Severe fractures up to 25 mm in width may indicate structural damage and should be evaluated and fixed by a professional. Very severe: any fracture wider than 25mm indicates substantial structural damage and would need extensive repair work, which may involve underpinning and rebuilding. Wider than 50mm indicates a very poor foundation and must be repaired or replaced. The width of a crack is called the spall line and it's important to understand that it can vary greatly depending on what type of structure we're talking about. In a building with concrete slabs, such as an apartment block, the spall line will usually be between 20-50 mm wide. However, in a house with a solid stone base, such as a cob or adobe house, the spall line could be much wider - up to 100 mm or more. That's why it's important to look at all aspects of your home, not just the ground floor, when checking for structural problems. There might be issues with the first floor room layout, for example, that would require major renovation work if the floor was damaged. In conclusion, large cracks in bedrock foundations are often due to natural causes, but small surface cracks on concrete floors could be signs of potential trouble below. It's best to call a professional before anything serious happens so that you can find a fix soon after.