Wood Block Flooring Technical Information

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Ponderosa pine (Pinus ponderosa) wood block flooring.

Quality wood block flooring begins with selecting the right tree species. Requirements include durability (particularly compression parallel to the grain) and the ability of the wood to hold its shape and size. But what are the specifics on minimum durability and maximum shrinkage? Unfortunately, the science for wood blocks just hasn’t been tested. The Handbook of Wood (2010), the authority on properties and mechanics of wood, offers a suite of information on wood strengths and shrinkage, but how does this apply to wood block flooring?

Wood is known as an orthotropic material or it has unique and independent mechanical properties in the directions of three mutually perpendicular axes: longitudi­nal, radial, and tangential. In the case of wood flooring, these properties determine not only the strength of the wood, but also how the wood shrinks and deforms with increases and decreases of moisture content. 

Durability

Addressing the issue of durability of wood block floors requires an understanding of the function of the floor itself. One setting that best presents the need for durability is in factory flooring. Factory floors are frequently required to withstand pressures of 15,000 lbs or more. This weight is based on average maximum loadings across a range fork-lift models and using average surface areas of both three and four wheel fork-lift’s results in a mean of 75 psi when converted. According to The Handbook of Wood ponderosa pine can withstand weights 2,450 psi (green) and 5,320 psi (12% moisture). Also important to note is that these numbers represent an average for the species and standard deviations are not reported. Soft wood tree species, such as ponderosa pine, will fall above and below these figures dependant on growth rate. Increases in growth rates usually result in decreased specific gravity and lower mechanical strength. This extremely important in areas of the southwest U.S. where ponderosa pine forests are notoriously dense and growth rates are extremely slow.

Another factor affecting the durability of wood is the presence of knots. There are two main types of knots (inter-grown and encased) that affect durability. Inter-grown knots have more of an effect on durability, given that it is caused by a living branch that continues to grow along with the tree and thus causing a greater disruption of continuous grain. Encased knots are caused by dead branches, which are no longer growing and cease to cause a disruption of continuous grain. Ponderosa pine is well known for its small encased knots compared to other pines used in flooring and in particular, the ponderosa pine of the Southwest has fewer and smaller encased knots because of dense growing conditions.

Finally, the presence of pitch pockets in wood will also decrease the durability of wood blocks. Again, arid, competitive environments of the Southwest result in less pitch production and fewer pitch pockets in ponderosa pine.

Shrinkage and deformations

Equally as important as durability is the ability of a wood block to hold its size and shape. Wood will not only shrink and warp during kiln drying, but also between seasons and fluctuating humidity levels. Under changing moisture conditions wood block floors can experience gap formations and become disjointed. This phenomenon expresses the need for the use of woods that have smaller fluctuations in shape and size with differing moisture conditions. Of the most popular soft wood block flooring species, ponderosa pine experiences the least amount of shrinkage and deformation of the commonly used soft wood species (Table 1).

Table 1. Percent loss (shrinkage) from green to 12% moisture for popular wood flooring soft wood species.

Past management and climactic conditions across the southwest United States have resulted in slow growing, dense ponderosa pine forests. This has also resulted in a high percentage of stems being in low diameter classes. These small diameter stems are ideal for wood block production, given their abundance and the need for wood market development in the Southwest. The benefits of ponderosa pine as a reliable, durable wood block material are supported by current literature. Add in the fact that ponderosa pine is the preferred soft wood species aesthetically and a strong case is made for ponderosa pine as an elite wood block flooring material.