When a Building Surveyor is inspecting a crack in a wall he is basing his opinion on what he can visually see at the time of inspection. Cracks can open and close over a season and a minor crack that is now visible to the building surveyor may be in existence since the property was built or could have recently appeared and will be ongoing. To be certain about the extent of any crack and more importantly whether it is likely to become worse over time the crack itself may need to be opened up to determine exactly what is happening. In cases were there are drains in the immediate area of the foundation, a drain test maybe necessary.
To determine the reason behind certain cracks is not always straight forward as the evidence may have been removed, such cases include trees that may have been removed before the property was built resulting in heaving under the foundations when the soil expands. Also heavy furniture or exercise machines that were previously stored or used in the property can result in vibrations leading to cracks in walls. Cracks in masonry walls are almost always structural cracks, however in most cases these cracks will not affect the stability of the structure and only minor repairs and normally monitoring is required.
The main concern with cracks in walls is to determine whether or not the crack/s are likely to be ongoing.
All large cracks start off as minor hairline cracks and deteriorated over time, hence the reason for on-going monitoring of the structural walls. A small leak in a drain close to the foundations can occur at any time. This can pull away soil under the foundation result in settlement in the foundation and cracking in walls.
If you suspect any new cracks or changes to existing cracks in walls as you reside in your property. You should take pictures detailing the cracks and also record the previous / recent weather conditions and building works that may have being carried out in the immediate area. This will assist your building surveyor in determining what may be happening without the need to open up the structure.
Finally, if you wish to be certain that all cracks including hairline cracks are not likely to deteriorate, opening up of certain parts of the structure and foundations maybe essential. As previously mentioned a drain test and CCTV inspection of drains in close proximity to the foundations maybe required before you purchase of the property.