How To Cut An Internal Corner For Cornice
Learning how to cut an internal cornice corner is the single biggest confidence hurdle for most DIYers but it's far simpler than it looks.
The vast majority of corners in a standard room are internal 90-degree joints, so once you've got the cutting angle sorted, the rest of the room falls into place quickly.
In this guide we'll show you how to cut a clean internal mitre on your XPS cornice, the same lightweight material that makes the whole job easy to handle on your own.
Watch the short video below to see the cut done start to finish, then scroll down for a step-by-step breakdown with photos so you can follow along at your own pace.
How To Cut An Internal Cornice
How To Cut An Internal Corner
Follow these simple steps to cut a internal 90 degree corner.
Place Your Cornice In The Mitre Box
Measure the wall length and mark your cornice, allowing a little extra at the corner end it's easier to trim back than to start again.
Position the cornice in the mitre box the same way it'll sit on the wall ceiling edge up, wall edge down. Getting the orientation right is half the battle.
First Cut
Place your blade in the 45 degree slot as shown. The finger is pointed towards which part will be your internal cornice piece.
Cut with steady, even strokes. XPS foam cuts cleanly with a fine-tooth saw, so let the blade do the work rather than forcing it.
Second Cut
Repeat the cut in the opposite direction for the matching length the two 45-degree faces meet to form your 90-degree internal corner.
Dry-fit both pieces before gluing. A clean join should sit tight with no gap; a quick trim now saves mess later.