Why Important
Many Georgian, Victorian & Edwardian buildings have cornices. These carved stone projections at the top of the wall have a significant bearing on the appearance of the façade and are thus very important to maintain, or replicate, and reinstall when missing.
What to look for
Cornices are always made from ashlar stone and the joints between the stones are very small (2-3mm).
Over time, the lime mortar filling the horizontal joints between cornice stones becomes eroded, leaving a path for water to find its way into the wall-head, where it can start to affect the roof timbers, which are often built into the wall.
As more water enters the fine joints they become eroded and increase as does the problem of water ingress. Structural movement can also affect the way water runs off them.
Sometimes projecting cornice stones can deteriorate through mechanical damage, because there is a fault in the stone or due to delamination of the stones if wrongly bedded (see The Care and Conservation of Georgian House).
Gutters were also sometimes formed in the stone cornices.
When to look
Lean (partly eroded) joints will allow wind-driven water in and will erode the arises (edges of the stones). Junctions between stone gablets and slaterwork behind are often finished with a mortar skew. It is therefore important to inspect the joints regularly.
How to fix
Re-point open joints with a suitable lime mortar mix. In cases of structural movement, it might become advisable to cover the cornice in lead sheet.
Avoiding creating problems
Do not use Cement Mortar for repairs, as this mortar would be to inflexible over time and crack, adding to problems.