Timber defects: know the names, reject the bad stuff

Timber defects are the flaws in a board — knots, splits, warps, rot and pests — that can weaken it or make it a pain to install straight. Knowing the names (from BCITO Unit 24360) helps you reject below-grade timber at the merchant before it ever gets to site.

Knots, splits and other grain defects

These are flaws in the grain itself. Some are cosmetic, some are a reason to send the pack back.

• Knots — branch intersections. They affect strength if large or loose. Tight knots are usually OK; encased or decayed knots mean reject.
• Splits / checks — separation along the grain, showing as end checks or surface checks. The length of the check determines whether it counts as a defect.
• Wane — bark or a rounded edge left on from the outside of the log.
• Pith — the soft centre of the tree. It’s often weaker and prone to splitting, so avoid pith-centre in structural timber.
• Resin pockets — in pine or spruce, sticky pockets that can leak through paint.
• Compression wood — denser wood formed on the underside of leaning trees. It’s brittle and can fail unexpectedly under load.
• Honeycomb — internal checks caused by rapid drying, which weakens the timber.

Warp: cup, bow, twist and more

Warp is distortion of the board’s shape. Each type has its own name, and it pays to know which way a board is bending:

• Cup — a curve across the width of the board, so one face is concave. More than 3mm per 100mm counts as a defect.
• Bow — a curve along the length of the board (longitudinal).
• Twist — a corkscrew distortion, with both ends rotated relative to each other.
• Crook — an edge bend along the length, making the board hard to install straight.
• Spring — a curve in plan view; it looks straight from the edge but bends when laid flat.

Rot and borer

Decay and pests are the ones you can’t accept in structural work — they only get worse.

• Decay (rot) — fungal rot. Incipient rot is early and soft; advanced rot is punky and crumbly. Neither is acceptable.
• Borer — the common house borer (Anobium punctatum) leaves 1.5–2mm exit holes, with larvae running a 2–3 year life cycle. H1.2 boron treatment kills it.

Sapwood vs heartwood

Knowing which part of the log you’re looking at explains a lot about durability and how treatment behaves.

• Sapwood — the pale outer rings, which are less durable.
• Heartwood — the dark inner wood, which is more durable.
• Treatment is mostly absorbed by the sapwood, so heartwood pine can look ‘untreated’ even though it’s naturally more durable.