Repairing External Cladding: Render

ABOVE: Water ingress can cause render to lose adhesion between individual coats, or even from the wall entirely in some cases.

There are many varieties of exterior render, with the most common make-up for traditional stucco being hydraulic lime, sand and hair. More modern renders are made from Portland cement, sand and water, although lime is often added to increase the permeability and workability of it. Sometimes, acrylics and glass fibres are also added to improve its durability.

Causes of render problems

Buildings clad with render are at risk of similar problems to brickwork, including water ingress, which will lead to a loss of adhesion between individual coats of render or between the render and the substrate beneath. This will lead to an obvious bulging and, ultimately, for the render to come away from the wall.

During periods of frost, trapped water can freeze and expand, causing damage.

One of the most common causes of damage is the failure of rainwater goods, leading to salt efflorescence, staining and the growth of moss and algae. Efflorescence can cause render to crumble if the salts crystalise. Another cause is inappropriate repairs. Using modern materials to repair a traditional finish can speed up decay rather than prevent it.

Movement can result in cracks, although minor hairline cracks commonly appear whilst render is drying out.

ABOVE: Efflorescence can cause render to crumble. The powdery residue should regularly be brushed away and the source of the problem identified.

How to fix it

As soon as any cracks appear they should be fixed, as even the smallest can allow water in.

Care should be taken that a modern mix will be compatible with a more traditional one. With modern render mixes, small cracks can be filled with acrylic or siliconeacrylic sealants. If a larger crack appears, this can be temporarily filled with silicone caulk, which can later be peeled away for a more permanent solution.

If the render has been subject to a number of patch repairs, a new coat of exterior paint will ensure it looks fresh.

In the case of more seriously damaged render, all damaged material should be chipped away (a cold chisel and short-handled sledge hammer are ideal for the job). If the steel mesh reinforcement beneath is also damaged then this should be repaired by cutting a new piece of mesh and attaching it to the substrate with galvanised nails. When you are ready to apply new render, mix more than you need and use a trowel to apply it to the mesh, leaving it slightly thicker than the surrounding render. Using the surrounding wall as a guide, smooth the patch by sliding a straight-edged board back and forth, drawing the straight edge across the wet render mix.

Maintaining render

As with brickwork, the most common reason for deterioration is simply a case of neglect. Regular checks should be carried out on rainwater goods and care should be taken to keep on top of moss and vegetation growth on the face of the building.

Find out how to identify and repair deteriorating brickwork...

Further reading:

• Cladding Options: Buying Render
• A Guide to External Appearances
• Plastering: A Cost Guide