A common problem in both old and new houses is cracking in the exterior walls.
Cracks can result from:
Settlement is caused by the evaporation of moisture from the bricks; this occurs most often in new buildings. Settlement can be seasonal; in the winter, moisture in the ground increases and in the summer the ground dries out; cracks can open and close during that time.
Thermal movement is usually seasonal; in the summer the walls expand, and in the winter the walls cool and contract.
Subsidence happens when the ground loses its load-bearing capacity. This usually happens if the soil is rich in clay and is worse in houses with shallow foundations. The problem can be exacerbated by plants and trees such as eucalyptus, oak, Leylandii, poplar and willow. Be warned that the complete removal of trees can cause cause 'heave'; moisture is no longer drawn from the ground and the sub-soil swells. It is safer to remove trees gradually, ideally over 10 years; prune them first, then remove one or two trees, prune again and then complete the removal.
The other cause of subsidence is where water washes away a sandy or gravelly soil, for example when a drain has cracked.
Cracks can be very minor such as one above a lintel or at the join between a bay and the main wall. More worrying cracks are in main walls, and extending for more than one metre or so. The size and shape of the crack is also important. Most experts agree that a crack of less than 5mm is usually unimportant, if there is no sign of bulging or sideways shifting. If the crack is dirty and has cobwebs in and over it, then it is likely to be long-standing and stable.
Cracks in or above lintels or below cills are usually caused by minor settlement; a 'tell-tale' can be used to determine if the crack is stable. The lintel can be taken out, the damaged area rebuilt and the lintel re-instated.
Cracks where a bay meets the main wall are common; the smaller bay has settled less than the rest of the house. If the crack is less than 5mm and dirty, then no attention is required. Lime mortar can be used to fill the crack if you wish.
Long cracks in walls, away from windows and doors, particularly if the brick or stone is cracked, rather than just the pointing, are of more concern. If the wall is bulging, wall-plates or ties can be used to restrain the wall. Vertical movement is usually caused by subsidence or heave. Both can be caused by major changes in the moisture level in the ground. These result from climate change or from the removal or growth of trees.
If you are in any doubt on these issues, please seek the advice of a surveyor or specialist.