Damp in Sandstone Buildings

“Private residence, Long Marton, Appleby-in-Westmorland”

Red sandstone typical of the Eden Valley is highly porous – but this is not what causes damp walls. The addition of concrete floors and pavements during modern conversions leads to a build-up of moisture in the ground beneath, which can only escape through walls. The used of cement pointing and other ‘tanking’ materials only serves to limit the ability of these walls to release this water, driving damp further and further up.

red sandstone building in Cumbria repointed with lime mortar to protect eroding stonework and reduce damp

↣ Lime pointing of red sandstone wall to reduce damp and protect eroding stonework.

Controlling Damp

Cumbria Stone Walling was asked to address one such damp problem in a red sandstone barn conversion. Damp was occurring extensively around the lowest part of the building, which had had concrete floors laid internally and concrete hard standing externally. Additionally, two garages had been constructed next to the building, which were likely impeding the natural drainage.

concrete slabs and cement pointing causing damp in sandstone wall - near Appleby, Cumbria.

↣ Concrete paving prevents groundwater evaporating naturally, causing a build-up of moisture which is forced to escape through the building.

For reasons of practicality, it was decided to remove some of the external concrete and replace this with a free-draining gravel. This would allow some of moisture trapped beneath the hard-standing to evaporate into the open air, rather than soaking into the walls. The cement pointing was also removed and replaced with a hot-mixed lime mortar to encourage the walls to dry out.

replacing concrete paving with free-draining gravel and repointing in lime mortar

↣ The free-draining gravel should reduce the amount of water entering the building, whilst repointing in lime will allow the walls to dry out.

Lime Pointing

Mortars and pointing in traditional solid wall buildings are designed to allow any moisture in the fabric of the wall to evaporate. Lime mortars perform this function well. Cement mortars are effectively impermeable, and therefore prevent damp from drying out. The stonework becomes saturated – encouraging algae and mosses, which traps further moisture.

cement pointing causing walls to become damp

↣ Cement pointing traps water in the wall causing the stone to become damp and encouraging algae growth.

Repointing in lime mortar allows moisture to be released on dry days and in the wind. On this project, the mortars where made on site from hot-mixed quicklime, limestone dust and sharp sands. Once set, these mortars actively draw existing moisture out of the stonework, where it can evaporate harmlessly.

hot-mixed lime mortar repointed of sandstone wall

↣ Re-pointing in lime mortar (hot-mixed) reconnects individual stones, allowing moisture to evaporate over a wider area.

Both the loss of original mortars or the use of cement pointing can cause soft red sandstone to erode. Lime pointing protects such stonework from further erosion by controlling moisture levels and acting sacrificially in extreme weathers. Replacing lime pointing is considerably cheaper than replacing eroded stones.

lime mortar used on eroding sandstone wall

↣ Soft sandstones can erode badly in original pointing is not maintained. Repointing in a hot-mixed lime will prevent further erosion.

Limewashing

Controlling damp inside a building can be difficult, because the high humidity and lack of airflow makes natural evaporation quite slow. Limewashes draw moisture from within the wall and spread it over a large surface area, where it can evaporate more easily. This is only true of limewashes; ordinary emulsion paints won’t allow evaporation to occur and will make damp problems worse.

rising damp caused by non-breathable concrete floors

↣ Non-breathable concrete floors in old buildings forces moisture beneath to be wicked up the porous walls.

In this case, I removed the existing mortars – including some very hard tanking renders at the base of the wall – and re-pointed in lime.

Dubbing out an old sandstone wall with lime mortar

↣ Pointing the walls flush (‘dubbing out’) helps the walls dry and makes a good surface for limewashing or plastering.

The worst affected areas were limewashed with a mixture of quicklime and limestone dust, applied freshly mixed. The limewash was extended well beyond the immediate damp areas to help encourage the walls to dry at depth.

As limewash (like all lime mortars) can only set fully when sufficiently dry, this process can take some time if the existing damp is very bad.

limewashed sandstone wall with excessive damp problems

↣ Excessing levels of damp can take a long time to dry and will slow the setting of lime mortars and limewashes.