April 29, 2023
If you would recall your science classes, you might remember the meaning of density. It is the mass of the substance per unit volume. Similarly, wood density refers to the amount of wood material in a given unit of wood volume. Wood may vary in density depending on its Moisture Content (MC). Moreover, you can get the MC by dividing the mass of oven-dry lumber by the wood’s green volume. Green volume, however, refers to freshly cut wood, and it has the maximum possible water volume a wood can have.
The density of wood varies from one species to another. This density also gets impacted by the environment where the tree grows. Besides, you will discover that, in the same tree, one part may have a different density than another part of the tree. Plus, the trunk may have a higher density than the branches.
The Density of Various Wood Types
If you’re the type of person whose occupation involves utilizing various species of wood, you might want to know the densities of various types of wood. You can categorize wood species as heavy hardwoods, medium hardwoods, light hardwoods, and softwoods. Below is a short description of these different categories:
Softwoods usually have low density. Compared to hardwoods, they don’t have high density, so they get referred to as softwoods. Softwoods come from coniferous trees that lack fibers and vessels in their branches.
Light hardwoods do not exhibit very tight fibers, and they are relatively lightweight. Their densities range anywhere from 400 kg/m³ to 720 kg/m³ when air-dried. They also carry around 15% MC when freshly cut. These timbers are not durable when used in tropical climates, and some are pretty durable in temperate climates.
Medium hardwoods range in density from heavy constructional timbers to moderately heavy. Their densities range anywhere from 720 kg/m³ to 880 kg/m³ with 15% MC. Some medium hardwoods can be classified as solid and heavy, yet, they fall short of being categorized as Heavy Hardwoods. They may be characterized by a lack of natural durability, especially when you expose them to harsh weather or when they get exposed to moisture. Thus, you must treat them with additives and preservatives to make them survive harsh outdoor factors and elements. However, they exhibit better durability when situated in temperate regions of the planet.
Heavy hardwoods, of course, are the heaviest of all wood types. They are perfect for heavy constructions, and their density ranges anywhere from 800 kg/m³ to 1,120 kg/m³ when air-dried or when they have around 15% MC. They can remain intact and durable even if you don’t treat them with preservatives. Nevertheless, their sapwood doesn’t exhibit exceptional durability.
Things You Should Know About Wood Density
When learning about wood density, there are several facts you should consider and understand. These facts are the specific gravity of the wood, its density and strength, and workability. Below is a short description of these factors:
Specific Gravity and Wood Density
Specific gravity refers to the density of something relative to the density of water. Thus, the wood density gets usually measured in comparison to water density. This measurement usually indicates whether a block of wood would float in water. The specific gravity of water is 1.00. If a block of wood has a density of less than one, then it will float in the water. But if its density is higher than one, it will sink in water.
Strength and Density
The strength of the wood is usually commensurate with its density. The denser the wood block is, the stronger and more durable it will be. When choosing wood, you must consider the Modulus of Rupture (MOR), or what we call Flexural strength. It measures the stress in a material before it ruptures or yields.
Woods get tested for their MOR, which gets recorded to indicate the MOR of timber. You can use the MOR to figure out the overall strength of a particular wood species. It shows the ultimate strength of the wood in contradistinction to the Modulus of Elasticity which indicates the deflection of the wood.
Density and How a Wood Holds Fasteners
The density of wood usually impacts how the wood would hold fasteners. The denser a wood and the more compact its fibers per area, the more difficult it will be to drive fasteners into the wood. Thus, denser wood will likely split if you drive large fasteners into it.
Wood with lesser density, however, has fewer fibers. So, when you nail them, the nail will easily pierce them. Yet, they are less likely to hold nails tightly. Nevertheless, even if you drive a large fastener on those wood, they will not snap quickly.
Other Elements Affecting Degree of Wood Density
Aside from the things you should know about wood density, it will also help if you are cognizant of the following factors to consider about wood density:
Proportions of wood
As mentioned above, wood density may vary in density. The density of wood found in temperate regions ranges from 0.3 grams per cc to 0.9 grams per cc. One species of wood may have varying densities depending on how the wood material gets distributed along its branches or trunks. Besides, it varies depending on the void volume and its extractive contents.
Wood substance’s density is around 1.5 g/cc, and there is no significant difference in the value of wood substance’s density among various species.
Growth Rings’ Thickness
You can observe wood and approximate its density by looking at the widths of the growth rings as well as the latewood proportion. However, the latewood is usually denser than the earlywood because of its smaller cell cavities and thicker walls of cells. Besides, with its larger ring width, its proportion also lessens in softwoods, while in hardwoods (ring-porous), its proportion increases.
So, visually, rings of softwoods with wider gaps have a lower density, while rings with wider gaps in hardwoods have a higher density. Furthermore, most diffuse-porous hardwoods have latewoods with no definite demarcation, and their ring width doesn’t indicate density.
The thing is, wood density may vary from one species to another species of tree. Besides, it may vary according to the portion you are measuring and the growth conditions of the tree. Thus, if you examine the main stem of a tree, it usually has a higher density than the tree’s higher branches.
As a meticulous woodworker, you might benefit from learning more about wood density and the related factors to consider because your very profession requires you to often engage and work with wood. With better knowledge of wood density, you can select better wood for your projects.
Jason is a 40-year-old woodworker, carpenter and author who have been involved in the woodworking and woodcraft industry with 17 years of experience. He is expertise in technical aspects, woodcraft and furniture building projects.