Guides / Grades and Specs

Hardwood Lumber Defects: Wane, Knots, Heart Center and What Each Grade Allows

February 19, 2027

Hardwood lumber defects are the features that stop part of a board counting as clear wood: knots, wane, checks, splits, decay, stain and similar marks. NHLA grades limit them by measuring how much clear face a board can yield, so a defect matters only where it falls inside the cuttings you need. This post is part of our guide to hardwood lumber specifications, and it shows what each defect is, what a grade tolerates and how to write a limit into the order. Grade, thickness and moisture numbers in this guide are typical values, and the NHLA rulebook governs. Its text was not read for this post: the grade figures come from the Wood Handbook and a Purdue summary of the 1998 rules.

Hardwood Vendor is a matching service, not a sawmill. We own no mills, kilns or stock, and the limits below are for you to put in your own contract with a supplier.

The defect table

The table lists the defects a buyer meets most often, what causes them and how a standard grade treats them. Read the last column as a risk map, because the rules decide what is a defect and the contract decides who pays.

Defect	Likely cause	Treatment in a standard grade	Who usually carries the risk
Wane	Round shape of the log, bark left on the edge	Restricted by grade, with special limits for FAS 1-Face and Selects	Seller, if outside the grade at shipment
Sound knot	Branch base, solid across the face	Not allowed in a clear cutting, allowed in a sound cutting	Seller, if the grade is exceeded
Unsound knot or decay	Loose knot with a pith center, fungi	Defect, not counted as sound	Seller, if present at shipment
Check	Rapid or faulty drying	Defect, unless planing removes it	Drying quality: seller at shipment
Split	Separation as the wood dries	Defect	Seller, if outside the grade
Shake	Separation between growth rings	Defect	Seller, if outside the grade
Warp (bow, crook, cup, twist)	Distortion, usually from drying	Does not affect grade if planing removes it	Depends on storage and contract
Sticker stain	Strips between layers during drying	Defect in clear grades, sound in 2B Common	Seller if wet drying, buyer if from storage
Bird peck, worm hole	Insect or bird damage in the tree	Counted as sound, allowed in 2B Common cuttings	Seller, if outside the grade
Mineral or gum streak	Natural color variation	Characteristic, not a defect	Written sort, if you care

Caption: AHEC pages on characteristics and defects, Purdue FNR-130 (summary of the 1998 NHLA rules), Wood Handbook chapter 5 and the NELMA-authored grading glossary. Treatment is summarized from those pages; species exceptions are in the rulebook, and the last column is the author’s reading of normal practice, not a rule.

Wane

AHEC defines wane as bark or missing wood caused by the round shape of the tree or log. The NELMA grading glossary words it as missing wood on an edge or corner from bark or any other cause. Wane costs width, and on an edge it can cost the straight reference edge a rip saw needs.

Purdue notes that special wane restrictions apply to FAS 1-Face and Selects. The wane limits themselves are in the rulebook, and I could not open them, so I give no numbers. If wane matters to your parts, such as a nosing edge or a panel that is glued edge to edge, ask for edges that are ripped or trimmed, and write the width you need after trimming. Straight line ripping and gang ripping are the usual ways to get true edges.

Knots and pin knots

AHEC separates two kinds. A sound knot is solid across its face and shows no decay. An unsound knot is a circular area where a branch joined the trunk, with a pith center and sometimes missing wood. In a clear cutting neither is allowed on the cutting face. Sound cuttings, which the lower common grades use, may carry sound knots, bird pecks, sticker stain and worm holes.

Small knots depend on the species. Purdue gives one example: in cherry, small knots or their equivalent not exceeding 1/8 in in diameter are admitted in the cuttings. Terms such as pin knot are used on mill sheets for small knots, and I found no single public definition that all mills share. For visible parts, write the limit as a size (“no knots over 1/8 in on the face”) or as a rule (“no knots on the show face”), and attach photos of what you accept.

Heart center

Heart center is the wood at and around the pith, the small soft core of the log. AHEC lists pith among defects, and the Wood Handbook abbreviation list includes FOHC, free of heart center, which tells you it is something buyers ask for by name. The NHLA rulebook text on heart center was not read, and I found no limit for it in the sources I could open.

As practical advice, treat it as a sort. Say whether heart center is allowed on the face, on the back or not at all, and say where it is judged: on the board as sawn or on the face after ripping. Without that, a mill sorts to its own habit, and quotes that look the same will not be.

Drying defects: checks, splits, warp and stain

AHEC defines a check as a lengthwise opening on the face caused by rapid or faulty drying. A split is a lengthwise separation that forms as the wood dries, and its glossary defines warp as distortion from the original plane, usually from drying. AHEC’s kiln drying note explains the mechanism: heat is raised gradually while humidity stays high so the outer cells do not dry too fast and trap moisture inside, which can cause irreversible damage. Drying schedules are species specific.

The practical consequence is that a defect can come from the drying, not the log. Stickers cause stain and marks: AHEC calls sticker stain a greyish discoloration and treats a sticker mark that planing removes as a characteristic. Stress from drying shows later, when a board is ripped and the strips bow. See kiln drying hardwood and sticker stain in hardwood lumber for how those are prevented and judged.

What grades do not treat as defects

The grade is about clear area, so several features are outside it. AHEC lists as characteristics, not defects under NHLA rules: heartwood and sapwood, burls, gum streaks (in cherry), mineral streaks, glassworm patches and sticker marks that planing removes. Purdue adds that plain red and white oak are graded to the standard rule apart from a consideration for mineral stain, and that gum streaks and spots are admitted without limit in cherry.

So if your product cannot take sapwood, mineral streak or color change, the grade does not protect you. Write the sort next to the grade, for example “sapwood allowed up to a stated share of the face” or “no mineral streak on the show face”. The NHLA grade guide on this site covers the sorts that sit on top of grade.

Writing the limit into the contract

A limit needs a standard, a place and a time. This wording is an example, not a recommendation, and the values in brackets are for you to set:

• “Grade: FAS, NHLA Rules for the Measurement and Inspection of Hardwood and Cypress, [edition year], graded rough, before kiln drying.”
• “Additional limits: no heart center on the show face; sap stain not accepted; sound knots on the show face not over [size].”
• “End checks and end splits trimmed within [length], measured on [percent] of boards.”
• “Moisture [range], measured with [method] on [number] boards per pack.”
• “Claims: within [days] of receipt, lumber unaltered, rule book and inspector as named in this order.”

The Wood Handbook adds two points that belong in the order. First, the grade stamp shows the quality at the time of grading, and later poor storage, improper drying or careless handling can drop the lumber below its original grade. Second, working or recutting a graded product to a pattern may change or void the grade, so say whether acceptance is on the rough or the worked lumber.

Who inspects and when

Inspection has two moments. Before shipment, a buyer can ask for photos of each pack, a sample board or an independent grader, which is the idea behind pre-shipment grading. On arrival, check a sample of boards from several places in the load before you plane, rip or dry anything.

Purdue’s summary of the 1998 NHLA sales code gives the shape of a claim, and those numbers are from the older edition, so confirm them in the current rules: the sales code binds only when the contract says so, the buyer had 14 days from receipt, and the lumber had to stay unaltered. NHLA says its National Inspectors give on-site rule interpretations and dispute resolution. Photos, the tally and the deadline are covered in lumber quality claim.

Next step

Defects belong on the order as limits and sorts, not as hopes. Start from the full stack in the hardwood lumber specifications guide, then send your lumber RFQ with the limits attached. We pass it to up to 3 mills or yards that say they carry it, and they quote you directly.

Sources

• American Hardwood Export Council, grading sawn lumber: characteristics and defects
• American Hardwood Export Council, glossary of terms
• American Hardwood Export Council, Technical Matters 3: kiln drying
• Purdue Extension, FNR-130 Hardwood lumber grades (summary of the 1998 NHLA rules)
• USDA Forest Products Laboratory, Wood Handbook chapter 5, Commercial Lumber (FPL-GTR-113, 1999)
• Building Products Digest, lumber grading glossary (NELMA)
• National Hardwood Lumber Association, grading rules page

Frequently asked questions

Keep reading

• End Sealing Hardwood Lumber: What It Prevents, Which Coating and When It Pays
• Powderpost Beetles in Hardwood Lumber: Which Boards Are at Risk and What to Ask