The Mortar Compatibility Principle
The fundamental principle of mortar specification for historic masonry is simple: the mortar must be softer than the masonry it bonds. This allows the mortar joint — the sacrificial element — to fail preferentially when thermal movement, freeze-thaw cycling or moisture causes stress in the wall. The brick or stone remains undamaged; the mortar is repointed when needed.
Victorian and earlier brickwork was made from hand-moulded clay fired at relatively low temperatures, producing a soft, slightly porous brick. Original lime mortar in these buildings is softer than the brick. Repointing in like-for-like lime mortar maintains this relationship correctly.
OPC cement mortar is significantly harder than Victorian soft brick — compressive strength 3–5 times greater. When cement-pointed joints eventually fail (typically 20–40 years after incorrect repointing), the failure path runs through the brick face rather than the mortar joint. The brick spalls, crumbles, or delaminations from the face — irreversibly.
The test: If you can scratch the surface of a mortar joint with a key and it powders easily, the mortar is lime-based and appropriate. If the key slides off without marking the mortar, or the mortar has a dense grey appearance, it is likely cement — and if the surrounding brick is soft Victorian stock, it is causing damage.
What This Means for Wiltshire Properties
Wiltshire has an unusually high proportion of historic masonry properties — Victorian and Edwardian brick in Chippenham, Bath stone in Bradford-on-Avon and Box, Cotswold limestone in Malmesbury, and a mix of oolitic stone in conservation area villages throughout the county. Every one of these property types requires lime mortar for all repointing work.
We carry NHL 2.5 and NHL 3.5 hydraulic lime as standard stock for all Wiltshire chimney and masonry repointing visits. We never use OPC cement on historic masonry. Where a previous contractor has used cement on a property we attend, we identify it clearly in our survey report and advise on the implications.
The NHL Specification
NHL stands for Natural Hydraulic Lime, specified by its hydraulic strength:
| Grade | Compressive Strength | Applications |
|---|---|---|
| NHL 2 | 2 N/mm² | Very soft stone, earth mortars, internal work |
| NHL 3.5 | 3.5 N/mm² | Most Victorian brick, Bath stone, soft sandstone |
| NHL 5 | 5 N/mm² | Harder brick, exposed positions, harder limestone |
For most Wiltshire Victorian properties — soft fired brick in Chippenham's terraces, or Bath stone in Bradford-on-Avon — NHL 3.5 is the appropriate specification. On very soft stone or in sheltered internal positions, NHL 2 or pure lime putty mortar may be more appropriate. We assess the masonry and specify the correct grade during the free survey.
Sand Specification
The aggregate (sand) used in the lime mortar also affects performance and appearance. Coarse, well-graded sharp sand produces a stronger mortar with good bond. The colour and texture of the aggregate affects the finished appearance — where we're matching existing lime mortar on a Corsham or Lacock property, we source aggregate that closely matches the original mix. For conservation area and listed building work, colour matching is part of our standard specification.
Modern Brick — When Flexible Mortar Is Correct
Post-1950s machine-made bricks — the type used on Hardenhuish, Pewsham and Sheldon — are fired at higher temperatures, are harder and less porous, and do not require lime mortar. For these properties, flexible polymer-modified mortar is the correct specification for chimney repointing and ridge work. It's more flexible than OPC cement, handles thermal movement better, and is appropriate for the harder brick it's used with. We assess brick type during the survey and specify the correct mortar before any work begins.