When working with traditional buildings, using the wrong mortar can do more harm than good. Hydraulic lime mortars are often promoted as a safe and durable solution, but their widespread use in historic masonry is raising serious concerns. At The Yorkshire Lime Company, we specialise in breathable, conservation-appropriate materials – and we often help property owners repair damage caused by inappropriate mortars. If you’re restoring a heritage structure, it’s essential to understand the limitations of hydraulic lime before using it in your project.
What Is Hydraulic Lime Mortar?
Hydraulic lime mortar is made from limestone that contains natural impurities such as clay and silica. When the lime is burnt and slaked, these impurities allow the mortar to set through a chemical reaction with water (hydraulic set), rather than relying solely on carbonation with air.
Hydraulic lime is graded by strength:
NHL 2: Feebly hydraulic (softest, slowest set)
NHL 3.5: Moderately hydraulic (middle ground)
NHL 5: Eminently hydraulic (hardest, fastest set)
Hydraulic lime mortars are often used in damp conditions or exposed locations because they set more quickly and perform better under harsh weather. However, their strength and speed come at a cost – especially in historic buildings.
What’s the Problem in Heritage Work?
Hydraulic lime may seem like a good option – but in conservation work by heritage builders, it frequently causes more harm than good. Here’s why.
It’s Too Hard for Traditional Masonry
Older bricks and stones are often soft and porous. They were laid in weak, breathable mortars that allowed the structure to flex and moisture to move freely. Hydraulic lime mortars, especially NHL 3.5 or NHL 5, can be significantly harder than the materials they’re bonded to. This mismatch puts stress on the surrounding masonry.
Over time, movement in the building or minor settlement causes cracks to form- not in the mortar, but in the brick or stone. That’s irreversible damage.
It Restricts Breathability
Unlike non-hydraulic lime, which remains porous and breathable after curing, hydraulic lime can form a more closed structure. This means moisture gets trapped within the walls. When trapped damp meets frost, it expands and leads to spalling, salt damage, and mould growth – especially on internal surfaces.
Moisture retention is one of the biggest hidden risks in using hydraulic lime where it’s not suitable.
It Alters the Moisture Balance of the Building
Breathable materials like lime allow traditional buildings to dry out naturally. When this process is blocked by hydraulic mortars or dense pointing, moisture builds up inside the walls, floors, and roof structures.
This imbalance affects the entire building fabric. You may notice peeling paint, musty smells, damp patches, or rotting timbers – symptoms that mimic rising damp, but are often caused by inappropriate materials.
It’s Often Overused or Misapplied
Even when NHL mortars are used with good intentions, they are often chosen by strength rather than suitability. It’s common to see NHL 3.5 or NHL 5 applied universally – even in sheltered or internal areas where a softer mortar would perform better.
Conservation experts recommend using the weakest suitable mortar for the job. That usually means a non-hydraulic lime mix, often with pozzolans added if needed, tailored to match the original fabric.
Are All Hydraulic Limes Inappropriate?
Not always. Hydraulic lime has a valid place in construction – particularly in:
- High-exposure coastal or upland sites
- Chimney stacks and copings
- Agricultural outbuildings with minimal detailing
- Areas subject to frequent water splashback or ground saturation
The key is understanding when and where to use it. Feebly hydraulic mortars (NHL 2) may be suitable in some historic buildings, but anything stronger must be applied with caution – and only after a full assessment.
At The Yorkshire Lime Company, we often create custom hot lime or blended mortars that meet the same durability needs as hydraulic mixes, without compromising the building’s breathability or long-term condition.
The Issue with Bagged NHL Products
Another major concern is the lack of consistency in bagged hydraulic lime. Many pre-mixed mortars contain additives, performance enhancers, or poorly graded aggregates. Since there is no fixed recipe across brands, the end product can vary significantly – even within the same NHL grade.
This lack of control makes it harder to guarantee compatibility with your building’s original materials.
A mortar that sets too quickly, or too hard, or doesn’t accommodate seasonal movement, will eventually fail. That failure may take 5 or 10 years to show – but when it does, it could mean costly repairs to bricks, stones, and plaster.
What’s the Alternative?
For most traditional buildings, the best choice is a non-hydraulic (or air lime) mortar made from lime putty or quicklime. These mortars set more slowly, allowing the wall to dry out gradually. They flex with the building and naturally re-carbonate over time.
Non-hydraulic lime:
- Encourages breathability
- Reduces the risk of salt crystallisation
- Is softer than the masonry, so it fails sacrificially (and predictably)
- Can be reworked, repaired, or replaced easily
In many cases, we also use hot lime mortars – a traditional mix made by slaking quicklime directly into the sand. These mortars are highly breathable and adaptable to the specific needs of your property.
How to Avoid Damage in Your Project
If you’re planning repointing, rendering, or internal repairs, here’s how to avoid using the wrong mortar:
- Request a building fabric assessment before work begins.
- Ask about the type of lime being proposed – don’t assume all lime is suitable.
- Avoid NHL 3.5 or NHL 5 for soft masonry unless absolutely necessary.
- Use the weakest mortar that’s appropriate for the exposure and location.
- Match the original mortar wherever possible using sample analysis.
Work with Conservation-Driven Specialists
At The Yorkshire Lime Company, we don’t follow modern trends – we follow what works. Our approach is based on material compatibility, breathability, and long-term performance.
We test, observe, and tailor every mix to your building. That means using hot lime, putty, or mild hydraulic blends when appropriate – but always with the goal of protecting the structure, not just patching it up.
Our aim is simple: to ensure your building lasts another hundred years with the least possible intervention.
Don’t Risk It – Use the Right Lime
Hydraulic lime might sound like the safe option, but in heritage buildings, it often causes silent damage that won’t show until it’s too late. Understanding the differences in strength, breathability, and compatibility is essential when working on older properties.
If you’re unsure what your building needs, get in touch. We’ll assess the materials, advise on the best mortar, and help you plan repairs or repointing that truly respect your home’s construction.
Speak to The Yorkshire Lime Company Today
Don’t leave the health of your building to guesswork. The Yorkshire Lime Company provides honest, conservation-focused advice backed by years of hands-on experience. Whether you’re restoring a cottage, repairing damp damage, or planning a full re-lime, we’re here to help.
Contact us today to discuss your project, arrange a site visit, or learn more about lime mortars that work with your building – not against it.
