I fill a crack in the wall and it opens again. Why?

FillerFillerA fine paste or powder that fills holes, fine cracks and dents so a wall, ceiling or piece of wood is smooth before it is painted. is made to fill small gaps before painting, not to hold something that moves. If a crackCrackA split or gap in render, concrete, a wall, a coating or wood. Fine cracks are usually only on the surface; ones that grow, keep reopening or run diagonally need attention. opens and closes, even slightly, with heat and cold or with the building's small movements, the filler splits again along exactly the same line.

Cracks that keep coming back usually sit where two different materials meet: brick against a concrete column, a bricked-up opening against the old wall, a patch against the plasterRender (plaster)A layer of mortar (sand, cement or lime, and water) spread over brick or concrete to give a wall a straight, sound surface ready for paint or tiles. around it. At these points meshReinforcement meshA mesh or fabric, usually glass fibre or polyester, set into render, a coating or liquid waterproofing to hold it together and stop cracks from opening. is set into the plaster to spread the movement. In a living room where a wall opening was bricked up, mesh went over the joints with the old wall before the second coat of plaster. On a courtyard wall, mesh went over the patches and where they met the old plaster.

Other times the crack reopens because the plaster underneath has come loose, or because something is pushing it from inside. On one ceiling, a straight-line crack hid rusty steel, which swells and opens the material again. Filler would never have held there: the crack was opened up to the steel so it could be repaired properly.

Keep in mind: diagonal cracks running from the corners of doors and windows, or cracks that grow from year to year, need an engineer to look at them before any repair.