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Why does render crack?

Typically, render is used to provide the building fabric with protection against the elements while also providing an aesthetically pleasing finish. Often, a render is applied to cover up the use of cheaper building materials such as blockwork rather than brick and stonework.

Unfortunately, a common stereotype of render is that it is liable to experience cracking. This is a huge concern for many of our customers, who want to invest in a new render facade but don’t want to find that a few years (or even months) down the line their investment isn’t holding true to its promise.

Cracks are the effect of a number of underlying problems, and if left to their own devices they can escalate a situation from bad to worse by drawing water through the building. There are a number of reasons that render may crack, and in this blog post we’re going to be looking into these reasons.


First thing’s first, is your render really old? Render does in fact have a lifespan, especially sand and cement render. If you are living in a fairly old property that is rendered in sand and cement, cracking is going to be inevitable due to the effect of weathering.

When driving rain, snow, frost and more are constantly attacking the exterior of your building, it’s likely that you will experience some kind of negative impact. This is particularly true for sand and cement render, which is incredibly solid, unforgiving and prone to absorbing water. So, consider how old the property is and when it is likely that it was rendered – the age of the render may be the reason for the wear and tear.

Incorrect materials for the substrate

We’ve previously written about a case study where sand and cement render was applied to high performance blockwork, and the resulting finish was a cracked mess within weeks. The reason for the cracked render is that high performance blockwork (this includes Ziegel blocks, AAC, Thermalite etc.) tends to be very soft and malleable, while sand and cement is incredibly solid.

The phrase ‘opposites attract’ just isn’t true in the case of renders and substrates; when choosing a render for your property, it’s imperative that it compliments the substrate so that they will work in tandem. This could be the reason your render is cracking. An installer should be able to advise you on the best type of render for the building fabric.  

Poor quality of workmanship – poor sealing at junctures, incorrect movement joints

A good quality installation from a knowledgeable installer can make all the difference to the durability of your installation. A trusted installer will ensure that correct reinforcement is applied around the weaker areas of a structure, especially around movement joints and reveals.

The render needs to be able to replicate and compensate for movements within the underlying substrate, otherwise it will crack. If this has not been taken into consideration then this could be the cause of the cracks.  

The render dried out too fast

Most common with dry-mix renders, if the water evaporates from the material too quickly it doesn’t leave enough time for the render to bond properly and the surface colour can appear patchy and discoloured.

A render that has dried out too fast is a dehydrated, weak surface and cracking is therefore inevitable. To avoid this, the render needs to be applied in moderate temperatures and if possible away from the direct glare of the sun, and for dry-mix renders if the mixture appears to be drying too fast simply sprinkle it with water as needed.

Fibreglass mesh wasn’t used

Best practice for rendering is to embed fibreglass mesh within the basecoat layer. The mesh is applied in strips that are each overlapped, reinforcing the entire surface of the basecoat and increasing its tensile strength. The result of this is that any movements within the building fabric will be absorbed by the mesh.

If Fibreglass Mesh isn’t embedded within the basecoat or even in the first pass of render, unfortunately this could be a contributing factor towards the cracking.

Structural problems

Sometimes it’s not actually the fault of the render, it’s the effect of underlying structural problems that are making themselves known by damaging the render finish. The structural problems can be numerous and difficult to pin down, whether it’s differential movement between an extension and the main house or due to water and rust.

Rising damp

Rising damp is the fear of all property owners as it’s often difficult to catch and identify. It’s relatively rare, but rising damp is essentially when water travels from the ground up through the walls by capillary action, causing bulging in the walls and floor boards and general structural damage. The structural damage that rising damp can cause is what encourages cracks to form in the external render. You will most likely notice signs of rising damp not just in your render but internally as well, so if this is the case then it needs to be identified and rectified immediately.

Leave a comment down below if you’ve experienced cracking in your render and how you fixed the problem!


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