This past week, we looked at the issue of sizing deck posts appropriately based on characteristics governed by the building codes such as span, depth, height, and thickness of framing materials. We talked mostly about posts that are 4×4 in or 6×6 inches.
The distinction between nominal and actual dimensions in lumber is a basic fundamental aspect of the construction industry that often confuses people who are unfamiliar with typical construction practices, for obvious reasons. Its counterintuitive. Although the technical elements of the construction industry are constantly changing, some of these practices like nominal vs. real dimensions, have roots deep in the history of building construction here in the United States, and some things even change significantly from one region of the country to another.
Nominal dimensions refer to the size of lumber before it undergoes final processing, while actual dimensions are the true measurements of the finished product. For instance, a piece of lumber with a nominal size of 2×4 inches actually measures 1.5×3.5 inches in its final form. This discrepancy between stated and actual dimensions has historical roots and practical considerations that continue to influence modern lumber production and usage.
The origin of this dimensional difference dates back to early sawmilling practices. When trees were first cut into lumber, the rough-sawn boards were indeed close to their nominal size. However, as the boards dried, they would shrink slightly. Additionally, to create a smooth surface, the rough lumber would be planed down, further reducing its dimensions. Over time, as wood processing became more standardized and efficient, the industry settled on consistent finished sizes that were smaller than the rough-cut dimensions. The nominal size became a convenient shorthand for ordering and specifying lumber, even though it no longer reflected the actual measurements of the finished product. This practice was eventually codified in industry standards and building codes, establishing the system we use today.
The difference between nominal and actual dimensions serves several practical purposes in modern construction. First, it allows for a degree of standardization across the industry. Regardless of the specific manufacturer or sawmill, builders can expect relatively consistent sizes when ordering lumber by its nominal dimensions. This standardization facilitates easier planning and design in construction projects. Second, the smaller actual dimensions contribute to more efficient use of raw timber resources. The reduction in size from rough-cut to finished lumber allows for more boards to be produced from a single log, minimizing waste. Additionally, the smoother surfaces of planed lumber improve handling safety and provide a more finished appearance in applications where the wood remains visible.
This difference is apparent here in Washington, DC almost as much as anywhere else in the country. Here in our city, we have an incredibly high proportion of historic buildings. In some specific neighborhoods alone there are thousands of buildings and homes that were built around the late 19th and early 20th centuries. In most of those buildings, the exterior envelope of the structure is made from historic brick, but often the interior framing for interior walls and structural floor systems will be built with historic wood joist and studs. These framing elements, made from locally available lumber, over 100 years ago, are often much larger than similar materials today. The floor joists and studs of our historic buildings are good examples of the contrast between historic construction and framing methodologies versus the modern alternatives. Many of the joists and studs used in this historic framing will be much thicker than their modern or contemporary counterparts. For example, many floor joints and historic homes built over 100 years ago will be over 2 inches thick, whereas modern Alternatives would only be 1.50 or 1.58 inches thick.
From an engineering perspective, the actual dimensions of lumber are important for structural calculations and building design. Contractors use the real measurements when determining load-bearing capacities, spans, and other structural factors in construction. Building codes and structural design tables are based on these actual dimensions as well, not the nominal sizes. For example, when calculating the strength of a beam or the load capacity of a floor joist, the true 1.5×3.5-inch dimensions of a 2×4 must be used rather than the nominal 2×4 measurements. This distinction is particularly important in deck construction, where accurate sizing of posts, beams, and joists is essential for ensuring the structure’s safety and longevity. Understanding the relationship between nominal and actual sizes is therefore critical for anyone involved in deck design and construction, from DIY enthusiasts to professional builders and engineers.
Use a contractor who understands and cares about doing things right. Always, feel free to reach out to us here at Dupont Decks and Patios. We are happy to help with almost all steps of the deck building and design process. Let us know about your ideas and talk to us if you have questions about possibilities . You can call us at (202) 774-9128. You can find us online at https://dupontdeckspatiosdc.com and you can email us there as well at https://dupontdeckspatiosdc.com/contact-us
