Plaster cracks: causes and how to prevent them

Shrinkage and no curing

Cement plaster shrinks as it dries. If it dries in the sun or wind in the first day or two, the surface shrinks faster than the wall below and cracks. Curing keeps the plaster damp so that cement can gain strength slowly. Most practice guides and site specifications give a minimum of 7 days of wet curing. In hot dry weather, sprinkle two or three times a day, and hang wet gunny cloth on exposed walls.

Start curing once the plaster has set enough not to wash off, usually the next day. Do not skip it because the wall looks dry.

Too rich a mix and too thick a coat

Many people think that more cement means stronger plaster. For plaster, it means more shrinkage. A mix of around 1:5 or 1:6 cement to sand is usual for internal walls, and 1:4 is used for outer faces or specified areas. See our cement sand ratio guide for the full list.

Plaster is applied in coats, and a coat that is too thick sags and cracks as it dries. A common guide for brick walls is about 12 mm for the first coat and about 8 mm or less for the finishing coat, with at least a day or two between coats. Where the wall is badly uneven, build it up in layers instead of one deep coat. Rake the first coat lightly so the next one bonds.

Joints between different materials

Brick, concrete and AAC expand and shrink by different amounts. Where a column or beam meets a brick wall, plaster over the joint will crack along that line almost every time. Fix this before plastering:

• Fix a strip of chicken mesh or fibre mesh about 150 mm wide, 75 mm on each side of the joint.
• Fill chases for pipes and conduits flush with mortar first, and put mesh over them.
• Cut a neat shallow groove, not deeper than about 6 mm, along the junction while the plaster is still green, to guide the crack into a line.
• Roughen smooth concrete surfaces with a wire brush or apply a bonding coat so the plaster grips.

Fresh brickwork and poor sand

Plastering on fresh masonry traps movement. Let new brickwork or blocks settle for at least 1 to 2 weeks if possible, and wet brick walls before plastering so that dry bricks do not suck water out of the plaster. For AAC, follow the plaster maker's advice, as AAC needs a different preparation.

Sand with too much silt or clay makes weak plaster that cracks and falls off. Do a simple jar test: put sand and water in a clear glass, shake, and let it settle. If the clay or silt layer on top is thick, wash the sand or buy a cleaner lot. Very fine sand also needs more cement to bond, so it shrinks more.

How to repair plaster cracks

• Hairline cracks: clean off dust, wet the area, and fill with a thin paste of cement and fine sand or a ready crack filler. Cure it. Paint over with a flexible putty.
• Wider cracks, over about 2 to 3 mm: widen into a V groove, remove loose plaster, fill with a polymer modified repair mortar, and cure.
• Long straight cracks along beams and columns: cut out a strip, fix a mesh across the joint, then replaster in two coats.
• If cracks are growing, wider than a few millimetres, or run through the wall, check the cause before repair. It may be settlement or load, so call an engineer.