The type of stone that is used in the construction of a building can have an impact on its long term performance. This was dramatically illustrated in an investigation carried out for Kearney Donald Partnership in Fortwiliam.
The geology of the Highlands and the Great Glen is amazing, throwing up huge mountains of dense hard rock, typically Granite and Gneiss.
When it comes to building, you might imagine that dense, hard durable stone was an ideal building material. In fact the opposite is true. Hard dense stones that are impervious to water an incredibly difficult materials to use and create all sorts of building problems.
The mortar that hods the building together and seals the buildings will not adhere to the stone, leaving open joists the wind to drive rain into the core of the wall. They don't breathe either. Once the water gets in, it cant evaporate back out again. The result is an ever increasing build up of water in the structure of the wall, until you get a waterfall on the inside face, as this photograph shows.
In contrast, sandstone is softer and breathes. Any water that is absorbed by the stone is evaporated out again very quickly. Crucially, because it is absorbent it will form a good bond with the mortar, creating a completely weather tight seal, preventing rain penetration.
In the centre of Fort William are two churches, built side by side and both completed in the same year 1861, by the renowned Victorian architect, Alexander Ross, provide a good illustration.
Fort William has a very wet climate and is surrounded by granite and gneiss.
The Mackintosh Church is built from local stone. The west facing walls suffer from serious damp penetration throughout its history. There have been various attempt to resovle these problems.
St Andrews church, is built from sandstone. This was all imported to the area. It is much more resilient to the damp, driving rain and the interior is in far better condition.
If you have a building made from granite, don't despair, there are solutions, but this is an area that requires the highest levels of conservation expertise from both the professionals and the tradesmen in order to get it right.
Speak to a conservation professional before you start.