Lime mortar
The soft, breathable, forgiving binder that traditional solid walls were built with - and still need.
Last updated 2026-09-05
What is Lime mortar?
Lime mortar is mortar made with lime rather than cement as the binder. It was the standard bedding and pointing material in the United Kingdom for most of the history of building, and it is still the right material for solid-wall and historic work. Lime mortars split into two broad families. Non-hydraulic lime, usually supplied as lime putty or as hot-mixed quicklime, sets by carbonation - it takes carbon dioxide back out of the air and slowly turns back into a form of limestone. Hydraulic lime contains clay impurities that let it set chemically in the presence of water, so it goes off faster and harder, and it is supplied in grades from feebly to eminently hydraulic. The general rule is that the further up the hydraulic range you go, the stronger and less permeable and less forgiving the mortar becomes, until at the top end it starts to behave more like a weak cement. Which family and which grade suits a particular wall is the designer's decision, taken from the strength and porosity of the masonry, the exposure and any analysis of the original mortar.
What makes lime worth the trouble is what it does after it is built in. A lime mortar is more permeable than the masonry around it, so moisture that gets into a solid wall - through the face, up from the ground, out of the air inside - can evaporate through the joints rather than through the units. That is what people mean by a breathable wall. Lime is also comparatively soft and slightly flexible, and it will accommodate small seasonal and settlement movements by developing fine cracks in the joint that then heal themselves, because free lime carried in solution recrystallises across the crack. A cement mortar does none of this: it is stronger than a soft handmade brick, it is far less permeable, and it moves the stress and the moisture into the unit. Lime is also gentle on the masonry when a wall is taken down, so bricks and stones can be cleaned off and reused, which is a large part of its low-carbon case.
The price of all this is time and care. Lime mortars set slowly - non-hydraulic mixes can take weeks or months to gain their strength through the thickness of a joint - and while they are setting they are vulnerable. Frost ruins them. Hot sun and drying wind ruin them by driving the water out before the set is complete. Lime work in the United Kingdom is therefore a broadly seasonal trade, protection with damp hessian and sheeting is standard, and the programme has to allow for it rather than pretend it away. Lime is also caustic in its fresh state and quicklime slaking is a genuinely hazardous operation, so the personal protective equipment and the method statement matter more than they do for a bag mix. On most projects the practical answer is to use the mildest binder that will do the job, mixed commonly somewhere between one part binder to two parts aggregate and one to three, and to let the specialist contractor and the designer set the detail.
How does Lime mortar work, step by step?
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Step 1: Establish what the wall needs
The starting point is the masonry itself. Soft handmade bricks, weathered limestone and rubble walling all want a weaker and more permeable mortar than machine-made engineering brick or a hard granite. The designer assesses the strength and porosity of the units, the exposure of the elevation, whether the wall is solid or cavity, and on historic buildings has the surviving mortar analysed for binder type, aggregate and proportions. Out of that comes the choice between a non-hydraulic putty or hot-mixed lime and a hydraulic lime, and the grade within that family. The wall is never given a mortar stronger than the units it is holding together.
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Step 2: Prepare the binder
Non-hydraulic lime arrives either as putty, which has been slaked and matured under water and improves with age, or as quicklime for hot mixing on site. Hot mixing - slaking quicklime directly with the damp sand - is a traditional method that produces a warm, workable, sticky mortar, and it is also the most hazardous operation in the trade, because quicklime and water together generate a great deal of heat and a caustic splash. It is carried out by people trained to do it, with full face and skin protection and a written method. Hydraulic lime is far simpler: it comes as a dry bagged powder and is batched like any other binder, though it has a limited shelf life once the bag is open because it will start to take up moisture from the air.
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Step 3: Batch and knock up the mix
Aggregate is a sharp, well-graded, clean sand chosen for colour and grain as well as for performance, because on exposed pointing the sand is what you see. Mixes commonly run somewhere between one part binder to two parts aggregate and one to three, with the specification for the particular job set by the designer. Lime mortars are mixed longer than cement mortars, often in a roller pan mill rather than a drum, and they benefit from being knocked up, left to stand and knocked up again before use. Water is kept to the minimum that gives a plastic, cohesive mortar - a wet lime mortar shrinks and cracks. Retempering a lime mortar with more water later in the day is normal practice and not the sin it would be with cement.
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Step 4: Build or point in the right weather
Lime work follows the weather more closely than any other masonry operation. On most projects the working season in the United Kingdom runs through the milder months, and work stops when frost is forecast within the vulnerable period, when the wind is drying hard or when the sun is beating on the elevation. Masonry is pre-wetted so it does not rob the mortar of its water. Lifts are kept modest, because a lime mortar has little early strength and a wall built too fast will squeeze the joints and go out of plumb. The specialist contractor calls the daily stop and start, and the programme carries the risk.
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Step 5: Protect and cure
Curing is where lime work is won and lost. New lime masonry and pointing are covered with damp hessian under polythene, kept damp for as long as the specification requires and shaded from sun and wind. The protection is left on for days or weeks depending on the binder and the weather, then removed gradually rather than stripped in one go, so the work does not dry out in a rush. Frost protection stays on through any cold snap in the early life of the mortar. Non-hydraulic mortars in particular go on gaining strength for months, and a wall that looks finished is not yet a wall that is strong.
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Step 6: Detail the wall so the mortar can do its job
A breathable mortar only works in a breathable wall. There is no point pointing an old solid wall in lime and then trapping it behind an impermeable cement render, a plastic paint, a modern gypsum plaster or a sealed internal lining. The designer keeps the build-up vapour open through its thickness, uses lime or clay plasters and mineral paints internally where the wall is solid, keeps external ground levels down and keeps rainwater goods working. Where a modern damp-proof course is absent, which is the normal condition in older solid-wall property, keeping the wall able to dry is the main defence, and the whole specification has to respect that.
What are the benefits of Lime mortar?
- Permeable, so a solid wall can dry outwards through the joints instead of through the face of the units
- Soft and slightly flexible, accommodating seasonal and settlement movement without cracking the masonry
- Free lime allows fine cracks in the joint to heal themselves over time
- Weaker than the units, so the mortar wears out first and the brick or stone is conserved
- Allows bricks and stone to be cleaned off and reused when a wall is taken down
- Lower embodied carbon than cement, with non-hydraulic mixes reabsorbing carbon dioxide as they set
What are the limitations of Lime mortar?
- Slow to gain strength, which limits build rates and lift heights and lengthens the programme
- Highly vulnerable to frost, drying wind and hot sun while curing, so the season is short
- Needs protection with damp hessian and sheeting as a matter of routine, which costs money and time
- Caustic in the fresh state, and hot mixing quicklime is a genuinely hazardous operation needing trained operatives
- Lower early strength means it is unsuitable for most modern loadbearing and high-exposure applications where the designer wants cement
- Skilled labour is comparatively scarce, and mixes and workmanship vary widely between crews
What is Lime mortar best suited for?
What plant does Lime mortar need?
- Roller pan mill or heavy paddle mixer suited to long mixing times
- Lime putty tubs or bagged hydraulic lime stored dry and in date
- Slaking bath, tools and full protective equipment where hot mixing is carried out
- Clean, sharp, well-graded sand stockpiled under cover and kept free of contamination
- Banker boards, hawks, pointing irons and lime brushes
- Hessian, polythene, sheeting and frost protection for curing
How is Lime mortar quality-checked?
- Binder type and grade approved by the designer against the masonry and the exposure
- Mortar analysis carried out on historic work before the mix is fixed
- Aggregate checked for grading, cleanliness and colour on delivery
- Sample panels approved for colour, texture and finish, and retained for comparison
- Daily weather recorded, with work suspended in frost, strong sun or drying wind
- Protection and damp curing maintained for the period the specification requires, and signed off before removal