Flaunching is the name given to the bed of mortar which holds chimney pots in place on top of the chimney stack. Flaunching also helps prevent rainwater permeating the top of the brick or stone of the stack.
In older homes, lime mortar was used for flaunching, and this will deteriorate over the years. Even when cement mortar has been used this may have cracked, allowing water to penetrate and weaken the bonding of the chimney pot.
It is therefore important to check the condition of flaunching. If it is unsound the chimney pots may be insecure and in extreme cases could topple in high winds resulting in damage to property or injuries to passers-by.
The condition of the flaunching usually can’t be readily seen from ground level and a Surveyor will often recommend that it is checked when repairs are being carried out to roofs and gutters. However it is easily possible to tell if a building was constructed with lime mortar rather than modern cement mortar, in which case it is more than likely that the chimney stacks will need detailed inspection.
Minor cracks in cement mortar may be repaired by filling with mastic, but if more extensive repair is required this is likely to entail erection of scaffolding to give safe access.