Lumber Dimensional + Grade Reference
Why this matters
Lumber dimensions are confusing - a "2x4" is actually 1.5" × 3.5". A "5/4 deck board" is actually 1" thick. Customer asks for "2x6 floor joists" but you need to know nominal vs actual + load capacity. Plywood thickness rated by nominal but sold by actual. Estimating materials requires knowing the real numbers. This card is the field card for non-carpenter trades that occasionally do framing OR estimate material.
Nominal vs actual dimensions
Lumber sold by NOMINAL size (what's stamped) but actual dimensions are smaller (planed + dried).
Softwood (Douglas Fir, SPF, hemlock):
| Nominal | Actual |
|---|---|
| 1x2 | 0.75" × 1.5" |
| 1x3 | 0.75" × 2.5" |
| 1x4 | 0.75" × 3.5" |
| 1x6 | 0.75" × 5.5" |
| 1x8 | 0.75" × 7.25" |
| 1x10 | 0.75" × 9.25" |
| 1x12 | 0.75" × 11.25" |
| 2x4 | 1.5" × 3.5" |
| 2x6 | 1.5" × 5.5" |
| 2x8 | 1.5" × 7.25" |
| 2x10 | 1.5" × 9.25" |
| 2x12 | 1.5" × 11.25" |
| 4x4 | 3.5" × 3.5" |
| 4x6 | 3.5" × 5.5" |
| 6x6 | 5.5" × 5.5" |
Rule of thumb: subtract 1/2" from each nominal dimension for actual.
5/4 deck board: 1" actual (5/4 = 1.25 nominal, ~ 1" actual after planing).
Hardwood dimensions
Hardwood sold by random width + length, rough OR S2S (surfaced 2 sides):
- 4/4 lumber: 1" rough; 13/16" S2S (after planing)
- 5/4 lumber: 1.25" rough; 1.0" S2S
- 6/4 lumber: 1.5" rough; 1.25" S2S
- 8/4 lumber: 2" rough; 1.75" S2S
Plywood thickness
| Nominal | Actual |
|---|---|
| 1/4" | 0.21" |
| 3/8" | 0.36" |
| 1/2" | 0.47" |
| 5/8" | 0.59" |
| 3/4" | 0.71" |
| 1-1/8" | 1.10" |
Subfloor plywood: 5/8" or 3/4" most common. 23/32" tongue-and-groove typical.
Hardwood plywood: actual sizes differ slightly; typically 11/32, 15/32, 23/32 (3/8, 1/2, 3/4 nominal).
OSB (Oriented Strand Board)
Common thicknesses: 7/16", 1/2", 19/32", 5/8", 23/32", 3/4".
Sheathing grade: 7/16" or 1/2" typical wall sheathing.
Subfloor: 19/32" (5/8" nominal) or 23/32" (3/4" nominal).
Drywall thickness + sheets
Thickness: 1/4", 3/8", 1/2", 5/8" (most common 1/2" walls + 5/8" ceiling/fire-rated).
Sheet sizes: 4' × 8' (standard), 4' × 10', 4' × 12'.
Fire-rated (Type X): 5/8" with glass fiber.
Moisture-resistant (green board): bathrooms + below grade.
Cement board: tile underlayment (Durock, Hardibacker).
Lumber grades
SPF (Spruce-Pine-Fir) + Hem-Fir most common framing. Grades (best to worst): Select Structural, No. 1 (visible/cabinet), No. 2 (most framing standard), No. 3 (cheaper, restrictions), Stud grade, Construction, Standard, Utility. Pressure-treated: stamps include treatment level + chemical. Most framing: No. 2. Visible: No. 1 or clear pine/oak.
Common framing materials
2x4 wall studs at 16" O.C. typical interior + non-bearing exterior. 2x6 at 24" O.C. for higher R-value insulation. 2x8 floor joists for 12 - 14 ft spans. 2x10 for 14 - 18 ft spans. 2x12 rafters/joists for largest residential spans. Engineered lumber (I-joists, LVL, parallam) for spans/loads beyond dimensional.
Span tables (residential floor joists, 40 psf live load)
No. 2 SPF, deflection L/360:
| Joist size | 12" O.C. span | 16" O.C. span |
|---|---|---|
| 2x6 | 10' 4" | 9' 4" |
| 2x8 | 13' 7" | 12' 4" |
| 2x10 | 17' 4" | 15' 9" |
| 2x12 | 21' 1" | 19' 1" |
Subtract 10% for 24" O.C.
Always verify span tables for specific application - local code AHJ has final say. Engineered for specific spans (always per stamped engineer).
Common lumber lengths
8 ft, 10 ft, 12 ft, 14 ft, 16 ft, 20 ft, 24 ft. Some yards stock to 32 ft.
Stud lengths: 92-5/8" (precut for 8 ft ceiling), 104-5/8" (9 ft ceiling), 116-5/8" (10 ft).
Pricing: per linear foot in some markets, per piece in others. 8' 2x4 typically retail.
Pressure-treated lumber + weights
Grade stamps include chemicals (ACQ, MCA, CA - current) + treatment retention level (e.g., 0.40 PCF ground contact). Use categories: above-ground (deck, fence), ground contact (posts, sill plates), below-grade (deck posts in soil). Higher treatment retention = more chemicals = more rot-resistant + more corrosive to ferrous fasteners - use hot-dipped galvanized or stainless.
Weights (8 ft length, dry): 2x4 8 - 10 lbs, 2x6 15 - 17, 2x8 22 - 25, 2x10 30 - 32, 2x12 38 - 42, 4x4 22 - 26, 4x6 35 - 40. PT wet 20 - 30% heavier. Plywood 3/4" × 4×8: 60 - 65 lbs. OSB 1/2" × 4×8: 45 - 50 lbs.
Common framing measurements
16" O.C. (on center) for studs: stud-to-stud center 16 inches.
Plate-to-plate height = stud length minus plate thicknesses (typically 4.5" for top plate + sole plate).
Wall framing rough opening for door: door width + 2.5".
Wall framing rough opening for window: window width + 1".
Header sizing (for wall openings):
- < 4 ft span: 2x6 header
- 4 - 6 ft: 2x8 header
- 6 - 8 ft: 2x10 header
- 8 - 10 ft: 2x12 header
- Always verify per local code
Common fasteners for lumber
Nails: 16d common (3.5" × 0.162") framing; 8d common (2.5" × 0.131") sheathing; 6d common (2" × 0.113") sheathing + trim. Screws: 3" exterior for deck framing, 2.5" GRK for structural, 2-1/2" subfloor screws, 1-5/8" drywall. Joist hangers: per manufacturer spec - Simpson SD screws, 10d common.
Common lumber mistakes for non-framers
Using "2x4" as 2" × 4": subtract for actual.
Ignoring grade: cheaper grade fails sooner.
Wrong fastener for PT: hot-dipped galvanized OR stainless only.
Underestimating weight: hurt back lifting wet PT 4x12.
Wrong subfloor thickness: 1/2" plywood subfloor too thin for 16" joist spacing.
Forgetting headers at openings.
Buying long lengths to cut (more expensive per ft typically).
References
- WCLIB (West Coast Lumber Inspection Bureau) grading
- SPIB (Southern Pine Inspection Bureau) grading
- AWC (American Wood Council) span tables
- ICC building code references
- Manuall internal: Building Envelope Fundamentals