Lime mortar is a mortar made from lime binder (either hydraulic lime or non-hydraulic lime putty) mixed with sharp sand, without Portland cement. It is softer and more flexible than cement mortar, allows moisture to move through it and evaporate (breathability), and is designed to be weaker than the masonry units it bonds. This means that as the wall moves with temperature and moisture changes, the mortar joints flex and eventually erode sacrificially rather than transmitting stress into the bricks or stone and causing them to crack or spall.

Why must lime mortar be used on old buildings?

Pre-1919 buildings were built with lime mortar and typically use soft handmade or stock bricks and natural stone that cannot tolerate the rigidity of Portland cement mortar. When cement mortar is used on these buildings, it traps moisture behind the hard impermeable pointing, causes frost damage and spalling of brick faces, and concentrates movement stress into the masonry units rather than the joints. Correct lime mortar repointing is essential for the long-term preservation of historic masonry - and is often a condition of listed building consent or work in conservation areas.

What is the difference between hydraulic and non-hydraulic lime?

Non-hydraulic lime (lime putty, CL90) sets only by carbonation - absorbing CO2 from the air - and is very slow to harden. It remains soft and workable for extended periods. Hydraulic lime (NHL - natural hydraulic lime, classified NHL2, NHL3.5, or NHL5) also sets by a hydraulic reaction with water, giving it greater early strength and the ability to set in damp conditions. NHL3.5 is the most commonly used for historic masonry repointing - strong enough to be durable but soft enough to be sacrificial relative to most traditional bricks.

What mix ratio should I use for lime mortar?

For repointing standard historic brick, a mix of 1 part NHL3.5 lime to 2.5-3 parts sharp sand by volume is typical. For softer or more friable bricks, NHL2 (weaker) or a lime putty mortar is preferred. For pointing exposed locations such as chimney stacks, NHL5 (stronger) may be specified. The sand grading, colour, and particle shape affect both the workability and the appearance of the finished pointing - always match the aggregate to the original mortar where appearance matters on a listed building.

Heritage & Conservation noun

Lime Mortar

/ laɪm ˈmɔːtə /

Also known as: NHL mortar, lime putty mortar, hydraulic lime mortar

Definition

Lime mortar is a masonry mortar made from lime binder - either natural hydraulic lime (NHL) or non-hydraulic lime putty - mixed with sharp sand, without Portland cement. It is softer, more flexible, and more breathable than cement mortar. On historic buildings, lime mortar is the only acceptable bedding and pointing material: it must be weaker than the masonry units it bonds, so that movement causes the mortar to flex and erode sacrificially rather than damaging irreplaceable bricks or stone.

In practice

Natural hydraulic lime (NHL) is classified by compressive strength: NHL2 (soft, for very friable bricks and stone), NHL3.5 (medium, the most versatile - used for most historic brick repointing and new build lime mortar work), and NHL5 (harder, for exposed positions such as chimneys and copings). A standard NHL3.5 repointing mix is 1 part lime to 2.5-3 parts sharp sand by volume.

NHL mortar must be kept damp and protected from frost for the first few days after application - it sets by a combination of hydraulic hardening and carbonation (absorbing CO2 from the air). Work should not be carried out below 3°C or in direct hot sun. Joints should be raked out to a minimum depth of 15mm before repointing, and the old mortar must be removed without damaging the brick arrises. On listed buildings, mortar samples from the original building are often analysed to match the aggregate colour, grading, and lime type precisely.

Building Regulations

StandardBS EN 459 - Building lime

Mortar standardBS EN 998-2 - Specification for masonry mortar

Listed buildingsLime mortar typically required by listed building consent

Conservation areasLime mortar often specified by local planning authority

The use of lime mortar on listed buildings and in conservation areas is typically a condition of listed building consent or pre-application advice from the local planning authority. Approved Document A permits lime mortar in masonry construction. Historic England publishes guidance on mortar specification and analysis for historic buildings. Using cement mortar on a listed building without consent may constitute an unauthorised alteration requiring retrospective consent or reinstatement.

Full Building Regulations guidance

Related Calculators

Masonry Mortar Calculator

Lime Mortar

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