How do I copy an angle that is not 90 degrees?

Almost no corner in a house is 90 degrees, and almost every mitre cut at 45 degrees in an old building opens up on one side. The professional answer is not to measure the corner more carefully — it is to stop using numbers at all and copy the angle physically.

What you'll need

• Sliding bevel
• Pencil or marking knife
• An angle finder or a protractor, for setting a saw (optional)
• Offcuts to test on

Step by step

1. Clean the corner first.Old caulk, paint build-up, plaster nibs and a bead of filler all sit in the corner and will be copied faithfully into your angle. Scrape the corner back to the real surfaces before taking anything off it.
2. Set the bevel in the corner.Loosen the locking screw, lay the stock flat along one face and swing the blade until it lies flat along the other. Both must be in full contact, and the bevel must sit flat on the surface the work will sit on — a bevel held at a tilt copies a different angle.
3. Lock it, then check it has not moved.Tighten the screw and offer the bevel back into the corner. If it no longer sits flat, it moved as you tightened. Reset it and try again. A bevel that shifts silently is the main failure mode of the whole method.
4. Transfer it straight to the work.Register the stock against the edge of the board and mark along the blade. You have now copied an angle you never measured, which means no rounding, no reading error and no arithmetic.
5. For a mitre, bisect the angle.Two pieces meeting in a corner each get half the corner angle. In a 90 degree corner that is 45 each, which is where the habit comes from. In an 87 degree corner it is 43.5 each, and cutting both at 45 leaves a gap on the inside face.
6. Bisect it without arithmetic if you prefer.Mark the corner angle onto a scrap board with the bevel, then use compasses to bisect the marked angle geometrically — equal arcs from each arm, and a line through where they cross. Reset the bevel to that bisector and it holds the mitre angle directly.
7. Or read the angle once and halve it.A digital angle finder or a protractor gives the corner angle as a number; halve it and set the mitre saw to that. Most digital angle finders will do the halving for you, which is what the mitre function on them is for.
8. Test on offcuts before cutting the real thing.Cut two scraps at the setting, hold them together in the corner, and look at the joint. Adjust and repeat. Two minutes of scraps saves a length of moulding, and no corner is ever quite what the first setting says.

Tips

• For internal corners on skirting and coving, scribing one piece to the profile of the other handles any angle at all and never needs measuring. Mitres are for external corners.
• A bevel with a locking lever on the end of the stock is easier to use than one with a wing nut in the middle of the blade, which gets in the way of the very surface you are registering against.
• Take the angle at the height the work will sit at. A corner is often noticeably different at skirting level and at picture-rail level, because walls lean.
• Write the setting on masking tape stuck to the bevel if you have several corners to do, so a knocked bevel can be reset without going back to the wall.

Common mistakes

• Cutting every mitre at 45 degrees — Only correct in a corner that is exactly 90. Every degree the corner is out shows as an open joint on one face, and it is always the visible face.
• Setting the bevel on top of filler or old paint — You copy the decoration, not the corner. The mitre then fits the paint and not the wall, and the gap appears once the work is pushed home.
• Setting the corner angle on the saw instead of half of it — It doubles the error into the joint. The mitre saw scale is asking for the angle of the cut, and each piece takes half the corner.

Questions people ask