First find the total angle of the corner (Screwfix, code 79209, is an example) then divide by 2 to get the angle to cut each of the two pieces that meet.
If you haven't got an electric compound mitre saw, they start around £30, or less, and are invaluable, but if you can afford more, do so - you get what you pay for.
4 golden rules:
1. Experiment with a length of scrap timber before attemping to chop up expensive mouldings - it will actually make the job more enjoyable coz the pressure will be off to get it right first time, and result in less wailing and gnashing of credit cards.
2. Remember that you can mark and measure till the cows come home, but don't expect it to be exactly right first time - allow a bit of extra length on the skirtings for taking a 'smidgin' more off here and there.
3. If it's a deep skirting remember to ensure it will be vertical when fixed in place - may sound obvious, but the application of nail or screws can pull it in more than you might expect when compared to holding or pressing it in place by hand.
4. If you've got mitres on BOTH ends of a length, start with the most awkward, and allow extra length for BOTH ends, for that wee 'smidgin'!
good advise by jack,i will add that when you buy your skirting look at the endgrain and make sure that it is straight as often you will find that it is slightly bowed and this will make mitres less perfect when the two pieces come together.
Agreed. Also because timber is often "cupped" I would suggest if you are going to paint it use mdf - its more stable. Don't forget when pinning the mitres to glue them first. Helps prevent shrinkage and opening of the joint.
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