Seal a crack between the wall and ceiling

Rake out the old crack, then seal the gap with flexible caulk so it can move without cracking again.

SC-01Check the crack before you touch itLook along the crack. A thin, straight gap running along the wall-ceiling line, especially one that opens up every winter, is normal movement and fine to seal yourself. If the crack is wider than about 5mm, stepped, diagonal, or turns up alongside sticking doors or a sloping floor, that can point to structural movement rather than simple shrinkage. Get a builder to look at it before you fill anything in that case.

SC-02Clear the room and protect surfacesMove furniture away from the wall and lay a dust sheet along the skirting below the crack. Open a window, since caulk and filler both release fumes as they cure. Wipe down the wall and ceiling below the crack so dust does not stick to wet caulk later.

1 each · Dust sheet

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SC-03Work out what the wall and ceiling are made ofThis changes how gently you can work and which filler to use, so check both sides of the crack. A solid brick or block wall feels hard and dense when you knock it, with no hollow sound. Plasterboard on timber studs sounds hollow between studs and solid roughly every 400-600mm, where a magnet run along the wall will stick to the screw heads fixing the board. Plasterboard on metal studs also sounds hollow but with a slightly tinnier knock, and a magnet or detector often picks up a continuous metal track running along the ceiling line. Dot-and-dab plasterboard sounds hollow over most of the wall even though solid masonry sits behind it, and lath and plaster ceilings (common before the 1930s) feel slightly soft or springy and the plaster is often powdery rather than solid.

SC-04Rake out the old crackDrag a filler knife or scraper along the crack to remove loose paint, dust and any old filler, opening it slightly so new caulk has something to grip. On solid brick or block and on timber-stud plasterboard, you can work fairly firmly. On dot-and-dab plasterboard, press gently, since the board is only held to the wall by adhesive dabs and can flex or crack if you push too hard. On lath and plaster, be extra gentle, over-raking can bring loose chunks of old lime plaster down; stop and get it checked if plaster keeps crumbling away rather than staying put. Vacuum the crack out afterwards so the caulk sticks to a clean surface.

1 each · Filler knife or scraper

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1 tub · Fine surface filler (small tub, for deep gaps in lath and plaster)

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SC-05Mask the edges for a neat lineRun a strip of masking tape along the wall and another along the ceiling, leaving a gap of about 5mm either side of the crack. This is optional but gives a much straighter finished line, especially useful on solid masonry where the surface can be uneven.

1 roll · Masking tape

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SC-06Apply flexible decorator's caulk along the crackCut the caulk tube nozzle at a 45° angle and load it into the caulk gun. Run a steady, even bead along the full length of the crack, keeping the gun moving at a constant speed. Always use a flexible decorator's caulk here rather than a rigid filler, since this joint moves with the seasons on timber-stud, metal-stud and dot-and-dab plasterboard, and a rigid filler will simply crack out again within months.

1 each · Caulk gun

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1 tube · Flexible decorator's caulk, 310ml

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SC-07Tool the caulk into a neat lineWet your finger or a caulk shaping tool and draw it along the bead in one smooth pass, pushing the caulk slightly into the joint and leaving a shallow concave line. Wipe off excess caulk from the tool onto a rag as you go rather than smearing it onto the wall.

1 each · Caulk shaping tool

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1 each · Damp cloth or sponge

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SC-08Pull the masking tape off while the caulk is wetPeel the tape away slowly, straight off the wall, while the caulk is still soft. Leaving it until the caulk skins over will pull the finished line apart when you remove it.

SC-09Leave it to cureCheck the cure time on the caulk tube, most need at least 24 hours before painting. Keep the window open and avoid touching or knocking the line while it sets, especially on ceilings where dust can fall onto it.

SC-10Sand and paint to matchOnce fully cured, lightly sand any ridges or excess with fine sandpaper, taking care not to score the caulk itself. Paint over the line with a matching colour, feathering the edges into the surrounding wall and ceiling so the repair disappears.

1 sheet · Fine sandpaper

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1 each · Matching paint, small tester pot

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SC-11Clean up and put the room back togetherWipe the caulk gun nozzle and shaping tool clean before the residue sets hard, and seal any leftover caulk tube tightly. Fold up the dust sheet, checking for dropped bits of old plaster or filler, then move the furniture back once the paint is dry to the touch.

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