Traditional lime render on old buildings should never be patched using cement this will prevent moisture in the wall from evaporating, forcing it out to the edges of the new patch, eventually causing render to fail around the edges of the repair. However, even in experienced hands, ‘invisible’ repairs are hard to achieve, so you may need to allow for redecoration of the whole wall. This will expose bare patches of wall which will need time to dry out before new render is applied.Īlthough rendering a large area is a skilled job, patching is less difficult. In most cases, only localised render repairs will be required, and the first job is usually to hack off any hollow, loose and/or cracked render. The strength of each coat (the ratio of sand to cement powder or lime, if it’s used) is important and will vary depending on the type of brick or stone it’s being applied to. Today it’s standard practice to apply two coats of render. Render used to be applied in three layers that got progressively thinner - these layers were known as the render, float and set coats. But all types of house rendering have one thing in common - they wrap the building in a protective skin. Interestingly, the smooth stucco finishes adorning many grand Georgian townhouses were originally applied as a means of disguising cheap brickwork and to emulate expensive cut ashlar stonework. Render comes in several different forms, including: roughcast, pebbledash or the smooth, plain variety. Either way, once damp becomes trapped behind the surface it can penetrate through solid walls, setting up the conditions for rampant timber decay within the building.

In some cases, missing lumps of render are also simply down to physical damage.

Cracking due to shrinkage can also develop where the original render mix was too strong. This is why small hairline cracks should be sealed when they first develop, otherwise water penetrating through can freeze and expand, blowing the render surface loose - resulting in even larger cracks.

The most common cause of render cracking and bulging, however, tends to be frost getting to moisture trapped between the render and the wall. In fact, cement render should never be applied to old solid wall buildings. Traditional lime render is more flexible and forgiving than modern cement. So the cause is sometimes linked to structural movement. If the wall behind the render surface moves or cracks, so will the render.

Hairline cracks in your render? This tends to happen over time.

What does render and replace do