Link to our main lime page for more information on

• Lime plastering and rendering
• Environmental and health benefits of lime
• Mechanical properties of lime
• Limewashes and eco-paints

Throughout history, lime mortars have held together everything from Egyptian pyramids and medieval castles and cathedrals, to rural farms and town houses. Lime mortar mixing was an important and technically demanding trade but has gradually died out since lime was overtaken in popularity by sand and cement mortars.

In recent years there has been a revival in the use of lime mortars in the sympathetic conservation and renovation of traditional buildings but also new builds and green-eco homes. In part this is for the mechanical properties of lime but also because of its environmentally friendly credentials.

It is commonly supposed that purpose of lime mortar is to stick masonry units such as brick and stone together. Although this is the case with sand and cement mortars, lime mortars do not set super-hard. Instead, the purpose of lime mortar first and foremost is to provide a cushion to spread individual masonry loads evenly, particularly when dealing with softer stones, brick and in Devon, cob blocks. As lime does not ‘stick’ stonework together, this means that traditional stonework produced with a lime mortar will be held together through craftsmanship rather than cement.

The golden rule governing the relationship between mortar and the materials that it binds is that the mortar should be softer or equal to those materials.(Find out more about the mechanical properties of lime). A soft lime mortar will take the brunt of the weather, form a porous surface for water to escape the walls and draw harmful salts away from masonry.

When set, sand and cement mortars are non-porous and harder than many rocks. As water cannot be released through mortar joints, it is often forced out through the building material. As well as the possibility of forcing damp back inside buildings, common affects of this are salt stains and the ‘popping off’ of exterior brick and stonework. This leaves distinctive pock-marks and can also be identified by sand and cement pointing standing proud of stonework.

Many older stone buildings, or those incorporating cob or timber, shift and move subtly over time or with the seasons. As sand and cement mortars set hard and are rigid, this natural movement can cause cracks, especially when it has been used to repair traditional materials. Soft, durable and breathable lime mortars however, will move with other materials, and have self-healing properties.

The durability, porosity and breathability of lime mortars promote healthy buildings (more on the health benefits of lime) and can also be treated as a ‘sacrificial’ material, cheaper and easier to replace than the materials they bind. The inherent breathability of lime does mean that lime mortars degrade over time-measurable in decades if not centuries- which is why it is often necessary to repoint stonework within older buildings and walls.

It is important to re-point walls where the original lime has degraded. Not just a cosmetic effect, stone and brickwork can destabilise if left too long without pointing.

Lime mortars consist of sand mixed either with Non Hydraulic lime putty or bagged Natural Hydraulic lime. Although Non-Hydraulic lime is softer and more breathable, Natural Hydraulic Limes (NHLs) are suitable for re-pointing stonework in particular where limework is close to ground moisture (more about NHLs).

Sympathetic conservation work often demands like for like replacement of building materials. This means that the colour and texture of the aggregate must be in keeping with its surroundings. This can mean sourcing sand from local quarries and/or the scientific analysis of old mortar to find an appropriate match.

The re-pointing process has three main stages. This first, ‘raking out’, involves cleaning out old mortar, plant life etc. between the masonry units to be re-pointed. This can be quite laborious in particular if sand and cement mortar has been used in the past. The spaces created between stones should be at least twice as deep as they are wide, and left with a square (cornered) edge so the lime has a key and doesn’t fall out. The second stage is filling in, pushing the lime mortar between the stones until it is stands slightly proud of the stonework. The mortar is then left to cure until leather hard, when it is brushed off to the required depth and finish.

Link to our main lime page for more information on

• Lime plastering and rendering
• Environmental and health benefits of lime
• Mechanical properties of lime
• Limewashes and eco-paints