Cutting mitres for skirting, architrave and coving

Learn to cut accurate 45-degree mitres so skirting, architrave and coving meet neatly at any corner.

MC-01Understand what a mitre joint isA mitre is an angled cut on the end of a piece of trim, usually 45 degrees, so two pieces meet to form a neat corner instead of butting together square. Skirting, architrave and coving all use the same idea. Get this picture in your head before you pick up a saw: you are cutting a slope, not a straight end.

MC-02Choose a mitre box for your first attemptsA mitre box is a U-shaped guide, usually plastic, with slots pre-cut at 45 and 90 degrees to guide your saw. A mitre saw is a powered version with a spinning blade that does the same job faster. Start with a mitre box and a tenon saw (a small saw with a stiff back and fine teeth for wood) - it is slower but far easier to control while you learn, and there is no blade spinning near your fingers.

MC-03Mark your cutting line on the show faceMeasure the length you need along the front face of the board - the side that will be visible once fitted - not the back. Mark it clearly with a pencil and square the mark across the board with a try square (an L-shaped tool for checking and drawing 90-degree lines). The beginner mistake is marking on the back edge, which ends up a different length once the angle is cut, leaving a gap.

MC-04Cut slowly in the mitre boxPlace the board flat in the box against the back fence, and line your pencil mark up with the 45-degree slot on the side you need. Hold the board firmly against the fence with one hand, and saw with slow, full-length strokes, letting the teeth do the cutting rather than pushing down hard. Beginners rush and press too hard, which makes the saw wobble out of the slot and gives a ragged, uneven cut.

MC-05Check the joint before you fix anythingHold your two cut pieces together as they will sit in the corner, without fixing them. You have done it right if the front faces meet in a tight, thin seam with no visible gap, and the top edges line up flush. If you see a gap at the front or the back, the angle is slightly off - recut rather than force it, since gaps only get worse once painted or filled.

1 sheet · Fine sandpaper (for smoothing cut edges)

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MC-06Learn which way the angle pointsInternal corners (like most room corners) and external corners (like an outside wall edge or a chimney breast) need the mitre cut in opposite directions. On an internal corner, the point of the cut sits at the back of the board; on an external corner, the point sits at the front. Get this backwards and the two pieces will not meet at all - always offer the piece up to the actual corner before cutting to check which way the slope should run.

MC-07Keep your hands clear of the bladeKeep the hand holding the board well back from the saw's path, and never let fingers stray in line with the cut. For short offcuts, clamp the piece to a bench rather than holding it by hand, since short pieces are easy to lose control of. If you move on to a powered mitre saw later, keep both hands outside the marked danger zone and let the blade stop spinning fully before lifting it clear.

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