Know What Lumber You're Buying: Actual Sizes and the Grade Stamp

The Problem

Lumber names are nominal: no board in the rack measures what it's called. Frame a layout assuming a 2x10 is 10 inches tall and everything downstream — beam heights, notch depths, stair math — lands wrong by 3/4 of an inch. The stamp on the board face also carries grading and drying information most buyers never read.

The Main Types and When to Use Them

Actual dimensions. Every "2x" board is 1-1/2" thick. Widths:

• 2x4 → 1-1/2" x 3-1/2"
• 2x6 → 1-1/2" x 5-1/2"
• 2x8 → 1-1/2" x 7-1/4"
• 2x10 → 1-1/2" x 9-1/4"
• 2x12 → 1-1/2" x 11-1/4"

The small sizes lose half an inch of width; from 2x8 up the loss grows to three quarters.

What each size does. Interior walls typically frame in 2x4. Exterior walls frame in 2x6 in much of the country (some regions still allow 2x4). The 2x8/2x10/2x12 sizes work as headers and joists. LVL (laminated veneer lumber) is the engineered option for load-bearing beams and long headers — above garage doors, for instance.

Reading the grade stamp. On a typical stamp:

• The agency mark (SPIB, for example) is the grading authority.
• The big number is the grade: SS (select structural) is best, then 1, then 2 — what you'll see 95% of the time on a jobsite — then 3.
• "Prime" marks higher quality within that grade.
• KD-19: kiln dried to 19% moisture content or less.
• HT: heat treated.