Lime or cement on old walls?

Why old walls need breathable materials

Most old buildings were built to let moisture move through them. SPAB calls this their permeable nature, or “breathability” ¹. Water that soaks into the wall can then evaporate again ¹.

SPAB says lime was used in the construction of nearly all old buildings in Britain ². It says lime lets old structures “breathe” and move slightly, which suits them ². Its advice is to use repair materials that match the originals ².

Lime also gives historic surfaces their soft texture ². This matters for the look of a building as well as how it works.

What can go wrong with cement

SPAB says a modern cement render is incompatible with the construction of most old buildings ¹. Cement render does not let water out ¹. Moisture can then be trapped in the wall ¹.

Cement is also more rigid than the wall behind it. SPAB says hairline cracks form, water gets in through them and becomes trapped in the fabric ¹. Timber-framed and earth-built structures are especially vulnerable to serious damage ¹.

The way the two fail is different. SPAB says cement renders often fail in patches and come away from the wall. Lime renders wear away gradually and evenly ¹.

SPAB also says Portland cement, gypsum plaster and plastic paints can seriously harm historic fabric ². It notes that these became popular partly because lime is slow and skilled work ².

What to use instead

SPAB’s lime page gives one example. For external walls with moderate exposure, a weak hydraulic lime may suit pointing or rendering ². So may a non-hydraulic lime with a pozzolanic additive, such as tile dust ². This is an example only. The right mix depends on the building, so get advice from someone who knows older buildings before choosing.

Listed buildings and consent

Government planning guidance describes conservation as an active process of maintenance and managing change ³. It says heritage assets should be conserved in a manner that is consistent with their significance ³.

Listed building consent is separate from planning permission ³. It is needed for works to demolish any part of a listed building, or to alter or extend it in a way that affects its character ³. The law says no one may do this unless the works are authorised ⁴.

Doing such works without consent is a criminal offence under section 9 of the Planning (Listed Buildings and Conservation Areas) Act 1990 ³.

Minor repairs are treated differently. North Somerset Council says like-for-like repairs usually do not need consent ⁵. It also says changes in materials or appearance may need consent ⁵. We could not find a primary source that says directly whether replacing a lime mortar with cement, or the reverse, needs consent. The government guidance we read does not address repairs ³. If you are not sure, ask your local council before you start. North Somerset Council offers a pre-application advice service to help decide whether consent is needed ⁵.

North Somerset Council states that it is a criminal offence to carry out works on listed properties without permission ⁵. It also says that if you own or live in a listed building, you need listed building consent from your local council to make changes or alterations to it ⁵. Its page points to its “Listed buildings” page for checking whether a building is listed ⁵.

Each council runs its own process. Check the website of the council that covers your property.

Where to get help

• Your local council’s conservation or planning team can tell you whether consent is needed ⁵.
• SPAB publishes advice on lime and on lime and cement renders ¹².