4 Types of Wood: Heartwood, Sapwood, Boxed Heart, and Free of Heart Center
There are various types of wood that sawyers work with on a regular basis. Four of the most common kinds of wood include heartwood, sapwood, boxed heart, and free of heart center (FOHC).
But what exactly do these terms mean and is one better than the other?
Before we get into understanding the differences, we need to better understand the basic anatomy of a tree. The center of a tree is called heartwood. This innermost layer is usually darker in color that the rest of the tree. That’s because heartwood contains cells that are no longer living. These dead cells are no longer capable of producing chlorophyll, which gives leaves its green color. Heartwood also contains more tannins, which are pigments that make wood appear darker.
As a tree grows, the woody tissue is split into heartwood and sapwood. Unlike heartwood the cells in sapwood are alive. These living cells help to transport water and dissolved minerals from the roots of the tree to its crown. Sapwood is new wood and as rings of sapwood form, the inner cells lose vitality and turn into heartwood.
Outside of the sapwood is a tree’s cambium cell layer, which is the growing part of the trunk. This layer is found between the old wood and the bark. Trees grow by adding a new layer of wood in the cambium layer every year. Trees also have inner bark, also known as phloem. The inner bark only lives for a short time. When it dies it becomes part of the protective outer bark, which protects the tree.
The primary thing a sawyer looks at is how much sapwood and heartwood a tree contains. Some tree species contain higher quantities of sapwood than heartwood. One way to determine the amount of sapwood and heartwood in a tree is to look at the tree trunk.
Tree species with thin layers of sapwood include:
Chestnut
Mulberry
Sassafras
Black locust
Northern White Cedar
Tree species with wide bands of sapwood along with central bands of heartwood include:
Maple
Ash
Hickory
Beech
Pine
Tree species in which the color difference between hardwood and sapwood include:
Hemlock
Cottonwood
Aspen
Spruces
Firs
Why does the amount of sapwood and heartwood matter? One reason is that heartwood generally has less moisture, which means it can be easier to kiln dry. Also, heartwood can be used for both structural and nonstructural building projects. Heartwood is what gives wood its strength and makes it resistant to decay. Sapwood is considered the secondary wood of trees. It’s softer than heartwood and usually only used to practice carving or cutting. Besides being softer than heartwood, builders don’t use sapwood because it’s prone to fungi and insect attacks.
Boxed heart wood and FOHC are the most common kinds of wood used in timber framing, which is a style of construction in which heavy beams frame the structure. But what are the differences between these 2 types of wood?
Boxed Heart Wood
What some may not realize is that heartwood is not the same as the heart of the tree. The heart is the very center of the tree and is usually called pith. Both pith and heartwood are types of woody tissue, but they are not used interchangeably. When someone says boxed heart wood, it simply means the heart of the log is left intact. In other words, the heart is enclosed within the four surfaces of the timber throughout its entire length. Sawyers do this by taking off the edges (or the bark) of the log and mill out or “box” the center.
Pith is much softer than heartwood and is not as strong or durable. For this reason, beams made from boxed heart wood tend to twist and crack. Still, certain species of wood can be used for structural purposes. These species include:
Douglas Fir
Tamarack
Redwood
Bald Cypress
Eastern White Pine
Cedar (Incense cedar is stronger than Western Red Cedar)
Of these kinds of wood, Tamarack is king for structural uses, with Douglas Fir coming in a close second. According to Carolina Timberworks, 60 percent of timber frames in North America are built from Douglas Fir.
Free of Heart Center (FOHC)
Free of heart center (FOHC), on the other hand does not include the heart of the tree. When the heart of the tree is removed, the timber is straight, durable, and less prone to twisting and cracking. For this reason, FOHC is the strongest kind of wood and best for beams. If, however, the beam cannot be Free of Heart, the next best option is to box it.
The worst thing a sawyer can do is split the heart. For example, if you have a 6x12 and split it down the middle for two 6x6s. This is not a good scenario. It’s better to take two smaller logs and box the heart on each one.
