The use and abuse of concrete and mortar

In a masonry wall, concrete or mortar is usually essential to bind the stonework together. In contrast, a dry stone wall is designed to settle into a durable structure without any cement. The fact that many miles of completely unmortared wall have stood intact for centuries, often with no maintenance or repair, proves that with proper construction and materials no binding substance is necessary. However, old and new walls may incorporate concrete or mortar, and this is not always a mistake as mortar has its uses. These include:

1. To strengthen the coping on an otherwise dry wall. Buck-and-doe coping, and upright or flat coping using small, light stones are vulnerable to being dislodged by animals, vandals or traffic vibration. Coping adjacent to stiles is also likely to get knocked.
2. To strengthen the entire wall against vibration from heavy traffic. Roadside walling is often mortared throughout, and given a solid concrete topping or a heavy layer of mortar below the topstones.
3. To strengthen walls made of fissile or very regular stones which would otherwise tend to slip apart on settling.
4. To make a sturdy coping easier and faster to place.
5. To provide a solid foundation where there is a lack of suitably large footing stones.
6. To strengthen a wall head made of small stones, or a stile which uses stone stoops or wooden posts.
7. To anchor fencing standards into the wall.
8. To create a horse jump or other special feature.

However, in most cases the disadvantages of concrete and mortar in otherwise dry walls far outweigh their advantages. Drawbacks include:

1. Materials and transport costs are considerably higher.
2. New walls settle by about 3″ (75mm) in their first few years. As this happens, dry walls increase in strength and cohesiveness, since every stone shifts into the tightest relationship with its neighbours. Dry stone walls with mortared topstones tend to settle away from the rigid copings, and without the weight of the topstones to anchor them the stones below are easily dislodged. The result is often as shown.
   
   Vibration from traffic may cause more than normal settlement, so the likelihood of gaps forming beneath a mortared cope is greatest on roadside walls.
   If mortar is used in the body of the wall, the coping may settle at the same time as the wall, but the wall itself may develop humps or bulges due to reduced flexibility.
3. Water expands by one twelfth when it freezes. Dry walls are built so that water drains freely through and into the subsoil. Concrete or mortar can trap rainwater which may break and dislodge stones when it freezes.
4. Gaps are more difficult and dangerous to repair if a mortared coping is still in place across the gap. You may need to use a sledgehammer to knock the coping down.
5. Vehicles which hit partly mortared walls are likely to suffer more damage than those which run into dry walls. Damage to the wall is also likely to be more extensive, as the wall tends to be dragged out in a unit.

Cement-based mortars are rigid and brittle and especially prone to poor settlement. In some cases cement mortar is stronger than the stones themselves, so that the stones deteriorate before the mortar. Lime and sand mortar should be used rather than cement and sand, to retain some flexibility. This is also preferable botanically. However, where a wall head or other part of a wall is to be mortared throughout, a small proportion of cement can be added to the mix for extra durability.