Lime

Love Lime, Hate Cement  - why?

The walling of old buildings in this country is usually stone, brick, timber or earth, which are all absorbent. These buildings rely on allowing the moisture in the atmosphere to be absorbed by the fabric and then evaporate from the surface as the temperature rises. The thickness of the wall may have been relied upon to achieve acceptably dry conditions internally – most of our medieval churches have really thick walls.

The mortar used to construct these old buildings was usually of lime and sand – this is soft and the porosity lets the dampness evaporate through joints between the stone, brick, timber or earth. Over time the mortar decays.  Old buildings also tend to move around in reaction to ground conditions and high winds. Soft mortars can take up these small movements without us noticing.

The whole point of the mortar joint is to decay in preference to the more costly wall materials and be replaced from time to time.

Cement mortars are very hard, not very porous and also quite rigid. If cement is used to point up walls in old building, the cement may crack as it cannot take up the small movements, and these cracks let the moisture get in. Once inside the wall, the moisture cannot evaporate as the cement is impermeable, so its tendency is to evaporate though the stone, brick, timber or earth. These decay more quickly and then need to be replaced – a much more costly exercise.

The problems of cement mortars can be put right relatively easily - by being removed and the wall re-pointed in lime and sand. But the problems of damp are made much worse if the walls are rendered in a cement mortar. Because it is very hard, a cement render can be difficult to remove and cause severe damage to historic fabric during the process of removal. In many cases it is less damaging to leave the cement render to age and decay naturally, but the wall will tend to be damp.