Knots

Also called: dead knot, live knot, loose knot

A knot is the base of a branch, preserved in the surrounding trunk wood, at the point where the grain changes direction to flow around it instead of running straight along the piece.

Why every knot is a weak point

Live or dead, every knot weakens timber's mechanical strength, because it forces a sudden change in the direction of the wood fibres around it. A live knot has grown from a branch that was still alive and attached when the tree was felled, so its fibres are intergrown, fused into the surrounding wood; a dead knot comes from a branch that had already died, so it sits encased in the trunk rather than bonded to it, and is more likely to work loose. Around any knot the grain runs distorted, or cross grain, and that zone has less stiffness along the length of the piece than straight grain does, so knots create localised weak spots. Knots nearer a beam's edge weaken it more than knots nearer the centre. Whether a knot actually reduces a piece's strength depends on where in the timber it sits, and the safest way to judge knotty lumber is to picture the knots removed and ask whether the wood left behind would be strong enough on its own. In glue-laminated beams, built up from separate boards glued together, knots do not run continuously through the member the way they do in one piece of sawn lumber, so their effect on strength is judged differently; and around a knot, the grain departs less from the surface angle in a naturally round log than it does in a sawn piece.

Grading and choosing timber

Because of this, knots rank among the most important factors used to grade timber for both strength and appearance. Lumber with sound knots, ones solid and fully held in place, is fine for low-stress work such as siding, trim, cabinets or shelving, where strength matters less than in a structural beam; there, knots simply add extra work to shape and are unwanted in fine finished work as well as a source of weakness. Knots can also be classified by how the saw happens to cut through them relative to the tree they came from. Spruce cut from a tree's inner trunk, or bole, can contain many small pin knots, tiny knots either intergrown or encased. Large knots are best kept high in a log, so that when it is sawn into boards most of them come out clear, with no knot showing in a plank's centre.

Species and their knots

Different timbers carry knots differently. Memel fir has few knots, but where they occur the surrounding grain is irregular and tends to tear when planed. Among deals, sawn planks of pine or fir, yellow deal suits general building work best, having fewer knots, shakes and sap streaks than other deals, with a tight, hard grain and a good wearing face, though it resists damp poorly; Baltic white deal is known instead by its small, hard, dark knots, its woolly texture straight off the saw, and its tendency to weather to grey. Loose knots can also cause trouble when a timber is worked with machine tools, and a timber with hard knots can badly blunt hand tools even where the wood itself works easily. One timber favoured for joinery in America works cleanly and carries little knot or sapwood content; another, which smells resinous when freshly cut, has large, somewhat translucent live knots. A further timber, compared with burr, a swirl-grained decorative growth found on some trees, has knots that are smaller and tighter than a yew burr's, with a silkier grain.

Working around knots

In the workshop, knots take extra patience: running a plane over one calls for extra care and persistence, since the reversing grain tears easily. But knots are not always unwelcome. Small-shop woodworkers can readily work around them, or use them deliberately as a design feature, and many fine craft and artwork pieces are built around showing knots off rather than hiding them. Where a knot must be hidden, high-class joinery work has long filled the cut-out hole with a putty of white lead, japan and turpentine.