When it comes to maintaining and restoring historic brick buildings, one material plays a crucial yet often overlooked role—lime mortar. Unlike modern cement-based mortars, lime mortar has been used for centuries in traditional construction and is uniquely suited for the conservation of heritage buildings. In London and other parts of the UK, many older homes and structures were built using lime mortar, making it essential for any repointing or restoration work to respect this traditional technique.
In this post, we explore why lime mortar is so important, its unique properties, and why it should be the top choice when working with historic brickwork.
What Is Lime Mortar?
Lime mortar is a type of mortar composed of lime (calcium hydroxide), sand, and water. Historically, lime was the primary binder in masonry work long before Portland cement became common in the 19th and 20th centuries. Lime mortar sets more slowly than cement, but this slower curing process allows it to flex, breathe, and bond in ways that modern mortars cannot replicate.
There are two main types of lime used in mortars:
- Hydraulic lime, which sets through a reaction with water.
- Non-hydraulic lime, which sets by reacting with carbon dioxide in the air—a process called carbonation.
Both types offer benefits, but the choice depends on the building’s age, condition, and exposure to the elements.
Why Lime Mortar Matters in Historic Buildings
1. Breathability and Moisture Regulation
One of the most valuable properties of lime mortar is its breathability. Unlike cement mortars, lime mortar allows moisture to pass through the walls and evaporate naturally. This is vital in older buildings that were designed to “breathe,” without damp-proof courses or modern insulation.
Cement traps moisture within the walls, which can cause bricks to deteriorate over time. Lime, on the other hand, allows buildings to stay dry and healthy, significantly extending their lifespan.
2. Flexibility and Movement
Historic buildings settle, shift, and expand with time and weather. Lime mortar is naturally flexible and can accommodate this movement without cracking. Cement mortar, being rigid, tends to crack under these stresses, leading to structural weaknesses and unsightly damage.
Using lime mortar helps preserve the integrity of the structure while adapting to natural movement in the building fabric.
3. Compatibility with Old Bricks
Traditional London stock bricks and other handmade bricks used in historic buildings are typically softer than modern bricks. Lime mortar is a perfect match in terms of softness and porosity. Cement-based mortars, on the other hand, are much harder than these bricks and can lead to irreversible damage.
When repointing with cement, the brick often erodes faster than the mortar, which defeats the purpose of protecting the wall. Lime mortar wears evenly with the brick, making it the appropriate and sympathetic choice for conservation work.
4. Environmental Benefits
Lime mortar is also a more environmentally friendly choice. Its production uses less energy than Portland cement and absorbs carbon dioxide during the curing process (carbonation), partially offsetting the emissions created during its manufacture.
This makes lime mortar a sustainable option for homeowners and conservationists looking to reduce their carbon footprint.
Common Applications of Lime Mortar in Restoration
- Repointing: Carefully removing old, failing mortar and replacing it with new lime mortar that matches in colour, texture, and performance.
- Brickwork Repairs: When replacing damaged bricks in historic walls, lime mortar is used to retain the building’s original characteristics.
- Rendering and Plastering: Lime-based plasters and renders are used on period homes for breathable and flexible wall finishes.
In all these cases, the goal is to preserve the character and function of the building without introducing incompatible materials.
Risks of Using the Wrong Mortar
Too often, well-intentioned repairs are carried out using modern cement-based mortar, which can cause more harm than good. When lime mortar is replaced with a rigid alternative, it traps moisture inside the walls and leads to spalling bricks, cracks, and interior dampness.
Such mistakes can be costly to reverse and potentially damaging to the building’s historical value. That’s why any restoration work on a period property should involve experienced professionals who understand the nuances of traditional construction.
Choosing the Right Lime Mortar
The selection of the appropriate lime mortar depends on the building type, location, exposure, and original materials used. A skilled contractor will often carry out testing to match the new mortar with the existing one, ensuring consistency in strength, appearance, and breathability.
For listed or heritage properties, it’s also essential to comply with local conservation guidelines and work with specialists who can provide traditional lime mortar services.
Conclusion
Lime mortar isn’t just a building material—it’s a critical element in the long-term health and preservation of historic brick buildings. Its flexibility, breathability, and compatibility with old bricks make it the best choice for repointing, repair, and conservation work.
If you’re planning to restore a historic property in London or surrounding areas, make sure to consult a professional who understands the importance of using lime mortar. Doing so will help protect the beauty and integrity of your home for generations to come.
