Cracks in a freshly plastered wall are the last thing anyone wants to see. It can be very frustrating trying to assign fault for this. For both the customer and the plasterer, who is often the first port of call when this arises.
After having been a plasterer for 10 years I have only seen this once. Hairline cracks appeared all around the room. Before plastering we noticed the walls had hairline cracks. Scrim tape was used over old cracks and the walls were heavily sealed and stabilised with a standard water/PVA solution. The walls did not stand out as having high suction rates either.
The house is not old enough to have wood and lath. The walls seemed solid, not blown or crumbling within the backing coat.
After having plastered the walls were allowed to dry out slowly. In fact it was winter time and there was no radiator in the room. When fully dry the walls were decorated and there was no problem. Slowly over the next 3 months hairline cracks began to appear. The exact same process was carried out in another room and the hallway/landing of the same property, where the walls are still perfect.
Knowing that the plastering was done perfectly, walls were sealed, low suction and slow drying times all adhered to I can only assume this is due to very slight movement in the building.
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 windows 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.