Nowadays it is unusual for self-builders to build from new in solid stone. For reasons of insulation, as well as the cost of stone and the labour involved, it is considered impractical. But what if you have a stone house already and it is in need of regular maintenance and occasional repair?
Most of us have seen mismatches in stone replacement and pointing, but it is not only the colour that can look wrong. If the mortar in an extension or a repair is mixed with the wrong materials and is not softer than the stone that surrounds it, damage can result.
“It is very common for householders to think they are protecting an external weakness in the stone wall of their house by applying an external patch of cement render but this can easily have the reverse effect,” says Douglas Kent, chartered building surveyor and technical secretary of SPAB the Society for the Protection of Ancient Buildings.
He explains that buildings with solid stone walls need to breathe. They are usually built of two skins of stone, with a core of rubble in between. “This core, to which the outer and inner layers of stone are attached by a lime mortar, acts as a wick,” Douglas says. “It is the key to the breathability of the building. Walls like this work by allowing water absorbed by the fabric to evaporate out. This is totally different from modern houses, which rely on a series of barriers to keep the water out. Therefore if you patch this with an impervious layer of sand and cement you will more than likely exacerbate the problem.”
A high proportion of the calls he receives at the SPAB helpline concern damp problems in old stone houses. “If you have a bad patch of damp in a stone house and the cement render outside is too well keyed in to detach without making the stone flake then it may be wise to leave it, as it will cause even more damage to remove.”
When the time comes to repoint and replaster on the inside, Douglas recommends a lime-based mix for solid walls on account of their pervious construction. “Avoid sand and cement,” he says. “A cement-based mortar will always lock in the water on these walls.”
Damp proof membranes are another cause of concern to SPAB. Douglas frequently gets calls from people who have recently moved into stone walled houses and discover damp problems a month or two later. “It is often because a damp proof membrane has been installed during the renovation and the membrane is of an unsuitable type,” he says.
Very often the inclusion of a damp proof course is a result of a bank or building society stipulation. The reason an unsuitable damp course is installed is usually because the owners or the lender calls in a remedial specialist who is tied to one particular product. SPABs recommendation is that householders should use an independent surveyor who specialises in old buildings. Rising damp in old stone houses can very often be eradicated quite simply by lowering the exterior ground level or digging a French drain outside.”
Another area of concern is the use of colourless external water repellents on houses with solid walls. SPAB urges caution in the use of these because it says they can very easily reduce the rate of evaporation from porous stone walls and trap moisture inside.
“When you have spalling patches of flaky stone on the outside wall, it may not be too serious,” Douglas says. “If there is plenty of material left and the damage is only superficial, it may be best to do nothing. One thing we do urge, however, is to try to discover the cause. It may be nothing more than a build-up of earth on the outside.”
Another problem in old properties with solid stone walls is a tendency for the rubble core to crumble and push the two skins out. “This is really quite common in old houses with solid stone walls,” says Adrian Dobinson, of Renaissance Architectural Consultancy.
“The usual solution is to grout it with a lime mortar. In more difficult cases flexible steel ties can be used to help hold the skins together, while in extreme cases stone stitching might be needed. This may involve replacing the eaves plate, which usually means the roof has to come off a complicated process that is best avoided unless absolutely necessary!”