How to Measure and Cut a Skirting Board for an External Wall Joint by Bisecting

In this guide, we will showcase how to measure and cut a skirting board for an internal wall joint by bisecting an angle. We have included a step by step guide and a video below:

When fitting skirting board you may come across instances where you need to cut boards to fit external angles which are not 90 degrees. This guide is designed to show you how to calculate the required angle for each cut, as well as making the cuts on a mitre saw.

To begin take a sliding bevel and recreate the angle you need to cut, locking the bevel in place when you’re happy it’s a snug fit.

Find the angle with a sliding bevel

We’re essentially trying to work out the value of half of the angle. The reason for this is that you need to make two cuts on the two pieces of skirting which fit together to create the whole angle.

Once you have the angle locked into place, using a pencil transfer this angle to either scrap wood or a skirting board offcut.  To transfer the known angle to the offcut, place the bevel onto the offcut so that the handle of the bevel is flush with the bottom edge of the offcut. Then draw along the metal edge with the pencil (see image). This should leave a line from the bottom edge of the offcut to the top and should look like the angle you are trying to replicate.

Mark bevel angle on skirting board offcut 

Once you have this marking drawn out, the next stage is to add further markings to calculate the angle for cutting the MDF skirting. Using a compass insert a pen or pencil and extend the arms to roughly 50 percent (see picture). Position the compass needle at the bottom of the angled line (base of the board, see picture). Then draw an arc with the compass intersecting the line. Keeping the compass where it is, draw a second arc intersecting with the base of the board (see images).

Mark arc on the drawn line

Mark opposite angle on opposite side

The final set of arcs are used to work out the bisect line, you might need to extend your compass slightly to around 75% (see pic if required). Taking the intersect points of the lines you’ve just created place the compass needle in the middle and draw the first arc in the middle of your angle. Repeat the same process from the middle of the second intersect point. This should produce two arcs which cross over forming the bisect point.

Marking the first bisect arc

Marking the second bisect arc

Using either a rule or the metal edge of the sliding bevel, draw a line from the base of the line (from the sliding bevel) to the middle of the bisect point you’ve just created. This is called the bisect line.

Marking the bisect line

To work out the angle you require you will need a protractor, place the protractor with 0 degrees across the first line we drew from the bevel. The bisect line should then give off a reading for the angle. This measures in the video as 67.5 degrees, however, we need to subtract this from 90 degrees to find out the angles of our cuts. In this case 22.5 degrees.

Place protractor on initial marking

Calculation of final angle

To make the cuts in this case you will need to set your mitre saw to 22.5 degrees. Using offcuts of skirting and cut it at the first angle, then set the mitre to the same degrees but in the opposite direction. You can then put them into position on the wall to check if they are correct. If you’re happy you can then make the final cuts on the pieces of skirting board.

Cut first piece to the angle you've calculated, in this case it was 22.5 degrees.

Test first piece to check correct angle and if it aligns.

Cut second piece opposite angle

Final Joint complete

About the Author

Kieron Miller is the Managing Director of Skirting 4 U and has been working in the skirting board and online retail industry since 2012. Kieron has been using his extensive product knowledge and problem-solving experience to help both trade and private consumers all over the country find the perfect skirting board products for their projects. He continues to innovate new solutions and product ideas as the market continues to grow.