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I.  WOOD'S STRUCTURE AND MOISTURE

A.    The Internal Structure of Wood

1.     The roots of a tree collect moisture and nutrients from the soil and carry them through vessels up the trunk and branches to the leaves.  The leaves mix the moisture, nutrients, carbon dioxide, and sunlight:  photosynthesis gives oxygen into the atmosphere and provides food for the tree.  The food is then carried through other vessels to all parts of the tree and back to the roots.  The vessels or fibers are aligned vertically in the standing tree.  When the tree is cut, sawn and milled into flooring, and nailed down, the fibers are horizontal, running the length of the boards.

2.     The fibers in a standing tree are full of moisture.  After it is cut, the tree begins to dry out.  As the fibers dry, they shrink in thickness or diameter, but almost none lengthwise.  The percentage of shrinkage varies in different woods, but it is a characteristic of all woods and is important in the understanding of wood flooring.

B.     Impact of Vessel Structure

1.     A close look at the cut ends of a red oak and a white oak board illustrates differences in vessel structure.  Red oak appears to have a band of tiny holes running across the end of the board, while white oak has none. The tiny holes are vessels, which in red oak stay open.  As a white oak tree matures, the vessels in the inner portion of the tree fill up with a deposit called tyloses.  It is this difference in the vessel structure which impacts on moisture absorption as well as on stiffness, hardness, sanding, staining and appearance.

2.     The large vessels that you can see with the naked eye are fibers formed during the early spring months when rains are plentiful.  This less dense portion is called "springwood”.  As rainfall lessens and days get warmer, growth slows down and produces denser "summerwood".  The tree remains dormant through the winter.  One year's growth, called an annual ring, is the combination of one springwood and one summerwood.  If you follow the springwood in a piece of flooring from the cut end around to the face of the board, you will see that the springwood gives the board its grain pattern or face character.

3.     Summerwood is the denser, more uniform area between the grain flair.

 

 
















C.     Controlling Movement Due to Moisture

1.     The most important aspect of controlling movement in wood flooring is proper air and kiln drying before milling.  When a tree is felled for flooring, it is first cut into boards.  The boards are stacked and separated from each other with stacking strips of uniform size.  This keeps the boards straight and allows better air circulation. The board stays in the stack for four to six months and loses about half its weight in water in that time.                 

2.     The boards are then put into a dry kiln.  Within the kiln there are both a source of heat and fans to circulate the air.  Some kilns have steam pipes to create heat and use live steam to control moisture;  others use a dry heat system.  The boards are processed in the kiln from 7 and 30 days, depending on the wood and kiln type, until they reach the optimum moisture content for flooring of 6% to 8%.  During the process the temperature in the center of the board should also reach 105°F to sterilize it, killing any insect eggs or fungus.  By now the boards have lost an additional 15% of their weight in moisture.  A board that began the process as 1" thick, 10" wide and 8' long would now measure 31/32" thick, 9 ˝" wide and 8' long.  The small amount of moisture remaining in the wood keeps the wood "alive" and flexible.

D.     Acceptable Levels of Moisture Content

1.     National standards have been established by various manufacturers and professional associations (NOFMA, NWFMA, NMFA, APA, etc.) for acceptable levels of moisture in wood flooring. 

2.     Moisture content, as determined by a moisture meter, must be at least 6% but not more than 9% with up to 12% moisture allowed in 5% of the material. 

E.     Plain or Flat Sawn Versus Quarter Sawn 

1.     Wood boards expand and contract with moisture change parallel to the annual rings.  The denser, stronger, summerwood holds the springwood in place, while the springwood is freer to swell "sideways" between the stronger grain. 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" ("rift sawn" is a few degrees off quartered and has no rays).  Both types of boards can be cut from the same log, as shown below. 

2.     The difference in how flooring behaves is significant since plain sawn expands and contracts across the width/face of the board, while quartered moves primarily through the thickness/depth.  This makes quartersawn wood considerably more stable dimensionally and less responsive to ambient moisture change.  Saw mills cutting the logs into boards will produce about 80% plain to 20% quartered lumber and most flooring comes mixed cut.  Quarter and rift sawn can be ordered but each board must be hand-selected and a premium is charged






















F.     Hydrostatic Pressure

1.     Dry wood is “hygroscopic” like a blotter:  the boards really want to regain some of their lost moisture.  Each vessel and fiber is smaller in diameter and has shriveled up.  This is why a dried board is narrower and a little bit thinner. but not shorter.  Introduce moisture to the wood and the moisture will be drawn into the end grain (vessel ends) quickly.  The fibers will accept this moisture and start to swell.

2.      Wood expands and contracts across the grain with moisture change.  This swelling is called "hydrostatic pressure". 

3.     The hydrostatic power generated by each little wood vessel swelling is so powerful that it was utilized by early Romans to mine stone.  The huge blocks of granite and marble used to build Roman cities were "cut" from mountains by drilling holes with a hand auger, hammering dry wood pegs into the holes, and soaking the peg ends with water.  Expansion of the pegs forced the stone to crack and separate, thus "cutting" it. 

G.    Causes of Cupping and Separations

1.     If wood flooring is only subject to moisture for a short time, the ends of the boards will swell, get wider, and display what the industry calls "fishtail".  However, long-term dampness will cause the entire board to swell, which is called "expansion".  When the boards in an area of flooring expand they press against each other.  This hydrostatic pressure causes two things to happen: 

a.     The edges of each board may curl-up slightly, which is called cupping.

b.     The force of the expansion of each board against the other may cause the edges of each board to compress slightly.  When the floor dries out and each board relaxes this compression may not ease out all the way and "shrinkage" occurs, resulting in fine cracks between boards.

2.     Shrinkage can also be caused by excessively dry conditions: these cracks usually disappear when moisture returns to normal.

H.     Expansion Space; [see IV, C , 2, below (link?)] One of the most significant causes of failure in wood floors is failure to install flooring with adequate expansion space which allows for the hygrostatic movement of wood.

I.       Controlling Moisture after Manufacture

1.     Precision kiln-drying and control of moisture content up to the point of shipment are among the responsibilities of the flooring manufacturer.

2.     After final milling, responsibility for control of expansion in wood flooring rests with those delivering, handling, storing, installing, finishing, maintaining and cleaning.  Keeping a consistent level of humidity during and after installation is critical.  To insure that the proper limits to moisture content are maintained, the flooring should not be exposed to high humidity or moisture.

a.     Flooring should not be transported or unloaded in rain, snow or excessively humid conditions. If the atmosphere is foggy or damp, thoroughly wrap wood with a tarpaulin.

b.     The moisture content of wood should be metered as it is delivered to the site and pieces showing over 8% moisture should be removed.

c.     Before delivery, check the job site for the following conditions and, if they exist, correct them before delivering flooring:

(1)   water or excess moisture underneath or in the house

(2)   green or wet concrete, wood subfloors or joists.

(3)   Basements and crawl spaces must be dry and well-ventilated.  A ground cover of 6 mil polyethylene film is essential as a moisture retarder over dirt or other moist surfaces and over a concrete floor when it is less than 5 feet below the wood flooring.

(4)    Surface drainage should be directed away from the building.

(5)   Before flooring is delivered to the job site, the building should be closed, with outside windows and doors in place.  In warm months the building should be well-ventilated and, if the house is to be air conditioned, it should be on.  During winter months heating and humidifying should be maintained near occupancy levels at least five days before flooring is delivered and until sanding and finishing are completed.

d.     Plywood subfloors and joists should be clean, straight and thoroughly dry: not more than 12% moisture.

e.     Subfloor construction:  With joist construction (16" on-center), subflooring should be a minimum of 3/4 inch plywood, 5-score (layers) or better.

(1)   On concrete slab, ˝ inch 5-score plywood can be used, although 3/4 inch is recom­mended.  Other special requirements may be involved, especially where dampness may be an issue. 

(2)   Over any type joist spaced more than 18" on center, 3/4" multi-score plywood is the minimal acceptable and 1" or thicker may be required.

(3)   Composition board or chip board, in any form, is not acceptable.

(4)   Subfloors not properly nailed may become loose, squeak or buckle. Plywood subfloor panels should be aligned with edges over a joist or block and nailed  or screwed on every joist every 4 to 6 inches.  Panels should be staggered so there are no four-way joints.  Gluing plywood to joists with an approved construction adhesive strengthens the subfloor and helps maintain a quiet subfloor system.

f.      Insulate over heating plant and uninsulated heat ducts. Use fiberglass batts or ˝" standard insulation board between joists. Over a heating plant, the insulation used should be non-flammable.

g.     Provide adequate expansion space next to all vertical surfaces, as well as when abutting tile, stone, concrete and other unyielding floors.  The minimum space is ˝ inch, but more may be required based on the size of the room and alignment of wood flooring.  Failure to do this may result in buckled floors:  the hydrostatic pressure from expanding wood has been known to move concrete block walls.

h.     Hardwood floors should not be laid until all concrete work (foundations, basement floors, etc.) is dry, which can take 90 days from date of pouring.  Floors should not be laid until two weeks after all dry wall or masonry work is completed.

i.       Proper nailing of wood flooring is essential.  Floors not properly nailed may become loose or squeak and will be much more susceptible to buckling due to moisture.  Strip and plank flooring should be nailed every 8" to 10", along each joist and midway between joists.  Flooring over 4" wide should have a heavy bead of construction adhesive on the back, the full length of the board.  Flooring over 6" wide should be pegged, keyed or top nailed.

   j.     Proper maintenance and cleaning procedures must be followed, and little or no water should be used in cleaning.