How to Read a Lumber Stamp: Decoding the Markings

That Stamp Tells You Everything

Pick up any framing stud or structural board at a lumber yard and you'll find an ink stamp on one face. Most people ignore it, but that small cluster of letters and numbers is a complete quality biography of the piece. Building inspectors check these stamps carefully, and understanding them helps you buy smarter.

The Five Key Elements

A typical lumber grade stamp contains five pieces of information, though the layout varies by certifying agency. Here's what each element means:

1. Certifying Agency

The outer mark identifies the grading agency that certified the mill. Common agencies include:

• SPIB — Southern Pine Inspection Bureau (southern yellow pine)
• WCLIB — West Coast Lumber Inspection Bureau (Douglas fir, hemlock)
• NLGA — National Lumber Grades Authority (Canadian lumber)
• TP — Timber Products Inspection (various species)

This mark tells inspectors and engineers that the lumber was graded by a recognized, accredited body — not just stamped arbitrarily by the mill.

2. Mill Number

A number identifying the specific sawmill that produced the board. This is a traceability tool. If a batch of lumber shows consistent defects, the agency can trace it back to the source mill. You don't need to memorize mill numbers, but their presence confirms the lumber is part of a quality-controlled supply chain.

3. Grade

This is the most important element for your buying decision. Common grades for dimensional lumber include:

• SEL STR (Select Structural): The highest structural grade. Tight knots, straight grain, minimal wane. Used where appearance and strength both matter.
• #1: High quality with slightly more allowable defects than Select Structural. Excellent for any structural application.
• #2: The workhorse grade for framing. Allows larger knots and some wane. Perfectly adequate for walls, floors, and roof framing where drywall or sheathing covers the lumber.
• #3 and Stud: Lower grades suitable for non-structural or lightly loaded uses. Stud grade is specifically intended for vertical wall framing up to 10 feet.

4. Moisture Content

Lumber stamps indicate the moisture condition at the time of surfacing:

• MC15 or KD15: Kiln dried to 15% moisture content or less. This is premium framing lumber with minimal shrinkage risk.
• KD or KD-HT: Kiln dried (19% or less) and often heat treated, which is required for export under international phytosanitary rules.
• S-DRY: Surfaced dry — 19% moisture content or less at the time of planing.
• S-GRN: Surfaced green — above 19% moisture. This lumber will shrink as it dries, which can cause nail pops, drywall cracks, and floor squeaks if not accounted for.

5. Species or Species Group

The stamp identifies the wood species or species combination:

• SYP: Southern Yellow Pine
• D FIR or DF: Douglas Fir
• HEM-FIR: Hemlock-Fir combination
• SPF: Spruce-Pine-Fir (extremely common in Canadian imports)

Species matters for structural calculations because each has different published strength values. Your engineer or building inspector needs to see the correct species for the load tables to work.

Why This Matters to You

Structural lumber from a reputable supplier should carry an approved grade stamp. If you're working with a building inspector — and in Chicago, you will be — those stamps are what inspectors look for at framing inspection. Buying ungraded lumber from a random source can cause problems at inspection, costing you far more in delays and re-work.