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A lumber grade stamp identifies the grading agency, mill, grade, species or species group, and often the board’s seasoning condition. Read those fields separately: a species code such as S-P-F names a group, not necessarily the exact species of an individual board, and a grade stamp alone does not prove that a board is suitable for every structural use.
What does a lumber grade stamp tell you?
A grade stamp is an official mark connected to a grading agency and a grading rule—not simply a mill’s informal description of quality. It records how the lumber was identified and graded under the applicable standard. The U.S. Voluntary Product Standard PS 20-20 defines a grade as “a minimum standard specifying the permitted characteristics of lumber, having regard to the end use for which the lumber is intended.”
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That makes the stamp useful when identifying a board, checking its grade and species group, or asking an agency to clarify a code. It is not a guarantee that the board is defect-free, that it meets every project requirement, or that a grade label has identical design values across different species. For structural decisions, read the complete identification alongside dimensions, applicable design values, code requirements, and qualified design guidance.
Stamp layouts vary by grading agency, lumber product, and market. In the United States, PS 20-20 lists the mill number, name, or abbreviation; agency symbol; species or species combination; dry-or-green condition at manufacture; and grade as the normal information for grade marks, with exceptions and qualifications in the standard. Other rules may add or arrange information differently. Do not assume that every stamp uses the same order or abbreviations.
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The stamp is most useful when read as a set of fields, not as a single quality score. The agency and rule help establish who graded or supervises the mark; the mill field identifies the producer; the grade and species identify important product characteristics; and the seasoning field records a condition at surfacing. Supplemental designations may add context about treatment or intended product use. A missing, faint, or partial field limits what you can safely infer.
How do I read a lumber grade stamp?
Read it field by field rather than treating the stamp as one code. Start with the agency mark, then identify the mill, grade, species, and seasoning information; finish by checking any extra rule, treatment, size, or product designation.
- Find the agency symbol or trademark. This identifies the organization supervising the grade mark and is often the best starting point for verifying unfamiliar codes. If the mark is worn or incomplete, do not guess from a fragment.
- Locate the mill identity. It may be a name, abbreviation, brand, or assigned number. A number in this position identifies the mill; it is not a higher grade or quality rating.
- Read the grade field. A grade may be written as a name, number, or abbreviation. “Select Structural” and numbered grades are examples of possible designations, but grade names and rules depend on the product and grading system.
- Read the species field on its own. It may name one species or a recognized species combination. Do not expand an abbreviation beyond what the applicable rule establishes.
- Check the seasoning condition. Look for a designation such as S-Dry, KD, MC15, KD15, or S-Grn. These describe a classification at surfacing under the relevant rules, not a current moisture reading.
- Read every additional mark. A stamp can also identify a grading rule, heat treatment, unusual dimensions, or a product-specific limitation. Treat those as separate fields, not as part of the grade unless the governing rule says so.
As a U.S. reference, PS 20-20’s standard list applies to grade marks covered by that standard and specifies exceptions, including marks used with Certificates of Inspection or Reinspection. Product and jurisdiction matter: Canadian lumber, specialty products, and marks governed by other rules may follow different requirements. The 2025 NLGA Standard Grading Rules for Canadian Lumber are a primary reference for lumber graded under those Canadian rules.
PS 20-20 describes the usual fields in a specific list: mill number, name, or abbreviation; agency symbol; species or species combination; whether the lumber was dry or green when manufactured; and the appropriate grade. The sequence in that written list is not a promise that the physical stamp will present the fields in that order. Find each element by its notation and context, and use the applicable rules to resolve uncertainty.
When examining a board, first cleanly expose the stamp if it is merely obscured, then read the entire mark from one end to the other. Keep the original order in your notes or photo. This helps distinguish a mill number from a grade number and a seasoning mark from a rule designation. Do not treat a letter or number as meaningful on its own if its position and neighboring fields are unknown.
What does the species mark mean?
The species mark identifies either a species or a recognized group, so the code may not tell you which exact tree species a particular board came from. For example, S-P-F means Spruce-Pine-Fir, a designated group rather than a promise that every board in the group is one specific species.
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Clear out junk files and repair common Windows errorsFree Scan →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Other examples used in structural-lumber guidance include DF-L or D. FIR for Douglas fir/western larch, SYP for Southern yellow pine, and HF for western hemlock/fir. The meaning of a mark depends on the applicable grading rules. A partial code, a familiar-looking abbreviation, or a seller’s shorthand is not enough to establish a species where the distinction matters.
The American Wood Council’s guidance on determining species from a stamp recommends contacting the inspection agency when a species or combination is unclear. Identify that agency from the stamp and ask it to interpret the designation under its rules. This is more reliable than inferring a precise species from the letters alone.
Why does the distinction matter? A species group is a defined identification, not a shorthand invitation to substitute whichever member of that group seems familiar. Structural design values are associated with the lumber’s applicable species or species group and grade. The same grade wording across two different species or groups does not, by itself, establish identical design values. Do not use the species letters alone to calculate capacity or approve a substitution.
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If a project specifies a particular species, group, or design value, compare that requirement with the full stamp rather than only the grade field. If the stamp uses a combination mark and the exact species matters to the decision, ask the agency named by the mark what the combination means under its rules. The agency can interpret its code; structural suitability still needs to be checked against the applicable design information and project requirements.
What do S-Dry, KD, MC15 and S-Grn mean?
These marks describe the board’s seasoning classification at surfacing under the applicable grading program. They do not tell you the board’s moisture content today.
- S-Dry or KD: Surfaced dry. The cited agency explanations associate the ordinary classification with a maximum moisture content of 19%.
- MC15 or KD15: A surfaced-dry classification with a 15% maximum in the cited program or agency descriptions.
- S-Grn: Surfaced green, generally over 19% in those descriptions.
- HT: A heat-treated designation. HT is not, by itself, a moisture category; it may appear alongside a seasoning mark.
These labels are tied to the rules and classification represented by the mark. They are not a substitute for measuring present moisture when that matters to a project. A moisture meter can provide an additional check of current moisture, but it cannot decode the board’s species, grade, agency, or recorded manufacturing condition. The American Lumber Standard Committee’s grade-stamp guide, the Western Wood Products Association, and the Pacific Lumber Inspection Bureau explain these classifications in their respective guidance.
The practical distinction is between a historical classification and a measurement now. A dry or green mark tells you how the lumber was classified at the specified manufacturing or surfacing stage under its grading rules. It does not promise that the board has remained at that moisture level during storage or handling. If present moisture is important, measure it separately rather than trying to infer it from the stamp.
Likewise, do not combine HT with a moisture abbreviation as though the two marks were synonyms. In the Markham example below, KD-HT contains a kiln-drying designation and a heat-treatment designation. The moisture classification and heat-treatment status are separate information. Interpret the exact combination under the governing product rules.
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What do extra letters, numbers and marks mean?
Supplementary marks can identify the grading rule, a treatment, a mill, a dimension, or a product limitation. Their meaning depends on where they appear and which rule applies; do not assume that an extra number is a grade or that a treatment mark tells you the whole condition of the lumber.
A City of Markham example illustrates how several fields can work together: MLB is the grading-agency symbol, S-P-F the species group, No. 1 the grade, KD-HT kiln-dried and heat-treated, 99 the mill identification, and NLGA the rule designation. The example comes from Markham Builder Tip No. 79; it is an illustration of one stamp, not a universal layout.
Read the Markham example by function rather than by assuming a universal left-to-right sequence: MLB establishes the agency mark; S-P-F names the species combination; No. 1 gives the grade; KD-HT records the two treatment/seasoning designations; 99 identifies the mill; and NLGA names the grading rule. The example shows why a stamp should not be reduced to its most prominent item. A board bearing the grade No. 1 still has a species group, agency, mill and rule context that matter to interpretation.
When a stamp includes a rule designation, use the rule that governs that product and market to interpret it. The same-looking mark may not carry the same meaning under every grading system. The NLGA rules, for example, describe agency and mill or grader identity, grade, species, seasoning, and additional marks required by the rules.
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Some marks can refer to a product-specific limitation or unusual dimension rather than to a generic grade. If you cannot identify one, do not assume that it is decorative or harmless. Record it and ask the agency to interpret it. Similarly, do not infer treatment details from a single letter if the full designation or applicable product rule is unavailable.
Does the grade alone tell you whether a board is right for a project?
No. A grade is one part of the board’s identification, not a stand-alone approval for a particular structural application. Species or species group can affect design values, and dimensions, product limitations, applicable codes, and project-specific requirements also matter.
When comparing boards or checking a substitution, compare at least:
- Species or species group: Similar abbreviations do not necessarily mean the same group.
- Grade under the applicable rule: Do not assume a grade number is a universal strength scale independent of product and species.
- Dimensions and product designation: Confirm the actual size and any structural-use or product-specific notation.
- Seasoning condition: Distinguish the stamped classification from present moisture content.
- Agency and rule provenance: Check which organization and grading rule support the mark.
A grade stamp is an important identification and grading record, but it does not calculate capacity or establish that a substitution is acceptable. Consult the applicable design values, building-code requirements, and qualified design guidance before relying on lumber in a structural role.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesThe grade is not a universal ladder that can be read without context. A grade name or number is meaningful within the grading rules for the relevant lumber product. The stamp helps identify that grade; it does not make all boards carrying a similar label interchangeable. Even two apparently comparable marks should be checked for species group, dimensions, rule, and product-specific notations before treating them as equivalent.
For appearance expectations, the same caution applies. The standard’s definition frames a grade as a minimum standard for permitted characteristics in relation to intended end use. That is a grading threshold, not a promise of a flawless board. The applicable rule determines which characteristics are permitted for that grade. If appearance is important, inspect the lumber and consult the relevant grade description rather than assuming the stamp guarantees defect-free wood.
A sound comparison keeps identification and approval as separate questions. First establish what each stamp says and which rule governs it. Then check whether the resulting board description meets the plans, applicable design values, and code requirements. If the proposed comparison is a structural substitution, do not resolve it by comparing grade labels alone; use qualified design guidance.
How can you verify an unclear or damaged stamp?
Use the agency mark to find the responsible inspection organization, then ask it to interpret the complete stamp under the relevant rule. If the agency symbol or key fields are illegible, a guess based on one surviving abbreviation may lead to the wrong species, grade, or product interpretation.
- Record what is actually visible. Note the full text and symbols you can read, including their positions. Keep uncertain characters marked as uncertain rather than silently filling them in.
- Include the board context. Give the agency the product type and any visible size, grade, seasoning, or treatment marks. A photograph of the full stamp can help preserve the layout, provided it is clear and complete.
- Ask the identified agency. Request the meaning of the species or species-group code and any rule or supplemental mark you cannot identify.
- Use the right market reference. Consult the grading agency’s current rules for the product and jurisdiction rather than applying a code guide from a different market.
- Keep structural questions separate. Even after the stamp is decoded, confirm design values and project requirements rather than treating identification as engineering approval.
WWPA and PLIB offer stamp interpretation and mill-identification material; for Canadian-marked lumber graded under NLGA rules, consult the NLGA standard. An agency’s reference materials and direct interpretation are preferable to guessing from a worn mark.
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When contacting an agency, send or describe the entire stamp, not just the unfamiliar abbreviation. The order and position of fields help distinguish a mill identifier from a grade or rule mark. Include legible product or size information if available, and state which characters are uncertain. Do not fill in missing portions as though they were confirmed: asking the agency to work from the visible information is more reliable than presenting a guess as a transcription.
If you have a complete, readable stamp, use the agency’s guidance and current grading rules to confirm its fields. If the agency cannot be identified from the remaining mark, seek help from a relevant lumber grading reference for that market rather than applying an abbreviation list from another country or product. A mark that cannot be verified should not be treated as proof of a particular species or grade for a consequential decision.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What a grade stamp cannot tell you by itself
A stamp does not necessarily reveal the exact species when it uses a group designation. It does not report present-day moisture, guarantee an appearance without defects, or replace the applicable design criteria. Nor does one label, read in isolation, establish that a board is appropriate for every use that might be described as structural.
Use the stamp to establish the board’s graded identity, then match that identity to the application. If a code is unclear, seek confirmation from the named agency. If the decision involves structural capacity or substitution, use the relevant design values and qualified guidance rather than trying to derive a result from the stamp alone.
It is also important to distinguish the board’s mark from a complete project evaluation. A stamp documents grading under a rule; a design decision depends on whether that identified lumber, in its dimensions and application, satisfies the requirements for the specific project. The standard’s minimum-grade definition should not be read as a guarantee of suitability for all end uses. It describes permitted characteristics in relation to intended use within the applicable grading framework.
Frequently asked questions
What does SPF mean on a lumber stamp?
SPF, often written S-P-F, means Spruce-Pine-Fir: a designated species group. It does not identify one exact species for every board in the group. Consult the grading agency identified by the stamp if you need the applicable group definition or if an exact species distinction matters.
Does KD mean the lumber is dry now?
KD is a seasoning classification associated with kiln-dried or surfaced-dry lumber under the applicable program. It describes a condition at surfacing, not a present-day moisture reading. Measure current moisture separately if the project requires that information.
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Is S-Grn the same as green-treated lumber?
No such conclusion follows from the stamp alone. In the cited stamp explanations, S-Grn means surfaced green, generally over 19% moisture content in those descriptions. It is a seasoning-condition designation, not a treatment mark. Read any separate treatment notation under the applicable rules.
Does a higher-looking grade number mean stronger lumber?
Do not assume that a number is a universal strength scale. Grade names and numbers depend on the product and grading system, and species or species group affects structural design values. Use the applicable grade rules and design information rather than comparing numbers in isolation.
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What should I include when asking an inspection agency to identify a stamp?
Send a clear image of the entire mark, not just the unfamiliar letters, and transcribe the visible fields in order. Include any readable size or product information and identify characters you are unsure about; that gives the agency context without turning a guess into a fact.
Can a grade stamp tell me whether lumber is visually defect-free?
No. A grade is a minimum standard for permitted lumber characteristics in relation to intended end use, not a promise of a flawless appearance. The applicable grading rule determines what characteristics are allowed for that grade.
If a stamp is hard to read, can I identify a board from one surviving abbreviation?
That is not a reliable way to establish the board’s full grade or species, especially when abbreviations identify groups or rules vary by market. Treat the surviving mark as a clue and ask the grading agency to interpret it rather than filling in missing fields yourself.
Can I use a moisture meter to decode a grade stamp?
No. A meter can provide an additional check of present moisture, but it cannot identify the grade, species, agency, rule, or manufacturing condition recorded by the stamp. Those fields need to be read from the mark and, if unclear, verified with the grading agency.
Frequently Asked Questions
What should I include when asking an inspection agency to identify a stamp?
Send a clear image of the entire mark, not just the unfamiliar letters, and transcribe the visible fields in order. Include any readable size or product information and identify characters you are unsure about; that gives the agency context without turning a guess into a fact.
Can a grade stamp tell me whether lumber is visually defect-free?
No. A grade is a minimum standard for permitted lumber characteristics in relation to intended end use, not a promise of a flawless appearance. The applicable grading rule determines what characteristics are allowed for that grade.
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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteIf a stamp is hard to read, can I identify a board from one surviving abbreviation?
That is not a reliable way to establish the board’s full grade or species, especially when abbreviations identify groups or rules vary by market. Treat the surviving mark as a clue and ask the grading agency to interpret it rather than filling in missing fields yourself.
The Bottom Line
Read the whole stamp: agency, mill, grade, species or species group, seasoning, and any supplemental marks. Use the applicable grading rule to interpret unfamiliar codes, and treat the stamp as an identification record—not a real-time moisture reading, defect-free guarantee, or stand-alone approval for structural use.
