Why may a modern cement render be inappropriate for an old building?
A. Because it is incompatible with the construction of most old buildings and can cause or accelerate serious decay. Modern buildings generally depend on an impervious outer layer and cavities to keep out moisture. By contrast, old buildings tend to rely on their porous nature (‘breathability’) to allow water absorbed by the fabric to evaporate back out. The use of an impervious Portland cement render in place of a traditional lime-based covering restricts evaporation. Hairline cracks form due to the mortar being more rigid than the wall. These then draw in water that becomes trapped in the fabric. Timber-framed and earth constructed buildings in particular can suffer major structural damage if moisture builds up behind a cement render. Q. How do I tell whether an inappropriate cement render has been used? A. If the building pre-dates about 1800, the original render is likely to have been of ordinary (non-hydraulic) lime or natural hydraulic lime that has a weak chemical set. Lime-base
