flat sawn lumber

Why Does Dried Wood Absorb Moisture?

Wood and Moisture: It’s All Biology

The hardwood lumber that you buy at the lumberyard began as a living tree. As a living tree, the trunk of the tree provided a way to transport water from the roots to the leaves. When the tree is cut down that water moving system is still present in that tree and much of that water is still in the lumber when it is first sawn.

Hardwood lumber just cut, referred to as green lumber, can have a moisture content of 40% or more! After hardwood lumber is sawn, it is typically air-dried to around 15% moisture content. Hardwood lumber will be continued to be dried in a kiln to a moisture content of 6-9%.

Wood is hygroscopic, which means just like a sponge the moisture content will change depending on the relative humidity of the surrounding air. When humidity increases, the wood absorbs moisture from the air causing the wood to expand. When the humidity decreases, the wood releases water into the air and the wood shrinks.

What Direction Does Wood Expand In?

When wood expands and contracts because of changes in moisture content, hardwood will move in a predictable way. Wood shrinks most in the direction of the annual growth rings (tangentially), and half as much across the rings (radially), and only slightly along the grain (longitudinally). or from the center of the tree to the outer edge).

Hardwood shrinks most in the direction of the annual growth rings (tangentially), about half as much across the growth rings (radially), and only slightly along the grain (longitudinally).

Flat-sawn boards will cup away from the heart of the tree. The shrinking will occur mostly in its width.
Rift-sawn boards will warp and shrink into a diamond or trapezoidal shape.
Quarter-sawn boards will shrink slightly in both length and width.

What Do I Do About Moisture and Wood Movement?

The solution to this problem is quite simple: don’t stop the wood from moving, but rather account for its movement. With a little bit of knowledge, you can predict the degree of wood movement, and take action to accommodate the movement.

Tips for Buying Hardwood Lumber through a Distributor or Lumberyard

 

Are you planning your first visit to buy lumber? Once you’ve decided to go beyond buying lumber at a home center, there’s some major differences to be aware of when buying lumber at a lumberyard or distributor.

Lumberyards and distribution centers have come a long way, offering a wide variety of wood species, thicknesses, grades and machining options. Here are some pointers for how to buy lumber at a distributor or lumberyard.

 

 

THICKNESS

When buying hardwoods, you won’t see lumber thickness marked in inches, but instead the convention is to use a measurement in quarters. For example, a 1 inch thick board is typically written as 4/4.

One thing to keep in mind is that lumber is sawn and then dried so the board that started out 4/4 inches will be closer to 7/8” (.875 inches) – and that thickness is before any type of surfacing is done.


BOARD FOOT

A board foot is the unit of volume used to measure hardwoods. Since hardwoods are sold in a variety of lengths, widths and thicknesses, it is simpler to have a kind of measure that can account for all of those variables at once, like volume.

12” long x 12” wide x 1” thick = 144 cubic inches

Calculate a Board Foot
   special thanks to American Woodworker for this image.

Armed with a tape measure, calculator and this knowledge, you can tackle any board foot calculation on the spot. But what happens the day you leave the shop and forget your calculator? Use this woodworker rule of thumb:  4/4 board, 8 ft. Long and 6” Wide = 4 Board Feet


GRADE

For someone just beginning to purchase hardwood lumber, lumber grades can be overwhelming. Most lumber companies use the generally accepted grading rules set by the National Hardwood Lumber Association. Grades are based on the amount of usable clear material in a board. The highest grade boards are FAS and Select, followed by #1 Common and #2 Common. What grade you choose depends on your project. Some projects, such as tabletops and high quality furniture, may dictate the highest grade available. Many other projects are just as easily adapted to #1 Common (often referred to as cabinet grade) – kitchen cabinet doors, smaller projects and items where some character is acceptable.

For a full illustrated guide to the various grades:
http://www.ahec.org/hardwoods/pdfs/IllustratedGradingGuide.pdf

MILLING SERVICES

Rough lumber is rarely flat or straight. Milling your own can save you some money but it will take time, equipment and a strong back. It can be beneficial to look into what milling options your distributor can offer.

Most yards will offer the following milling services:

–       Rip one edge of a rough board straight (called SL/E, Straight-Line Edge)

–       Plane both faces lightly by taking approx. 1/16” off the surface (called Hit-and-Miss)

–       Plane both faces, leaving no little wood with rough cut marks, removing approx.. 1/8” of thickness (called S2S, Surfaced Two Sides)

–       Mill both faces and edges (called S4S, Surfaced Four Sides).

There are typically varying costs added to the price of rough lumber to cover the additional milling. Depending on your situation, these added costs may save you money in the long run through saved labor, faster lead times and a reduction in equipment needs.

What is the difference between Quarter Sawn, Rift Sawn and Plain Sawn Lumber?

When lumber is cut from logs, it is typically cut in one of three ways: quarter sawn, rift sawn or plain sawn. Each type of lumber is dependent on how the log is oriented and cut at the sawmill. The result is a particular orientation of the growth rings on the end grain of the board and is what defines the type of lumber.  The type of cut also determines the figure in a piece of wood and the wood’s mechanical properties.

Sound too confusing? This graphic will help…

PLAIN SAWN / FLAT SAWN
Most common, least expensive
Plain sawn, also commonly called flat sawn, is the most common lumber you will find. This is the most inexpensive way to manufacture logs into lumber. Plain sawn lumber is the most common type of cut. The annular rings are generally 30 degrees or less to the face of the board; this is often referred to as tangential grain. The resulting wood displays a cathedral pattern on the face of the board.  See pictures here.

QUARTER SAWN
More expensive than plain sawn material
Quarter sawn wood has an amazing straight grain pattern that lends itself to design. Quarter sawn lumber is defined as wood where the annular growth rings intersect the face of the board at a 60 to 90 degree angle. When cutting this lumber at the sawmill, each log is sawed at a radial angle into four quarters, hence the name. Dramatic flecking is also present in red oak and white oak.  Find out more.

QuartersawnRiftsawnLumberDifference_Diagram3

Comparison of typical cutting methods for each type of lumber

RIFT SAWN
Most expensive, least common
Rift sawn wood can be manufactured either as a compliment to quarter sawn lumber or logs can be cut specifically as rift sawn. In rift sawn lumber the annual rings are typically between 30-60 degrees, with 45 degrees being optimum. Manufactured by milling perpendicular to the log’s growth rings producing a linear grain pattern with no flecking. This method produces the most waste, increasing the cost of this lumber. Rift sawn lumber is very dimensionally stable and has a unique linear appearance.  Refer to additional pictures.

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