Cutting plasterboard cleanly

Learn to score, snap and finish a plasterboard cut so edges are straight, clean and ready to fix.

CP-01Set up your tools and workspaceYou need a retractable trimming knife (a handle with a sharp, snap-off blade), a straightedge (a metal ruler or spirit level works), a tape measure, and a flat surface bigger than the board, like two trestles or a workbench. Lay the plasterboard flat with the side you'll see (the smooth ivory face) upwards. Beginners often try to cut on a soft or uneven surface, which makes the board wobble and the cut wander.

CP-02Mark and score the lineMeasure your cut line and mark it with a pencil at both ends, then hold the straightedge firmly along those marks. Run the knife along the straightedge in one steady stroke, pressing hard enough to cut through the paper and just into the white plaster core underneath, not all the way through. It should feel like you're cutting card, not sawing wood. The mistake beginners make is scoring too lightly or in several wobbly passes, which gives a ragged, curved snap instead of a straight one.

CP-03Snap the board along the scorePosition the scored line at the edge of your workbench or trestle, with the cut line facing up. Push down firmly and evenly on the overhanging side in one motion. The board should snap cleanly along the score line with a crisp crack, like breaking a biscuit. If it doesn't snap easily, your score wasn't deep enough; don't force it, go back and score again a little deeper.

CP-04Cut the back paperAfter snapping, the two halves will still be joined by the paper on the back face. Fold the board slightly to open up the fold, then run your knife along the crease to cut through this paper. Keep the board flat as you do it. Beginners sometimes pull the halves apart before cutting the paper, which tears a rough, uneven strip off the back instead of a clean line.

CP-05Clean up the cut edgeRun a plasterboard rasp (a flat tool covered in coarse teeth, a bit like a cheese grater) along the cut edge to knock off any rough bits of plaster or paper fluff. Hold it flat against the edge and use light, even strokes. This step matters because a rough edge will stop two boards butting together neatly, leaving a visible gap once you fill and paint.

CP-06Cut an opening for a socket or pipeFor a hole in the middle of a board, mark the shape, drill a starter hole in one corner with a drill (a tool that spins a bit to bore through material), then use a padsaw (a narrow hand saw with a pointed blade) to cut along your lines. If the board is already fixed to a wall, treat it as if there's a cable or pipe behind it, even if you've used a detector: detectors can miss cables that run deep, sit in metal conduit, hide behind metal-backed plasterboard, or simply don't follow the usual safe zones, which older or amateur wiring often ignores completely. If there's any doubt at all, turn off the relevant circuit at the consumer unit (the fuse box) before cutting, and turn off the water at the stopcock before cutting anywhere pipework could run.

CP-07Check you've done it rightLay the cut piece against the space it's meant to fill, or against a straightedge if it's a straight cut. A good cut is straight, square (a perfect right angle where needed) and sits with no more than a 2-3mm gap against the neighbouring board or wall. If you see a wavy edge, a lipped corner, or a gap that widens along its length, go back and rasp or re-cut rather than trying to fill a big gap later.

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