The design of a lime mortar is vitally important when working on old properties. Mortars serve the essential function of binding masonry together in the construction of walls, but in traditional buildings they must do much more than this. Historic mortars were designed not only to bind but also to protect the masonry they hold.
For a mortar to function properly, it must be softer than the masonry it supports. This allows the mortar — rather than the bricks or stone — to absorb and release moisture. In this way, the mortar acts sacrificially, taking on the effects of weather and ageing while keeping the masonry safe. This process is what is meant when we describe old walls as “breathing.” A suitable mortar must therefore be both soft enough to protect the masonry and porous enough to absorb and shed water readily.
This is why cement mortars are often unsuitable: they are harder than the bricks they bind and do not provide the same breathable qualities.
Designing the Right Mortar
Good mortar design begins with understanding the characteristics of the masonry. For example, handmade bricks are softer and more porous than those produced in factories from the mid-19th century onwards. Granite is harder than limestone, while chalk is softer still. We design lime-based mortars with well-graded aggregates, ensuring the right level of softness and porosity to complement the masonry. This not only provides the necessary protection but also enhances the appearance of historic walls, where mortar is an important part of the building’s character.
Climate and exposure must also be considered. An exposed chimney may require a harder lime mortar than the sheltered base of a wall. Each situation is assessed to ensure the mortar is appropriate for both the building material and its environment.
Traditional vs Modern Methods
Older buildings differ significantly from modern ones. They were constructed with solid walls — often 50–60cm thick — without cavities, damp proof courses, or impermeable barriers. Walls were typically built with two facing layers (internal and external) and a core in between. In the early to mid-20th century, sand and cement mortars became fashionable, and many repairs were carried out with these harder, less sympathetic materials. Unfortunately, this trapped moisture inside walls and forced it to pass through the masonry rather than the mortar, accelerating decay of the historic fabric.
Our repair techniques always return to traditional lime mortars with well-graded aggregates. These mortars restore the wall’s natural ability to breathe, protecting the masonry and ensuring the building remains structurally sound.
Matching Historic Mortars
Mortars for repair must also match the existing ones, both in function and in appearance. We pay particular attention to colour, texture, and finishing. Different limes, local aggregates, and pozzolans can affect the shade and texture of the mortar.
Samples of historic mortars often reveal the addition of clays, muds, chalk, or coal dust, which we replicate where necessary to achieve an accurate match.
Finishing is equally important. As the mortar dries, we often use churn brushes to bring out the aggregates, soften the surface, and match the character of the original work. This careful approach ensures new mortars blend seamlessly with the old, preserving both the look and performance of the building.
