Lumber & Building Materials
Lumber dimensions, grading, engineered wood products, OSB, plywood, pressure-treated lumber, and material storage.
- Explain the difference between nominal and actual lumber dimensions
- Identify common engineered wood products including OSB, LVL, and I-joists
- Describe the grading system for Canadian lumber
- Select appropriate lumber species and treatments for interior and exterior applications
Lección 1
Lumber Dimensions & Grading
Nominal vs Actual Dimensions
One of the most fundamental facts in carpentry is that lumber dimensions are not what they seem. When you buy a "2x4," you are not getting a piece of wood that is 2 inches by 4 inches. The nominal dimension (the name) refers to the rough-sawn size before drying and planing.
After kiln drying and surface planing (milling), each dimension is reduced by approximately 1/2 inch. The typical dimension of a 2x4 after milling (actual size) is 1-1/2 inches x 3-1/2 inches. The nominal dimension (2x4) refers to the rough-sawn size before drying and planing - the actual dimension is always smaller. This is a frequently tested fact.
| Nominal Size | Actual Size (inches) | Actual Size (mm) |
|---|---|---|
| 2x4 | 1-1/2 x 3-1/2 | 38 x 89 |
| 2x6 | 1-1/2 x 5-1/2 | 38 x 140 |
| 2x8 | 1-1/2 x 7-1/4 | 38 x 184 |
| 2x10 | 1-1/2 x 9-1/4 | 38 x 235 |
| 2x12 | 1-1/2 x 11-1/4 | 38 x 286 |
Remember Actual Dimensions
A 2x4's actual size is 1-1/2" x 3-1/2". This matters for every framing calculation - wall thickness, header depth, stud length, and rough opening sizes all depend on actual dimensions.
Canadian Lumber Grading
In Canada, lumber is graded by the National Lumber Grades Authority (NLGA) under CSA standards. Structural lumber grades include:
- Select Structural - Highest quality, fewest defects
- No. 1 - Good structural quality, minor defects allowed
- No. 2 - Standard framing grade, most commonly used
- No. 3 - Economy grade, limited structural use
- Stud - Special grade for vertical load-bearing wall studs
Every piece of graded lumber carries a stamp showing the grade, species, moisture content, and the grading agency.
The actual size of a 2x4 is 1-1/2 inches x 3-1/2 inches after milling. Every dimension loses approximately 1/2 inch from nominal. No. 2 grade is the standard framing lumber grade in Canada.