Hardwoods
Hardwoods are the deciduous trees that (lose their leaves in the fall/cold months.) There are more than 200 varieties that prove both pliable and plentiful for woodworking. The most common ones are oak, maple, walnut, hickory, and mahogany. Hardwoods are classified by pore openings either Closed Grain (smaller pores) or Ring Porous (larger pores). Closed grain examples include maple and cherry. Ring Porous examples are species such as oak, poplar, and ash. Hardwood trees take longer to mature, so their density tends to be more expensive than softwood species. Hardwood is thus best suited for fine woodworking like cabinetry, furniture, and flooring.
Softwoods
Softwoods are coniferous (evergreen trees) like cedar, pine, fir, and spruce. Only about 25% of softwoods are used in woodworking. As a Closed Grain (smaller pores) product, softwoods don’t tend to have a very defined grain in the finished product. As a softer wood, they can be dented – using even your fingernail. Softwood absorbs moisture much easier, requiring treating for use exposed to the elements. Faster-growing than hardwood counterparts, these straight-grained species are less expensive and well suited for house framing, construction, and outdoor projects (when treated) like decks.
Moisture Content
The moisture content of wood is measured at the time of manufacturing. As a piece of lumber dries it shrinks. This shrinkage can lead to defects
S-GRN. Surfaced in green condition.
S-Dry. (Surfaced dry.) Moisture content measured less than 19% after manufacture.
MC15/KD15. Moisture was 15% or less at the time of manufacture.
KDHT. (Kiln-Dried and Heat-Treated.) Dried to < 19% moisture content with core brought to 56 degrees C for 30 minutes.
Because heat-treating lumber kills insects and fungi, lumber that is shipped globally must be heat treated.
Nominal vs. Actual (Sizes)
Structural/dimensional softwood lumber is referred to by its common nomenclature (such as 2×4). This size isn’t the exact dimensions of the board, this is the “nominal size”. Click the photos to see a larger version of the charts. Similarly, hardwoods have a standard thickness as it is cut into quarter-inch increments.