Do knots in timber matter?

A knot is where a branch was. The grain of the trunk had to flow round it, so the timber there is denser, harder, oriented differently and full of resin. Whether that is a problem depends entirely on what the piece is doing and where the knot is sitting.

Step by step

1. Tell a live knot from a dead one.A live knot was a living branch when the tree grew round it, so it is fully bonded to the surrounding wood and the grain sweeps continuously round it. A dead or encased knot was a dead branch, so there is a ring of bark between it and the wood and nothing holding it in.
2. Expect dead knots to fall out.Sooner or later, usually as the timber dries or when the piece is machined, they come loose and leave a hole. If the piece is visible or has to hold a fixing near that point, reject the board.
3. Judge knots structurally by position, not size.Timber in bending is in tension along one face and compression along the other. A knot in the tension zone — the underside of a joist near mid-span — interrupts the fibres that are doing the work. A knot at the neutral axis in the middle of the section matters much less. Edge knots are worse than centre knots.
4. Let the strength grade do the judging on structural timber.This is exactly what strength grading assesses. A C16 or C24 stamp means the knots in that piece have been assessed against the rules for that class, by eye or by machine. On graded timber, do not second-guess the grade; on ungraded timber, avoid heavy edge knots in anything carrying load.
5. Avoid knots where a joint or a fixing lands.A knot is hard, dense and differently oriented. A screw into one splits or wanders, a chisel skates off it, a mortise through one tears out, and a moulding cut across one breaks up. Set out your joints to miss them.
6. Seal knots before painting.Resin migrates out of knots, especially once the timber warms in the sun or against a radiator, and stains straight through emulsion, undercoat and gloss. Two thin coats of knotting solution or a dedicated stain-blocking primer over every knot, before priming, is the standard answer.
7. Treat a loose knot in a piece you have to use.Knock it out, clean the hole and either glue it back in with the grain aligned, or fill it — with a two-part wood filler if it will be painted, or a plug cut from matching timber if it will be clear finished.

Tips

• Whitewood typically has many small hard knots; redwood has fewer and larger, softer ones. If a rash of small dark dots would spoil the look, buy redwood.
• Knots blunt blades faster than clear timber and can throw a router or a planer. Feed slowly through them and expect a slightly rougher surface where the grain reverses round the knot.
• "Knotty pine" is sold deliberately as a look. If you like it, you are buying a defect on purpose and paying less for it, which is a good trade.

Common mistakes

• Painting over knots without knotting solution — It looks fine for a season and then brown or amber patches come through, worst on south-facing joinery and above radiators. Repainting without sealing produces the same result again.
• Cutting a mortise or a housing through a knot — The knot tears out rather than cutting cleanly and the joint loses both its fit and a good deal of its strength. Move the joint or move the timber.
• Using a board with a big edge knot as a joist or a rafter — An edge knot in the tension face is where a bending failure starts. Turn the board so the knot is in compression if you can, or use another board.

Questions people ask