Dimensional Lumber

Looking Beyond Lumber Shrinkage

The true answer lies a little deeper than just modern processing equipment and shrinkage of lumber. Depending on the species and grade of the lumber the boards will either go into a drying process or be sold as green lumber where the moisture content isn’t altered.

Pine and many softwoods which allow for quick drying times are commonly dried for dimensional lumber. Hardwood species take longer to dry due to higher densities and are designated for furniture or flooring markets. The rough sawing dimension required to cut depends on what market the lumber will be finally processed for.

When the logs are first sawn, the boards are cut with the anticipation of needing to dry and be machined into their final dimensions. Only lower grade lumber will not be dried. More dense wood species such as Oak would be first cut at much larger dimensions than lower density woods such as Pine. Hardwood lumber is dried for aesthetic markets where the lumber is graded on size of defects and overall appearance. Pine lumber is graded on size of defects but more focused on being structurally strong when used in building construction.

Hardwood Lumber (Oak)

Oak is often first cut 1" thicker over what the final dimension would be cut. Hardwood lumber that makes grade for flooring or furniture markets will be slowly dried. A typical moisture content for hardwood species would be dried to or below 10% moisture content. Dense wood must be dried slowly to avoid creating defects such as warping, bowing or checking. After the lumber has been dried, it will then be planed/surfaced and edged to its final dimension. This allows for mills to cut out defects such as knots, warping, etc. so the lumber grade is maximized. Hardwood lumber is more commonly bought and sold as 4/4 for 1" thick and every 1/4" upward.

Hardwood Summary

• Originally sawn 1" thicker than what it will be finally sold as
• Dried slowly to avoid creating defects reaching around 10% MC
• Drying times take 25-35 days
• Dried to be sold into flooring and furniture markets for aesthetic appearance

Softwood Lumber (Pine)

Pine would normally be sawn closer to 1/2" thicker than its final dimension. This is why pine lumber is commonly called a 2x4 when they actually measure 1.5 x 3.5. Pine is a less dense wood species and it allows for quick drying times. Pine designated for building components would be dried to 19% or below which is the standard for building products. The lumber will be dried at the original dimension of full 2" x 4" and after drying the boards will be surfaced with a planer head and trimmed down to 1.5" x 3.5".

Softwood Summary

• Originally sawn about 1/2" thicker than final dimensions
• Dried quickly in high volume drying kilns reaching 19% MC
• Drying times take 20-30 hours
• Dried to be sold into structural framing applications (not appearance graded)

Over time we've come to accept these nominal (approximate) dimensions as standard. But there's still a catch. Shrinkage can be highly variable. This makes it very difficult to manage and regulate dimensional accuracy.

Rules for Calculating Dimensions

So now you know that 1x3, 2x4 and 4x8 are really just names or nominal dimensions. But... it gets even more complicated. Dimensions change based on the width or the thickness. Here are the general "rules" for calculating softwood dimensions:

Nominal vs Actual Dimension Rules

Nominal Dimension	Actual Size
Less than 2 inches	Reduce by 1/4 inch
Less than 8 inches	Reduce by 1/2 inch
8 inches or greater	Reduce by 3/4 inch

Dimensional Lumber

Softwoods (pine, spruce, fir, etc.) are generally known as framing or dimensional lumber. This wood is commonly used for framing walls and floors in our homes and regularly used for building wooden crates and pallets. Since no one likes unexpected dimensional errors, the handy chart below outlines the changes in dimensions you can expect:

Dimensional vs Actual Lumber Chart for Dried Softwoods

Regulating Lumber Dimensions

The US Department of Commerce standardized softwood dimensions for green and finished lumber back in 1969.

Today, regulating dimensional accuracy in the US falls under the responsibility of the National Institute of Standards and Technology (NIST). The current standards are published by the government in a document titled the American Softwood Lumber Standard (PS 20). The current version is labeled the Voluntary Product Standard PS 20-20 .

Why is regulation so difficult?

State and local inspectors spot check lumber dimensions at mills and retail locations but determining if softwood boards are within limits is complicated. Here's why:

1. Shrinkage
   Green lumber has higher moisture content. As the wood dries (either in an oven or air dried) the dimensions reduce. Depending on wood species - and specifically wood grain direction - shrinkage can range from 2% to 15%. This equals anywhere from 0.012" to 0.6" for a 4 inch board.

2. Surfacing
   Boards with smooth sides are said to have a dressed surface. The process of smoothing out board surfaces further reduces the original width or thickness. Dimensional lumber is commonly labeled S4S. The designation stands for "surfaced 4 sides".

3. Inconsistent Testing
   Inspectors do not have nationally agreed-upon testing procedures. Everything from an individual inspector's sampling protocol to the device used for measuring can muddy the results. That's about to change.

If you're wondering about hardwoods, thickness measurements are slightly different, but most hardwood widths follow the softwood standard. As an example, a 6" hardwood board is actually 5.5".

New Testing Procedures Under Review

The Office of Weights and Measures (OWM) at the NIST has been working with the American Lumber Standards Committee (ALSC) since 2016 to develop new testing procedures for dimensional accuracy. The last update was in 2020. The next big step in moving the new procedures forward is a five-year revised standard (PS 20-25) currently under review.

Have you ever wondered about the impact of lumber's dimensional accuracy on wooden pallets and crates? Have your own wrong-size lumber story you need help solving? Send your details down below and let's connect.

12/12/2024 by Jonathan Haynes on Lumber