The Differences Between Interior and Exterior Dimensional Lumber

Along with stone, wood is one of the oldest building materials. It is used from immemorial times and even today, it is one of the most used building materials. For practical reason, construction timber is classified according with its suitability for use, indoors or outdoors, although the trees that produce this lumber are not.

However, there are species of trees, which are more suitable for outdoor lumber production. This is due to their high resin, sap and oil content that makes them much more resistant to the vagaries of weather and damage caused by insects. In addition to its natural qualities, exterior lumber will be chemically treated to make it more resistant to moisture and insects.

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Dimensional Lumber

The wood used in the construction industry comes at a huge range of sizes. We find sizes and thicknesses to suit any kind of job, the construction of houses and cabins, to building fences, sidings, furniture etc.

Anyway, to meet certain industry standards and building code requires that lumber be standard sizes. The standard lumber is called “Dimensional Lumber”.

Usually, the most used Dimensional Lumber is 2-by or (2”x …) such as:

  • 2”x4”; 2”x6”; 2”x8” and boards

However, this dimension is somehow confusing, because the real thickness is only 1 1/2 inches not 2 inches. Actually, should not be confusing, because the thickness is always invariable and only the length is different.

Of course, dimensional lumber is divided into two categories that are Exterior Dimensional Lumber and Interior Dimensional Lumber.

Interior Dimensional Lumber

Generally, Interior Dimensional Lumber is made from Pine.

Pine is softwood and is part of conifer family. However, pin does not contain large amounts of resins and oils, making it unsuitable for outdoor use.

It is mainly used for wall framing, floor and roof (rafters).

For interior is also used Spruce and Fir. They are also, softwood and are suitable for Interior Dimensional Lumber.

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Exterior Dimensional Lumber

Some species of softwood such as Cedar, Redwood, Cypress are particularly suitable for Exterior Dimensional Lumber. This is because they have a very high content of resins and sap.

However, Exterior Dimensional Lumber is more expensive than Interior Dimensional Lumber.


Exterior Dimensional Lumber is subject to various chemical treatments, which are inoculated by pressure, the wooden structure to make it more resistant to weather and insects.

Nevertheless, despite all chemical treatments, Exterior Dimensional Lumber turns gray in time. It is therefore, necessary to be applied annually one coat of exterior sealant.

Although more expensive, Exterior Dimensional Lumber is ideal for execution of decks, patios, gazebos, shed, playgrounds, etc.

The most used are cedar and redwood.