is concerned, each group has certain characteristics that are individual and infallible. White oak is usually yellow to gray in color' is harder-stiffer-brasher. It sands smoother than red, and comprises about 35-40/c of the forest (because that's the way Mama Nature planned it). Red oak is usually pinkish tan in color' is a little more flexible or soft, takes stain or bleach better than white, and comprises about 60 to 65% of the forest. Now Mama fools us! There are pink/tan white oak boards and yellow/ gray red oak boards. How can you be sure?

White Oak Preferred for Whiskey Barrels

Look at the vessels on the cut ends of the board. Red oak appears to have a band of tiny holes running across the end of the board. White oak has no tiny holes. The vessels in red stay open -take a piece of red oak and blow through it into a pail of water- bubbles! As a white oak tree matures the vessels in the inner portion of the tree fill up with a deposit called tyloses. Hence, the difference in appearance of cut ends, and stiffness, and hardness, and sanding, and stain appearance. White oak is used for whiskey barrels rather than red since the tyloses plug the vessels (straws) and who wants a leaky barrel?

The large vessels, (tiny holes) that you can see with the naked eye are fibers formed during the early spring months when winter has broken, rains are plentiful, and the earth rejoices with rapid growth. This less dense portion is obviously called "springwood." As rainfall lessens and days heat up, growth slows down and produces dense "summerwood." The tree remains dormant from November through February. The combination of one springwood and one summerwood is called an annual ring-one years growth. Read the rings, as we all know, and learn the age of a tree while the width of the rings tells of drought, fires, rains, etc. Looking at a piece of flooring, follow the springwood from the cut end around to the face of the board and you will see that the springwood gives the board its grain pattern or face character. Summerwood is the denser more uniform area between the grain flair. Graining

Plain or Flat Sawn Versus Quarter Sawn or Quartered-
See Wood Flooring CUTS & GRADES

With annual rings in mind, the great oak tree maker decided that oak boards will expand and contract with moisture change, parallel to the annual rings. The word in geometry is "tangential." And this too is logical, realizing the dense, strong, summerwood holds the springwood in place, while the springwood is freer to swell "sideways. " The oak flooring industry makes constant use of this property.

Flooring manufactured with grain or annual rings running across the width of the boards is called "plain," or "flat" sawn. Flooring with grain running at right angles to the face or across the thickness, is called "quarter sawn," or "quartered." Both types of boards can be cut from the same log, as shown in the diagram. Cuts

The difference in how the flooring behaves is significant since plain sawn expands and contracts across the width of the board, while quartered in theory get thicker and thinner. Quartered has considerably more dimensional stability and does not respond to season change as does plain. Using this property, many old Eastern homes in this country had their oak floor installed after the base was applied to the wall and the oak floor was fit tight to the base-years ago-with no problems-quarter sawn flooring only. To install in the same manner today guarantees failure. Why?

Today the saw mills cutting the logs into boards will produce about 80 percent plain to 20 percent quartered lumber. With the tremendous loss in productive capacity in the industry, the remaining flooring mills can no longer afford the expense of separating the lumber. Quartered oak flooring is extremely hard to find, and expensive. Most of all the production is mixed cut.

Controlling the expansion of oak has been one of the elusive goals of the industry since its inception. The use of quartered flooring was successful, but met an economic fate. To question the problems caused by expanding oak gives need for these examples.

  1. Expanding oak floors may cup, buckle (graphic), raise off the subfloor, even push walls out.

  2. Whiskey barrels swell tight and hold for years, drain and dry it-it leaks.

  3. And the power generated by each little fiber swelling was best illustrated by early Romans. The huge blocks of granite and marble used to build Roman cities were "cut" out of mountains by drilling holes with a hand auger, hammering dry oak pegs into the holes, and soaking the peg ends with water. The peg expanded and -pop - the oak splits the mountain.

The control of expansion in today's oak flooring rests with proper handling and maintenance, but mostly with the installer using proper methods.

New Products Minimize Expansion Problems

The oak flooring industry has, however, developed product types to minimize expansion problems.

  • 5/16 thick parquet.

  • Acrylic impregnated.

  • Laminated floors.

Making use of the nature of oak and the fact that 80% of their incoming lumber is plain sawn, manufacturers of 5/16 parquet gang rip the boards through their thickness, turn the resulting slat flat and now 80% of the slats are quartersawn (the other 20% are plain). Various Types of Cuts

To further aid in gaining stability, these slats, now called fillets, are spaced during assembly (into units) to provide hairline expansion space between fillets. The result of using 80~% quartered fillets and spacing creates a parquet floor with enough stability to permit installation with minimum expansion.

Acrylic Impregnated Wood is More Stable

Acrylic impregnation marries the properties of wood with the plastic technology developed in the early 50's. Using the absorption ability of wood, two chemicals in solution are forced into the dry wood fibers completely saturating the wood. An outside force is then used to combine the two chemicals (polymerize) into solid acrylic (plastic). This chemical reaction takes place throughout the thickness of the wood. The resulting wood floor product has increased density, hardness, ( see HARDNESS Table of Wood Floor Species ) and wear resistance; and coloration can be inserted uniformly through the thickness of the wood. The acrylic fills the fibers and deters moisture absorption, thereby providing increased dimensional stability. These floors do not require "refinishing," in the pure sense of the word, since the acrylic finish is throughout the thickness. Surface treatment performed occasionally will reveal a brand new floor!

Up to this point the properties discussed have pertained only to solid oak floors, one piece of wood through its thickness. Hold on, because now we're going to take those properties and use them as we change direction entirely.

Laminated Floor Blocks are Warp Resistant

Laminated floor products came into being in the 50's as the result of improved peeling and gluing technology. Again using the property of oak's tangential expansion, three layers of oak are glued together with the face (top) layer and back (bottom) layer grain running parallel or the same direction. The center (core) layer is turned 90 during assembly and the glue used in the assembly is stronger than the wood itself. Now, as the fibers in each layer absorb moisture and want to expand, each layer is restrained by the other and improved dimensional stability results. There are many laminated floors that have performed successfully, even when cut tight to walls. Laminated Cuts

Production of the laminated products begins with the felling of the tree, but after stripping off the branches, the trunk remains whole, rather than cut into logs as with solid flooring. The tree length logs are transported directly to the flooring mill and dumped into water filled ponds or stacked under constant sprinkling to keep them as wet as possible. The logs are then placed in huge vats filled with hot water and live steam and literally cooked for at least 24 hours. After removal from the vat, the full length tree is cut into bolts of varying lengths determined by the characteristics of the log and the mill's need for backs and faces or cores. The bolt is positioned in a large lathe and turned at high speed against a sharp blade that literally unrolls the log just as a roll of toilet tissue unrolls. The cooking of the log turns the fibers into (almost) a wet box of Kleenex, making this peeling operation possible. Peeling Process

Because the peeling requires soaking wet logs, no prior drying is required. This fact makes it possible for laminated products to respond quickly to increases in demand. The utilization of the log is greater since nearly the entire volume of wood is used. The high-quality portion for face layers, lower quality for cores and backs. The sheets of veneer pass through a dryer, are clipped for defect and size, glued and assembled into plywood sheets, generally about 48" x 60". It is conceivable that a tree is felled in the woods on Monday and is plywood by Thursday. The large plywood sheet then is ripped and diced up to produce a myriad of laminated (engineered) floor products and Prefinished floors .

There are several variations on the theme. The usual is three layers, however, some manufacturers use five. Oak cores are common, but other species are acceptable. Backs are normally oak but species of very similar characteristics are sometimes used. The bottom line, however, is always the same. An odd number of layers with the face and back of the same (or similar) species produces a dimensionally improved, warp-resisting floor product.

Laminated Blocks Installed Below Grade

Ultimately, the customer has for their floor, a product that displays far less expansion and contraction with moisture changes and therefore can be successfully installed below grade, in basements, in humid climates, and even fit tight to vertical obstructions. Very different from solid floors. The floor can be glued direct to concrete with several adhesive types. There is a visual difference between laminated and solid flooring that many customers will be unable to discern; as the log is peeled it opens up a grain flair that is much more pronounced than that found in a sawed product.

The width of the bands of springwood/summerwood is greater and the stain acceptance and less wear ability of the springwood is apparent.

The major concern of many who display resistance to the laminated products is longevity. Can a laminated floor be sanded and refinished? Consider the thickness of the top layer. The thicker the better. Some products entering the market in the late 80's have a very thin face veneer, and are doubtful as to their ability to be sanded and refinished. The entire life wear and appearance of the floor is in the face thickness. The thick-faced products, with proper maintenance, can be expected to last up to 20 years' end can be sanded and refinished at least twice by a pro, giving another 40 years. A 60-year floor! Yes, we do have something to sell.

Another variation of laminated flooring has come to our country from Europe and doubtless will soon be manufactured within our shores. The face is many pieces -- or one piece -- of sawn wood, not peeled veneer. The core is solid pine (or other species) fillets aligned across the width of the plank. The back is a peeled veneer with grain direction parallel to the face. This flooring is non-beveled and prefinished, and very salable in our market today.

And another variation -- an impregnated sawn oak multi-piece thin face, with a separated oak fillet core at 90 grain direction -- with no back -- for adhesive installation.

See Related Pages:
Hardness Table for Wood Floor Species
Wood Floors + Water = Trouble
Wood Flooring Cuts & Grades
Wood Flooring Species

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