The good news is that, generally, concrete, stone, brick and masonry walls and concrete or screeded floors that have cracks less than 1 mm wide (the thickness of a credit card) are common and usually do not warrant any corrective action. Most of these small tight cracks are caused by normal shrinkage as the moisture in the walls and floors evaporates over time or settlement of the structure which usually occurs within the first few years after construction.
Be warned, however, that changes in condition around the structure may also cause settlement many years later! Examples are planting a new garden or tree or removing a garden or tree that is against or close to the house.
Moreover, if a horizontal, vertical, or diagonal masonry crack is filled with hard masonry patching compound, any substantial future movement is likely to show up as a new crack in the patched area or nearby. Therefore, always use a non-shrinking grout to prevent stressing yourself!
Cracks that continue to move are a reason for concern! Continued movement in cracks should be evaluated as there may be a need for corrective action. Therefore, if you notice a crack has re-cracked or the crack has opened or gotten larger it should be monitored! However, first, make sure there is no shrinkage of the filler product. All cracks that are 5 mm and greater should be carefully monitored to ensure there is no continued movement.
Most homes develop cracks. Old and new homes can develop cracks as well depending on climatic and physical factors.
However, it is worth getting to the bottom of what is causing the cracking. A crack is actually the visible symptom of a possible problem and not the problem in itself.
What causes cracks?
In new homes, settlement may cause minor cracks. Normal foundation settlement occurs when the underlying soil compacts as a result of construction on previously undisturbed soil, changes in soil conditions and moisture. Typically small, hairline-sized cracking may be the result of a minor settlement, expansion and contraction, or changes in a season or cycle.
Usually, these defects, though often unsightly, are not structurally significant.
Thermal movements between different materials
Different materials such as timber, bricks, steel, concrete etc. have different coefficients of linear expansion. In other words, the differing materials contract and expand differently. This causes stresses in structures where they are combined and may cause cracks to form. The best way to combat these defects in plasterwork is to provide a joint between the different materials.
In older homes, changes in climatic conditions can affect the structure. Houses move with the climate. Heat and moisture will make them expand, and cold and dryness will make them contract slightly. This movement is normal and in most cases will not cause cracking. Unusually hot or cold spells can result in increased expansion or contraction of the structure which may cause cracks.
The rise or fall of the water table in very dry or very wet conditions can also affect the house’s foundations.