Put simply, it cracks.
Sand and cement, with or without lime, is put on 10 – 15mm thick and then painted. Sand and cement is not at all flexible, it will not allow for any movement.
It may sound strange, but all buildings move over time and seasons - it is these slight movements, or shrinkage, that start the cracking process.
Once sand and cement starts to crack, water will ingress and the coating will start to “blow” – to separate from the wall, often coming away in large sections.
Sand and cement has not always been a bad choice, in the past this was often the only option and became a historically traditional method.
Over the last ten years or so, the range of renders available has improved dramatically – there are many affordable alternatives. We would always recommend choosing a product that is guaranteed to last, not one that is guaranteed to crack.