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Lime Mortars, Renders & Plasters

Which lime to use, where it works, and what its carbon claims really mean

3 min readUpdated November 2026

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What is lime?

Lime is made by heating limestone or chalk, driving off carbon dioxide to produce quicklime, and then combining it with water. Mixed with sand, it makes mortars, renders, and plasters.

Lime remains popular for repairing older buildings because it is generally softer and more vapour permeable than cement. It can let a wall release moisture and accommodate small movements.

Types of lime

The right type depends on exposure, the wall, and how quickly the work needs to gain strength. Stronger is not automatically better, particularly with soft bricks and stone.

  • Lime putty or hydrated lime (non-hydraulic)

    How it sets
    Slowly, by absorbing CO2 from the air
    Typically used for
    Plaster, limewash, and sheltered conservation work
    Keep in mind
    Slow to harden and needs protection

  • Hot-mixed lime

    How it sets
    Mainly by carbonation
    Typically used for
    Conservation mortars and repairs
    Keep in mind
    Hot, so needs experienced handling

  • NHL 2

    How it sets
    Water and air
    Typically used for
    Soft masonry and internal work
    Keep in mind
    Weakest of the natural hydraulic grades

  • NHL 3.5

    How it sets
    Water and air
    Typically used for
    General mortar and render
    Keep in mind
    Check it suits the masonry

  • NHL 5

    How it sets
    Water and air
    Typically used for
    Exposed locations and damp conditions
    Keep in mind
    Can be too strong for soft masonry

  • Formulated or premixed lime

    How it sets
    Varies
    Typically used for
    Convenience and consistency
    Keep in mind
    Ask what the binder contains, as it may include cement

NHL numbers refer to the minimum 28-day compressive strength in MPa under BS EN 459-1.

Mortar, render, and plaster

Each use has different demands, but the general principle is to choose a mix that is no stronger or less permeable than the masonry needs.

TopicDetail
RepointingMatch the existing mortar where appropriate. Mortar should be the sacrificial part of the wall, so the bricks or stones do not take the damage.
RenderUsually applied in several coats, with each one allowed to cure. It needs sound detailing at the top, base, and openings.
PlasterTraditional three-coat systems take time, while premixed products can simplify the work.
LimewashA thin breathable finish that needs periodic renewal.

Watch out. Cement repairs on older walls can trap moisture and speed up decay. Before you repoint or re-render, find out what is on the wall and why any damp is occurring. For a listed building, check whether consent is required.

Is lime low carbon?

Lime is often described as carbon neutral or even carbon negative. Treat those claims carefully. Making quicklime releases process CO2 of around 0.75 tonnes per tonne before counting kiln fuel. Non-hydraulic lime reabsorbs CO2 as it cures, which offsets part of this over time, but how much and how quickly depends on the product and conditions.

For comparisons, use EPDs with matching declared units and scopes. Published figures vary considerably by product.

Tip. Ask whether a carbon claim includes carbonation, what timescale is assumed, and whether it has been independently verified.

Working with lime

Lime rewards preparation and patience, and most failures come from drying too quickly or frost.

TopicDetail
TimingAvoid frost and hot, windy conditions. Allow for this in the programme.
CuringProtect fresh work with hessian or sheeting and keep it damp as advised.
BackgroundDampen the wall to control suction, and remove loose or unsuitable material.
SafetyLime is caustic. Wear eye and skin protection and keep clean water to hand. Take specialist advice on hot mixing.
SkillsFor repairs to historic or exposed walls, use someone with lime experience.
SandIts grading affects performance as well as colour.

Trade-offs

Lime is not always the better choice. It can cost more in time and labour, and it may be unnecessary in modern cavity construction designed around cement-based products. Where a modern impermeable system is already in place, adding lime alone may not change how the wall behaves.

Using Matera

Tell suppliers about your wall type, exposure, and time of year. Ask for the product's strength class, binder contents, recommended sand and mix, and curing advice, along with an EPD if carbon matters to your project.

Sources

Where the facts in this guide come from. Standards and guidance change, and some sources are suppliers or trade sites, so check the original before relying on a detail.

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