Why Plaster Cracks: Drying, Thickness and Mistakes

Plaster cracks usually result from fast drying, overly thick coats, poor preparation, incompatible materials or movement in the substrate. The type of crack points to its cause.

Seeing fine lines appear on a wall a few weeks or months after renovation is very frustrating. Sometimes it is just a cosmetic defect; sometimes it signals a more serious process. Simply filling a crack and painting over it often does not work: if the cause remains, the crack reappears in the same place. So the first step is to understand what the crack is telling you.

Look at the type of crack

The appearance of cracks says a lot about their cause:

• a network of fine cracks (like a spider's web): usually from the surface layer drying too fast or shrinking;
• a straight crack along the junction of two materials: where brickwork meets a concrete column or beam, for example;
• diagonal cracks from the corners of door and window openings: in zones where stress concentrates;
• a horizontal crack where ceiling meets wall: from different movement of structural elements;
• a crack that widens, lengthens or appears on both sides of a wall: may indicate a structural problem.

In the last case a specialist's assessment is essential before any cosmetic repair. If a crack's width changes over time, recording it (marking the ends with a pencil and writing the date, for example) helps diagnosis.

Fast drying

Plaster mixes harden with the help of water. When the surface dries too quickly, the mix does not have time to cure fully, the surface layer shrinks and a network of cracks forms. Conditions that cause this include:

• working in hot weather or near heaters;
• strong draughts or wind through open windows;
• direct sunlight on the wall;
• a very dry, unprimed substrate drawing water out of the mix rapidly.

To prevent it, keep a moderate temperature, protect walls from draughts and direct sun, and dampen cement-based plaster in the early period as the method requires. Turning on heaters to speed up drying is a typical mistake.

Coats that are too thick

Every mix has a maximum thickness for a single coat. When a wall is very uneven it is tempting to exceed it, but a thick coat dries at different rates inside and out, shrinkage stress increases and the mix may slump under its own weight.

The correct approach is to fill large hollows separately beforehand, apply plaster in several coats, each after the previous one has set, and use reinforcing mesh in thick coats. For very large deviations, plasterboard lining can be more reliable.

Preparation and material mistakes

Poor adhesion to the substrate also leads to cracks and detachment. Typical mistakes:

• plastering over a dusty, dirty or flaking old surface;
• skipping primer or using one unsuited to the substrate;
• adding more water to the mix than instructed: it becomes easier to work but strength drops and shrinkage increases;
• re-tempering mix that has started to set by adding water;
• applying cement-based mix over gypsum plaster;
• combining materials from different manufacturers without checking compatibility.

Junctions of different materials and movement

Brick, aerated concrete blocks, cast concrete and metal react differently to heat and moisture and expand and contract by different amounts. Plaster over the line where they meet feels this movement and cracks.

To prevent it, reinforcing mesh extending onto both sides of the junction is embedded in the plaster. In newly built buildings it must be remembered that structures continue to settle and dry during the first years. Some cracks may appear later in such buildings, so final decorative finishes are chosen with caution (for example, materials with some flexibility or mesh under wallpaper).

How to repair cracks

Once the cause has been eliminated or the crack has been confirmed as stable, the repair steps are:

• widen the crack slightly and remove weak material;
• remove dust and prime the surface;
• fill the crack with a suitable repair compound or flexible filler;
• embed reinforcing tape or mesh and cover with skim coat;
• smooth, prime and repaint the whole area so no colour difference shows.

Where there is a network of many cracks, applying mesh over the entire surface and a new skim coat is more reliable than local patching.

In its renovation work Zaferoğlu İnşaat controls plastering methods, drying conditions and material compatibility. A site visit can be arranged to identify the cause of cracks in your walls and choose a repair approach.