The Evolution of Voluntary Product Standard PS 20
In the early 20th century, lumber dimensions were inconsistent. A builder in Chicago might receive 2x4s that were 1.75" thick, while a builder in Seattle received 2". The American Softwood Lumber Standard (PS 20) was developed to stabilize the market. It defines how much material can be removed during the surfacing process to ensure a "consistent" product for architects and engineers.
Moisture Content and Shrinkage Dynamics
Lumber is an organic, hygroscopic material. When it leaves the mill as "Green" (S-GRN), it has a moisture content above 19%. As it dries to "Kiln-Dried" (S-DRY) levels (approx 15-19%), it shrinks significantly. PS 20 standards require that a 2x4 surfaced at 19% moisture must measure 1.5" × 3.5". If the wood continues to dry to 6% in an arid environment, it will shrink further—often by another 1/16".
| Nominal | Actual (Imperial) | Actual (Metric) | Common Trade Use |
|---|---|---|---|
| 1 × 4 | 3/4" × 3 1/2" | 19 × 89 mm | Trim, Furring |
| 2 × 4 | 1 1/2" × 3 1/2" | 38 × 89 mm | Standard Framing |
| 2 × 6 | 1 1/2" × 5 1/2" | 38 × 140 mm | Exterior Walls |
| 2 × 10 | 1 1/2" × 9 1/4" | 38 × 235 mm | Floor Joists |
| 4 × 4 | 3 1/2" × 3 1/2" | 89 × 89 mm | Posts / Blocking |
Blueprints vs. Reality: The Tolerance Gap
Blueprints are usually drawn with nominal labels but *actual* spacing. For example, a "2x6" wall is drawn at 5.5" wide. If you are converting from metric drawings, use our length converter to ensure your metric-to-imperial stud spacing (e.g., 400mm vs 16" OC) remains within the tolerances of local building codes like the IRC Section R502.3.
Summary for Procurement
When ordering from a lumber yard, always specify the surfacing. S4S means surfaced on four sides (standard). Rough-sawn lumber might actually be the full nominal dimension, which is often used in restoration work or custom timber framing. If your order is large, always double-check the volumetric yield using our lumber calculator to prevent short-ordering on the job site.