Lumber grades explained
Two boards can be the same species, the same size and the same price, and one is structural while the other is not. The difference is the grade stamp, and it is the least-read label in the lumber yard.
The grades, in order
| Grade | What it allows | Typical use |
|---|---|---|
| Select Structural | Fewest defects, highest design values | Long spans, exposed beams, visible structure |
| No. 1 | Small tight knots, minor slope of grain | Joists, rafters, where spans are near the limit |
| No. 2 | Larger knots, some wane | Standard framing — most decks are built with this |
| No. 3 | Significant defects, still structural | Short spans, blocking, non-critical framing |
| Stud | Cut for vertical use, limited wane | Wall studs only, not joists |
| Utility | Lowest structural grade | Concrete forms, temporary work |
| Appearance grades | Graded for looks, not strength | Deck boards, railing, trim |
What is printed on the stamp
A grade stamp carries five pieces of information, and all five matter:
- Grade — the name above.
- Species or species group — SPF, Douglas fir-larch, hem-fir, southern pine. Design values differ by group.
- Moisture content — S-GRN (green, over 19%), S-DRY (19% or less), or KD (kiln dried).
- Mill and certification mark — which agency inspected it.
- Span rating for engineered and panel products.
A board with no stamp is not a structural board. It may be perfectly good lumber, but it has no design values and an inspector cannot accept it for a span that requires them.
Deck boards are graded differently
Decking is an appearance product. The grades are about the face, not the strength: Select, Choice, Quality, and for cedar Clear Heart and STK. A "No. 2" deck board does not exist in the structural sense.
What you should look for instead is the mill mark and the moisture content. Deck boards that are installed green (S-GRN) will shrink and cup; KD or S-DRY boards installed with a ⅛ gap stay where they are put.
Why the grade is not the whole story
Grade describes a single board's defects. It does not describe the span, the load or the connection. A No. 2 joist at 16″ centres on a 10 ft span is fine; the same joist at 24″ centres on a 16 ft span is not, and no grade of 2×8 makes it so. The span table is the other half of the decision, which is why this site publishes how to read one rather than a span calculator.
What the grade stamp does not tell you
- It does not give design values. Those come from the species group and the grade in the NDS supplement, and they are what an engineer uses.
- It does not cover treated lumber retention (UC3B vs UC4A), which is a separate rating about decay, not strength.
- It does not cover panel grades (sheathing span ratings), which are a different system.
- It does not replace the stamp. If the board has no stamp, this page does not make it structural.
Questions people actually ask
What grade of lumber should I use for a deck?
No. 2 is the standard for joists, beams and framing, and it is what most decks in the US are built from. Use No. 1 or Select Structural where the span is near the table limit or the member is exposed.
What does S-DRY mean on a lumber stamp?
Surfaced dry — the board was dried to 19% moisture content or less before planing. S-GRN means green, over 19%. Dry boards shrink less after installation, which matters for decking and railing.
Is No. 2 lumber strong enough for a deck?
For most residential decks, yes, at the spans in the span table for that species and spacing. Grade alone does not determine adequacy; span, spacing and load do.
Can I use Stud grade for deck joists?
No. Stud grade is intended for vertical wall framing and does not carry the design values for horizontal joist spans.