The Differences Between Interior & Exterior Dimensional Lumber

Dimensional Lumber

• Wood comes in all shapes and sizes, from ultra-thin veneers to heavy solid blocks. In construction, however, standard sizes are necessary for producing structures that are consistent with industry standards and building codes. Dimensional lumber fulfills that requirement by adhering to uniform sizes. The most popular dimensional lumber, commonly called “2-by,” includes 2-by-4s, 2-by-6s, 2-by-8s and larger boards. When you purchase 2-by dimensional lumber, you won’t get boards that are 2 inches thick, because 2-by is only 1 1/2 inches thick, but no matter where you purchase 2-by, it should be the same thickness and width, although the length may vary.

Exterior Dimensional Lumber

• Some types of softwood naturally resist moisture and insect damage. The reason is not because the wood grain is soft, but because these softwoods come from conifer trees that contain high sap and resin content. Examples of softwoods that are suitable for exterior building include cypress, redwood and cedar. You can find dimensional lumber in all three of these wood types, but it is expensive when compared to the dimensional lumber used to frame interior structures. The most cost-effective exterior dimensional lumber is chemically treated pine, which is infused under pressure with chemicals that seal the grain and deter insect infestation.

Interior Dimensional Lumber

• Pine, a softwood, is the dimensional lumber of choice for building walls, floors and rafters in the construction of most new homes. Although pine also comes from a conifer tree, it doesn’t contain enough natural oils or resins to protect it from water or wood-eating bugs. Two other softwoods, fir and spruce, are also available in dimensional sizes for building interior structures.

Considerations

• Except for treated lumber, wood used to build exterior projects is often structural and aesthetic. Redwood and cedar produce strong storage sheds, gazebos and decks, but they are also slightly aromatic and have attractive wood grain patterns that make them desirable for outdoor furniture. These woods will weather to a soft gray in time, and despite their weather-resistant properties, they will benefit from an annual coat of clear exterior sealer to maintain their good looks.