A lumber grade ranks a piece of lumber by its strength-reducing defects, and under 2021 International Residential Code (IRC) Sections R502.1.1 and R602.1.1 load-bearing sawn lumber must carry a grade mark showing it. The IRC span tables list four grades for each species group: select structural (SS), No. 1, No. 2 and No. 3.
A grade stamp is the mark ink-stamped on a board that states who graded it, the grade, the species, the mill and the moisture condition. The grade is what lets a designer, plan reviewer or inspector confirm that the lumber matches the cell of the span table used.
Key facts
- 2021 IRC Section R502.1.1 requires sawn lumber for load support to be identified by a grade mark of an accredited lumber grading or inspection agency, or to have a certificate of inspection.
- 2021 IRC Section R602.2 sets the minimum stud grade at No. 3, standard or stud grade lumber.
- 2021 IRC Tables R502.3.1(1) and R502.3.1(2) list grades SS, #1, #2 and #3 for Douglas fir-larch, hem-fir, Southern pine and spruce-pine-fir.
- 2021 IRC Section R502.2.2 allows blocking and subflooring boards of utility grade, and Section R507.2.1 requires deck wood to be No. 2 grade or better.
- PS 20 (the American Softwood Lumber Standard) requires the stamp on lumber under nominal 5 inches thick to show S-GRN, S-DRY or KD for its moisture condition.
What does a lumber grade stamp show?
A grade stamp has five parts: the grading agency mark, the mill identification, the grade, the species or species group, and the moisture condition. The Western Wood Products Association (WWPA) lists the same five elements as the parts of most grade stamps.
Parts of a softwood lumber grade stamp, from PS 20 Section 7 and the WWPA page on interpreting grade stamps.
| Stamp element | What it tells you | Example wording from the source |
|---|---|---|
| Agency mark | Which accredited agency supervised the grading | WWPA certification mark |
| Mill | Where the piece was produced | Assigned mill number, or the firm's name or brand |
| Grade | The grade of the piece | Grade name, number or abbreviation |
| Species | The commercial species or species group | Individual or combination species name |
| Moisture | Condition when graded | S-DRY or KD: 19% maximum. S-GRN: over 19%. MC-15 or KD-15: 15% maximum |
PS 20 Section 7.3.1 says the grade mark signifies that the lumber conforms to the size, grade and seasoning provisions of the rules it was graded under. Section 7.3.5 says the mark includes the commercial name of the species, or the group designation where the grading rules group species. Section 7.3.7 adds that ripping, resawing or surfacing graded lumber negates the grade mark, so a board resawn on site is no longer graded.
The dimensional lumber sizes guide explains what S-DRY and S-GRN mean for the actual size of the piece.
What does the 2021 IRC say about grade marks?
Sections R502.1.1 (floors) and R602.1.1 (walls) both require sawn lumber to be identified by a grade mark of an accredited agency, with design values certified by an accreditation body that complies with DOC PS 20. A certificate of inspection from a qualifying agency is accepted in place of the mark.
The same wording appears for roof framing in Section R802.1.1 and for wood foundations in Section R404.2.1. One related rule matters on site:
- End-jointed lumber. Sections R502.1.1.2 and R602.1.2 allow approved end-jointed lumber with a grade mark to be used interchangeably with solid-sawn members of the same species and grade. Where it is used in an assembly that needs a fire-resistance rating, the mark must include "Heat-Resistant Adhesive" or "HRA".
Used or unmarked lumber is a local matter, so ask the building official what is accepted before reusing it.
Which lumber grades appear in the IRC span tables?
The floor joist, rafter and ceiling joist tables in the 2021 IRC list SS, #1, #2 and #3 for each of four species groups. Header and deck tables use No. 2 only.
Grade names as they appear in the 2021 IRC and where each applies.
| Grade name in the IRC | Where the 2021 IRC uses it |
|---|---|
| SS (select structural) | Floor joist Tables R502.3.1(1) and R502.3.1(2), rafter Table R802.4.1(1), ceiling joist Table R802.5.1(1) |
| #1 | Same tables |
| #2 | Same tables, and the base grade for header and deck tables (footnote b to Tables R602.7(1) and R602.7(2); Section R507.2.1 for deck wood) |
| #3 | Same tables |
| Stud, standard | Studs, with No. 3: Section R602.2 |
| Utility | Blocking, subflooring boards (Section R502.2.2), blocking (Section R802.5.2.3), some studs (Section R602.2 exception) |
| No. 4 common | Subflooring boards (Section R502.2.2) |
SS is printed in the span tables as the abbreviation. The WWPA dimension lumber page spells it out as select structural.
No. 2 vs No. 1 lumber: how much do spans change?
A higher grade carries more span, and the gap depends on the species and the size. The table below shows 2x10 floor joists at 16 inches on center for all four grades and species groups.
2021 IRC Table R502.3.1(2): 2x10 floor joist, 16 inches on center, live load 40 psf, dead load 10 psf, L/Δ = 360. Maximum spans in feet-inches.
| Species group | SS | #1 | #2 | #3 |
|---|---|---|---|---|
| Douglas fir-larch | 17'-4" | 16'-5" | 15'-7" | 11'-11" |
| Hem-fir | 16'-5" | 16'-0" | 15'-2" | 11'-8" |
| Southern pine | 17'-0" | 16'-1" | 14'-0" | 10'-10" |
| Spruce-pine-fir | 16'-0" | 15'-5" | 15'-5" | 11'-8" |
Two things stand out. First, No. 3 gives up a lot of span against No. 2, so a stack that arrives No. 3 cannot simply replace a No. 2 callout. Second, Southern pine drops furthest from #1 to #2, while spruce-pine-fir shows identical #1 and #2 spans in this table. Do not read across species without checking. The floor joist span table has full #2 tables for all four species groups, and the header size chart and stud spacing guides use the grades the same way.
What is stud grade lumber?
Stud grade is an optional grade intended for vertical use, as in load-bearing walls, and the IRC accepts it for studs alongside No. 3 and standard. The WWPA lists Stud as its own category, separate from Structural Light Framing (select structural, No. 1, No. 2, No. 3) and Light Framing (construction, standard, utility).
2021 IRC Section R602.2 states studs shall be a minimum No. 3, standard or stud grade, and allows an exception for utility grade in bearing studs that do not support floors and in nonbearing studs, spaced per Table R602.3(5). Section R602.3.1 caps utility grade studs at 16 inches on center, supporting no more than a roof and ceiling, and 8 feet in height for exterior and load-bearing walls or 10 feet for interior nonbearing walls.
What are lumber species groups?
A species group is a set of species sold and graded together under one name, so they share design values. The IRC tables use four: Douglas fir-larch, hem-fir, Southern pine and spruce-pine-fir.
PS 20 Section 7.3.5 says where grading rules group species, each species in the group is identified in the rules and the stamp shows the group name. The IRC does not list which species belong to which group, so take that from the grading rules of the agency on the stamp. If the stamp names a species or group that is not one of the four, the IRC span tables do not apply directly. Section R502.3 sends other grades and species to the AWC span tables.
How do lumber grades appear on a framing plan?
A callout normally carries size, grade, species and spacing, such as 2x10 #2 DF-L at 16 inches on center, so a reviewer can find the same table cell. Add a general note that lumber is grade marked per the adopted code.
Framing Plan draws joist layouts, beams and openings and exports PNG, PDF and DXF. It does not check grades, span sizes or code compliance: its sample profile uses demo span figures and metric sizes, and the editor works in millimeters. The adopted code edition and local amendments govern, and engineered products such as I-joists and LVL use manufacturer tables. For the fasteners that tie graded members together, see the framing nail size chart.
Summary
Lumber grade, species and moisture condition appear on the grade stamp, and 2021 IRC Sections R502.1.1 and R602.1.1 require load-bearing sawn lumber to carry that mark or a certificate of inspection. The span tables list SS, #1, #2 and #3 for four species groups, with studs allowed at No. 3, standard or stud grade under Section R602.2. Read the stamp against the table cell before relying on a span. See PS 20 at NIST and the WWPA grade stamp guide.
Open the framing studio to lay out a preliminary joist and beam plan, then add grade and species callouts for your designer or plan reviewer.