When evaluating wood suitability, “weakness” is a relative assessment across several distinct physical properties, not a single measurement. Wood considered weak for flooring may be adequate for insulation or packaging. Understanding wood strength requires focusing on underlying metrics that quantify resistance to different types of force, moving beyond common labels like “softwood” and “hardwood.” These measurements identify species with the lowest density and least resistance to indentation.
Defining Wood Strength Metrics
Wood strength is primarily quantified using two standard metrics: density and the Janka hardness test. Density, a measurement of mass per unit volume, is a strong indicator of overall strength. Denser wood contains more load-bearing fiber and less air space, and density is directly correlated with nearly all mechanical properties. A low-density wood will almost always be a weaker wood.
The Janka hardness test measures a wood’s resistance to denting, wear, and surface damage. The test measures the force required to embed a 0.444-inch steel ball halfway into a wood sample. The result is recorded in pounds-force (lbf), where a lower score indicates a softer wood that is more easily dented. Because resistance to indentation is tied to fiber density, the Janka score serves as a practical proxy for material weakness.
The Weakest Woods By Density and Hardness
The weakest wood is Balsa (Ochroma pyramidale), a fast-growing hardwood tree. Balsa registers an exceptionally low Janka hardness rating, typically around 67 lbf. For context, Red Oak, often used for flooring, has a Janka rating of approximately 1,290 lbf, demonstrating Balsa’s softness.
Other commercially available species also rank low, often categorized as “softwoods” derived from coniferous trees. These species are easily marked and dented due to their low fiber density. Examples include:
- Paulownia, scoring around 300 lbf.
- Eastern White Pine, typically scoring around 380 lbf.
- Atlantic White Cedar, scoring around 350 lbf.
- Western White Pine, scoring around 420 lbf.
- Yellow Poplar (a hardwood), registering Janka scores around 540 lbf.
Structural Characteristics That Reduce Strength
The fundamental reason certain woods are weak is rooted in their cellular structure and growth patterns. Wood strength is determined by the amount of cellulose and lignin—the main components of the cell walls—present in a given volume. Weak woods exhibit low cell wall density and high porosity, containing a greater proportion of air pockets than solid fiber.
Rapid growth rate contributes significantly to low density. Trees that grow quickly, like Balsa or Paulownia, produce wider annual growth rings. This accelerated growth results in wood with thinner cell walls, making them less dense and structurally weaker than the thick-walled cells of slow-growing species.
The orientation of the wood fibers, known as the grain structure, also influences strength. Lumber with a diagonal or sloping grain, where fibers deviate from the board’s long axis, is considerably weaker than straight-grained wood. Additionally, knots, which are remnants of branches, interrupt fiber continuity by causing the grain to swirl and curve, creating weak points.
Common Applications of Softer Woods
A low-strength designation does not mean a wood species is useless; their unique properties make them desirable for specific uses. Balsa wood’s extreme lightness and workability make it the preferred material for model aircraft, surfboards, and crafts. The high percentage of air pockets contributing to Balsa’s low density also makes it an effective thermal and sound insulator.
Other low-Janka woods, such as Pine and Cedar, are valued for their ease of shaping and affordability. They are extensively used in the production of wood pulp for paper and packaging materials. Cedar, despite its softness, is naturally resistant to decay and insects, making it suitable for outdoor applications like fencing, decking, and siding. Softer woods are also commonly used for interior, non-structural framing and simple furniture.