How do I cut skirting for a corner that is not square?

Scribing handles any internal corner without measuring an angle at all, and external corners get a mitre cut to the true angle rather than an assumed forty-five degrees.

Difficulty: intermediate
Time: 1 hr
Read: 5 min

Short answer

Internal corners: scribe, never mitre. A scribe copies whatever shape the corner actually is and stays closed when the timber shrinks. External corners: mitre, but find the real angle by holding two offcuts against the walls, marking where their front edges cross, and cutting through both marks — never assume 45 degrees.

Almost no corner in a house is exactly ninety degrees, and a mitre depends on both halves adding up to the true angle. That is why mitred internal corners open into wedge-shaped gaps and scribed ones do not, and it is why the external corner technique is about measuring the real angle rather than trusting the mitre saw's detent.

What you'll need

a mitre saw or a mitre box
a coping saw with a fine blade
offcuts of the skirting for marking
a sharp pencil and a block plane or file
grab adhesive and pins or screws

Internal corners: a mitre compared with a scribe, and what to do when the corner is not square

Step by step

Fit the first length of an internal corner cut square.Push it hard into the corner with a square end. It needs no shaping at all, and it establishes the face the second piece will sit against.
Cut a 45 degree mitre on the second piece to expose its profile.This mitre is not the joint — it is a marking exercise. The line where the mitre meets the face of the board is the exact profile of the skirting.
Pencil along that exposed profile line.Some people run a pencil edge-on along it to make the line obvious. That line is what the coping saw follows.
Cut the waste away with a coping saw, back-bevelled.Follow the pencil line, angling the saw so you are removing more material from behind than from the front. Only the front edge should touch, which is what lets the joint close tight regardless of the corner's angle.
Test-fit and ease with a file or knife.Take material off the back, never the front. A scribe that is slightly tight closes with a few strokes.
For an external corner, do not assume 45 degrees.Hold two offcuts along the two walls so they overlap around the corner. Mark on each where the other's front edge crosses it, and where the back edges cross.
Cut through both marks.The line joining those two crossing points bisects the true angle, whatever it is. Cut both pieces on that line and the mitre closes.
Glue and pin external mitres.A mitre held only by being pushed together opens as the timber moves. Glue the mitred faces, pin through the joint from both directions, and the corner stays closed.
Fix into the wall, not into plaster alone.Grab adhesive plus fixings into masonry or studs. Skirting fixed only to a plaster skim pulls off with the plaster.
Deal with the gap at the bottom last.Where the floor is uneven, a flexible caulk closes a small gap; a larger one wants scribing to the floor or a beading.

Tips

Cut a spare piece and practise the coping cut before touching the actual skirting. It is a five-minute skill once seen and awkward the first time.
Do not cut every length to its measured size before fitting. Fit and mark in place, one piece at a time.
For very ornate skirting profiles, a coping saw with a very fine blade and patience beats any power tool.
Where a wall bows badly, a slight back-cut on the square end of the first piece lets it push tighter into the corner.

Common mistakes

Mitring internal corners — The joint runs the full depth of the skirting, so it depends on the corner being exactly the assumed angle and on the timber never shrinking. Neither is true. The joint opens into a wedge-shaped gap, and filling it works for one season before the timber moves again.
Setting the mitre saw to 45 degrees for an external corner — Plastered external corners are commonly a degree or two off ninety because of the corner bead. Two 45 degree cuts then leave a gap at the front or the back of the joint, and the gap is at eye height for anyone sitting down.
Easing a tight mitre by shaving the visible face — It shortens the face and changes the profile, so the joint no longer matches its partner. Take material off the back and the front edge stays as cut.

If it doesn't work

Internal corner has opened into a gap

Cause: Mitred rather than scribed — Fix: Cut the second piece back and scribe it properly. Filling a mitred internal corner is a repeating job.

External mitre open at the front

Cause: True angle greater than 90 degrees, cut as 45 — Fix: Re-cut both pieces using the two-offcut marking method to find the real bisector.

Scribed joint tight at the top and open at the bottom

Cause: Not enough back-bevel, or the wall leans — Fix: Relieve more material from behind the scribe so only the front edge bears.

Questions people ask

Why is a scribe better than a mitre inside?

Two reasons. It copies the actual corner rather than assuming an angle, so out-of-square corners are handled automatically. And when the timber shrinks across its width, the shrinkage happens behind the visible front edge instead of opening the joint, so the corner still looks closed years later.

Can I scribe an external corner too?

No — both faces are visible on an external corner, so there is no way to overlap one behind the other. External corners are always mitred, which is why cutting the true angle matters there.

What about a corner that is wildly out, like 100 degrees?

The method is the same: hold two offcuts, mark where their edges cross, cut through the marks. It works for any angle, which is exactly why it is better than trying to measure the angle with a protractor and halve it.