Trees have two classes – hardwood and softwood. Hardwood trees have broad leaves. Softwoods, also known as conifers, have needlelike or scale like leaves. While hardwoods shed their leaves at the end of each growing season, softwoods are categorized as evergreens. A few softwood species are not considered evergreens, such as cypress, larch, and other exotic species.
The terms hardwood and softwood do not apply to the hardness or softness of the wood. Some species of hardwoods have softer wood than species of softwoods. On the other hand, some softwoods are as hard as the medium density hardwoods.
Understanding the heartwood and sapwood can help lumber buyers look at the strength and density of the wood. There are three zones at the end of a log – the bark, the sapwood, and the heartwood. The heartwood is the inner zone which is darker in color while the sapwood is light colored just beneath the bark. The structural center of the heartwood is the heart center in the lumber trade.
Since all heartwood was once sapwood, there is no difference in weight when dry or in strength. Cell deposits make heartwood more durable when in contact with soil and under other conditions conducive to decay. When wood is treated with preservative, the area treated can have deeper and more effective penetration in sapwood.
There are two distinct areas visible in each ring of annual growth – springwood and summerwood. Springwood is the inner light colored portion and summerwood is the outer dark. Springwood is composed of the large cavitied thin walled cells formed during the early part of each growing season. Thicker walled cells formed later in the year is the summerwood portion.
Summerwood contains more solid wood substance than springwood and appears darker in color. Springwood and summerwood bands are more noticeable in dense softwood species. The proportion of springwood and summerwood present has an important effect on its strength properties.
The rate at which softwood trees used for structural purposes grow has important effects on their strength properties. An accurate measure of this can be provided by the width and character of the wood in each growth ring. Pieces with medium to narrow growth rings have higher strength properties than wide rings.
Wood pieces with rings that have dense, darker summerwood also have higher strength properties than pieces with a lower percentage of summerwood. When structural material is graded, the rate of growth, or the number of rings per inch, and the density, or proportion of summerwood, is considered and made part of the specification.
Solid wood substance is heavier than water regardless of the species. Dry wood of most species float in water because a portion of its volume is occupied by air filled cell cavities. Variation among species in size of cells and in thickness of cell walls affects the amount of solid wood substance present and its specific gravity.
Specific gravity of wood is a measure of its solid wood substance and an index of its strength properties. Specific gravity values may be somewhat affected by gums, resins, and extractives which contribute little to strength. Relationship of specific gravity to wood strength is evident when assigning higher basic stress values to dense lumber.
Lumber sawed so that the annual rings form a 45 degree angle or more can be described as edge-grain, vertical grain, or rift-sawn in softwoods. Flat grain or plain-sawed describes lumber which the annual rings are at an angle of 45 degrees or less.
Softwood lumber is better than hardwood in many ways. It is easy to convert and strong in tension. It is less expensive than hardwood and has a faster growth rate, meaning it can be harvested sooner than hardwood trees. Software is used as woodware for buildings and furniture. Hardwood is mostly used for trimmings.