Lime mortar is made by mixing lime with sand and water to create a breathable, flexible joint material used in historic restoration and soft masonry work. The basic ratio is one part lime to two or three parts sand, with water added gradually until the mix reaches a thick, paste-like consistency. The process is straightforward, but the type of lime you choose and the sand you use will determine how strong and workable your mortar turns out.
Choosing Your Lime
There are two main types of lime used in mortar, and they behave very differently. Your choice depends on what you’re building and how exposed it will be to moisture.
Lime putty (also called non-hydraulic lime) sets through carbonation, meaning it slowly absorbs carbon dioxide from the air to harden. This process is gradual, sometimes taking weeks or months to fully cure. Lime putty produces the softest, most flexible mortar, making it ideal for very old or fragile stonework. It needs air exposure to set, so it won’t harden in constantly wet conditions.
Natural hydraulic lime (NHL) sets through a combination of a water-driven chemical reaction and carbonation. It gains strength faster than lime putty and can set even in damp environments. NHL comes in grades: NHL 2 is the weakest and most flexible, NHL 3.5 is a general-purpose middle ground, and NHL 5 is the strongest. For most repointing and general masonry, NHL 3.5 is the standard choice. NHL 5 is reserved for exposed areas like chimneys, foundations, or anything below the damp-proof course.
If you’re working on a building constructed before the early 1900s, lime mortar (rather than modern cement) is almost always the right material. Cement is too rigid and traps moisture, which can damage soft historic brick and stone.
Picking the Right Sand
Sand does more than act as filler. It makes up the bulk of the mortar and has a major effect on strength and bond quality. Sharp sand (also called builder’s sand or coarse sand) with angular grains is the best choice. The rough edges interlock with the lime and create stronger bonds than smooth, rounded grains.
Coarser sand grading consistently produces stronger mortar. Research published in the Open Journal of Civil Engineering found that mortar made with coarser-graded sand had higher compressive strength and stiffness, largely because coarser sand requires less water to reach a workable consistency. Less water in the mix means a denser, stronger final product. Finer sand, by contrast, tends to lower bond strength.
A well-graded sand containing a range of particle sizes, from coarse down to fine, is better than sand that’s all one size. The smaller grains fill gaps between larger ones, reducing air pockets and improving density. Avoid sand with high clay or silt content, as this weakens the mortar. If your sand feels silky or muddy when wet, wash it before use or choose a cleaner source.
For restoration work, try to match the color and texture of the original mortar. Sand is what gives lime mortar most of its color, so sourcing a local sand similar to the original can make repairs blend in naturally.
The Mixing Process
Start by measuring your materials. The standard ratio is one part lime to two and a half or three parts sand by volume. For softer, more flexible mortar (on very old buildings), lean toward three parts sand. For slightly stronger mortar in more exposed locations, use closer to two parts.
Combine the lime and sand in a clean mixing tub, wheelbarrow, or on a mixing board. Mix the dry ingredients thoroughly until the color is completely uniform with no streaks or pockets of unmixed lime. This step matters more than it might seem: poorly blended dry ingredients lead to inconsistent mortar with weak spots.
Once the dry mix is uniform, add clean water gradually. Pour in small amounts while mixing continuously with a trowel, hoe, or paddle mixer. The goal is a stiff but workable paste, roughly the consistency of thick shaving cream. It should hold its shape on a trowel without slumping, but spread easily when pressed into a joint.
The most common mistake is adding too much water too quickly. An overly wet mix loses strength and shrinks as it dries, creating cracks. If the mortar looks soupy, add more dry mix in the same ratio to bring it back. You can always add a splash more water, but rescuing an oversaturated batch is harder.
Working With Lime Putty Mortar
If you’re using lime putty rather than NHL, the process has a few differences. Lime putty is sold as a wet paste, so you mix it directly with damp sand rather than blending two dry ingredients. The ratio is still roughly one part lime putty to three parts sand.
The best lime putty mortar is “knocked up” (thoroughly beaten and compressed) before use. This means working the mix aggressively with a trowel or beating it in a tub to remove air and improve plasticity. Well-knocked-up lime putty mortar is noticeably smoother and easier to work with. Lime putty mortar can also be made in advance and stored under a damp cloth or plastic sheet for days or even weeks without going off, which is a major advantage over cement-based mixes.
Applying and Curing Lime Mortar
Before applying mortar to any joint, dampen the surrounding masonry with clean water. Lime mortar needs moisture to cure properly, and dry stone or brick will pull water out of the mix too quickly, weakening the bond. Don’t soak the wall to the point of dripping, just mist or brush water on until the surface is evenly damp.
Press the mortar firmly into joints using a pointing trowel or a pointing iron, packing it tightly to eliminate air voids. Overfill the joint slightly, then trim it back flush or to a slight recess once the mortar has stiffened enough to hold its shape. The finish profile depends on the style of the building, but a slightly recessed finish is common for traditional work.
Curing is where lime mortar demands patience. NHL mortar will begin to firm up within a day, but full strength develops over weeks. Lime putty mortar takes even longer, hardening gradually over months as it absorbs carbon dioxide. During the first week or two, protect fresh mortar from direct sun, strong wind, frost, and heavy rain. Any of these can disrupt curing. Cover the work with damp hessian (burlap) or light plastic sheeting, and mist it with water in hot or dry weather to prevent it from drying out too fast.
Frost is the biggest threat. Water inside the mortar expands when it freezes, destroying the bond before it has developed any real strength. Avoid lime mortar work when overnight temperatures are expected to drop below 5°C (41°F), and keep protection in place until the mortar has had at least a week of above-freezing conditions.
Common Ratios at a Glance
- General repointing (NHL 3.5): 1 part NHL 3.5 to 2.5 parts sharp sand
- Soft historic brick or stone (NHL 2 or lime putty): 1 part lime to 3 parts sharp sand
- Exposed areas like chimneys (NHL 5): 1 part NHL 5 to 2 parts sharp sand
These are starting points. Adjust slightly based on your sand and working conditions. If the mortar feels too harsh or crumbly, add a touch more lime. If it’s too sticky or fatty, increase the sand proportion.