Flint is a fine-grained silica, very hard, impervious and occurring typically as small, irregular nodules. When freshly extracted from chalk, these are black with a white crust, but weathered flints – from fields, river-beds, beaches or gravel pits – can vary in colour from blue and grey to yellow, brown and orange.

Flint has been employed for a wide range of building types in southern and eastern England since Roman times. It has been used for thick, rubble-cored walls and, from the 19th century, facing skins on backings of brick or other stone. Walls can be approximately dated by their coursing, mortar composition and whether flints are used whole, fractured or knapped.

Problems are attributable to the impervious nature of flint, its unwieldy shape and small size. These characteristics can give rise to poor bonding that may be weakened further by rain penetration due to a lack of maintenance. Inappropriate repair methods and materials can also contribute to deterioration.

Problems range from the localised displacement of flints to loosely-bonded dressings and the detachment of facing flintwork from rubble cores or backings of stone or brick. Where there is doubt over the stability of a wall, a structural engineer should be consulted.

Flint and brick wall

No, this is a big misconception. Dense, cement-rich mortars shrink on drying, and fine capillary cracks develop that admit water and cause damp internally. This can accelerate decay and possibly lead to structural failure in a wall.

Lime-based mortars give the best results. Their porous nature allows walls to ‘breathe’, moisture to evaporate and fine cracks to ‘heal’. While a non-hydraulic lime may be appropriate for repointing flintwork, a mix with a faster and harder set is likely to be desirable for rebedding flints and copings, or filling cracks. This can be achieved by adding pozzolans (materials such as dust from soft and low-fired bricks) or using a natural hydraulic lime (strong hydraulic limes, however, should be avoided).