Cracks in cement based rendered finishes applied to outside walls are fairly common and often overlooked by property owners as being unimportant, especially as they do not repeat internally and so aren’t seen as serious or linked to issues of subsidence.
Cement / sand render mixes are prone to crack especially on warm south or south west facing walls. Such can be caused by shrinkage; either in the underlying masonry or in the render itself. It can also be caused by thermal movement of the underlying masonry. Such cracking will also typically manifest at relatively weak points in a wall and will most notably be around window and door openings as they expand and contract within the wall openings.
The poor initial preparation or application of the render can also cause cracking to develop; it may also have been allowed to dry out too rapidly between coats, or the topcoat of render may not have bonded with the undercoat. The latter can occur if the undercoat had cured before the topcoat was applied or had not been properly ‘keyed’. Cracking can also result if the mix was stronger than the substrate to which it was applied.
The danger is that if left unrepaired, the cracks can allow rainwater to penetrate and become trapped behind the render. When water is trapped in this way it may migrate to the internal face of the wall and manifest as damp.
In addition, when outside temperatures drop the trapped water will freeze, expand, and this will cause further damage. The render will eventually lose its key and fall from the wall.
The cracks should be repaired by cutting back the render on both sides, finishing with an undercut edge. If the render is blown then it should be cut back and removed back to sound material. Then, any cracked underlying masonry should be cut out and replaced prior to re-rendering.
If you are concerned about the state of your render then contact Woodward Chartered Surveyors to arrange for an inspection. As well as building surveys for home buyers, we also offer Structural investigations for home owners wanting to know more about a particular housing defect.