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The Surfaces of Wood and Their Properties

If you viewed our blog on different sawing patterns, you might have some more questions about the surfaces and their properties. In that blog, we talked about flat, quarter, rift, and live edge sawn sawing patterns. Every board has three different surfaces. Depending on the sawing pattern, these surfaces are located in different areas causing the lumber to have different properties. In this blog, we dive deeper into the surfaces of wood and their properties.

Cross Section/Transverse Surface

The first surface, and the most easily identifiable surface, is called the cross-sectional or transverse surface. This is the surface that is seen when looking at the end of a log or the top of a stump. You will see annual growth rings. This surface is most often referred to as end grain.

Radial Surface

The second surface is the radial surface. Radial surface is created by cutting along the radians of a round cross section. This surface has uniform, straight grain lines, and is always found perpendicular to the annual growth rings. If the ray plain is cut perfectly, ray fleck will appear on the surface. This surface is what people call quarter or rift sawn surface (they look different, but their properties are very similar.)

Tangential Surface

The final surface is the tangential surface and appears by cutting tangent to the growth rings. When looking at the cross-section/transverse surface, the growth rings will appear to be smiling or frowning at you (depending on which way you’re looking at it), forming a cathedral grain pattern. It is also referred to as the flat sawn surface.

Identifying the Surfaces

The easiest way to identify surfaces is simply by looking at their characteristics. The cross-section/transverse surface exposes all the pores and shows the annual growth rings. The radial surface will appear to have straight grain and possibly contain ray fleck. Identifying between quarter and rift sawn is a little more challenging. Not all woods show ray fleck when quartered. In these cases, the end grain gets inspected for the orientation of the growth rings. With rift sawn, you will notice the annual growth rings sit at an angle, whereas with quarter sawn, the growth rings will be 90 degrees from the surface. The tangential surface will show the cathedral grain pattern.

As I mentioned earlier, all boards, no matter the sawing pattern, contain all three surfaces. Solid wood products, such as lumber, are classified by the surface of the wood that corresponds to the widest face. So if the tangential surface is the widest, it will get classified as flat sawn, and if radial, then gets classified as either quarter or rift sawn.

Moisture Loss

Because all three surfaces expose the wood cells at different angles, each surface loses moisture at a different rate. Wood loses moisture the slowest from the radial surface. The tangential surface loses moisture about two times faster than the radial surface. Moisture is lost the fastest out of the cross-section, which is about 10-15 times faster than the radial surface. End splits and cracks are a result of the fast moisture loss of the cross-section.

Amount of Shrinkage and Movement

The amount of shrinkage and movement is also different across the three surfaces. The species of wood is another variable that affects shrinkage. Shrinkage in the cross section is the least at 0.1-0.2% from green, and because it is so small, we consider it not to shrink or move at all. The radial surface shrinks about 3-7% from green and is the most stable surface. Even though we don’t consider the cross-section to shrink or move at all, the surface isn’t reliable. Since it loses moisture 10-15 times faster than the radial surface, it is more prone to checking and splitting. The radial surface doesn’t shrink or move much. There’s an old saying that,” Wood cups towards the barks (meaning the outside of a log),” which is true. The radial surface remains fairly flat while it’s drying. And finally, the tangential surface shrinks the most at 5-11%. This surface will always cup.

How this Affects Your Products

Each surface has its pros and cons, and some of these variables are the craftsman’s preferences. The first thing is the appearance. You need to know what sawing pattern you want, and sometimes the style of the piece determines this for you. If you want ray fleck, source quarter sawn lumber.  If you want straight grain lines without ray fleck, opt for rift sawn lumber. Finally, choose flat sawn if you want the cathedral grain pattern.

Another thing you might consider is cost. All three sawing patterns have different price points. Flat sawn lumber will be the cheapest, and quarter sawn will be the most expensive. If you want more info on why the sawing pattern affect price, check out our blog, “Different Types of Sawing Patterns.”

But the biggest concern is the wood movement. Flat, quarter, and rift sawn lumber will shrink and swell over the seasons. Not accounting for wood movement can have devastating effects on your projects. Wood is a strong substance, which means wood movement is also very strong. The wood can crack or blow out joints if movement isn’t allowed. Accounting for wood movement is one of the most important parts of wood manufacturing.

This image depicts the location of flat, rift, and quarter sawn boards based on the orientation of the annual growth rings.

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