The SPAB’s annual National Maintenance Week campaign (20-27 November) reminds anyone who looks after a building – regardless of its age, type or purpose – of the simple, achievable steps they can take to prepare for the worst that winter can bring. However, when it comes to caring for old buildings it is beneficial to understand more about how they were built in comparison to modern construction. James Simpson OBE HonDSc FRIAS FSAScot Consultant to Simpson & Brown, Architects and committee member of the Building Limes Forum explains why…
Maintaining and repairing all buildings is essential and, in traditional buildings, keeping roofs and gutters in good order helps to keep masonry dry. If damp patches or green staining become evident, a problem has been ignored for far too long: dampness and possibly rot will ensue. However, walls still become wet on the outside from driven rain. This superficial dampness is normally retained in the outer surface of the wall, then evaporates and escapes back into the air. Paradoxically, impervious finishes always fail in this respect: small and sometimes invisible cracks enable moisture to penetrate; it is then unable to escape and eventually the whole wall becomes damp.
Traditional building began in the pre-industrial age, when buildings were made from natural, locally sourced materials. In the UK, earlier buildings of timber were superseded by walls of stone or brick, bedded in lime mortar, plastered internally and frequently also finished externally with a floated or thrown lime plaster coat and commonly finished with limewash.
These buildings were assembled by hand and detailed to exclude and shed water. Materials were, in relative terms, soft, weak, flexible and permeable. Rainwater was shed or carried away from roofs, and the walls, protected by overhangs, cornices or drip-courses, remained relatively dry. Moisture absorbed in driven rain penetrated the outer surfaces of the walls but evaporated out again when the rain stopped and the sun came out.
Portland cement mortars, renders and roughcasts are not good practice on solid walls: they simply do not work, for the reasons stated. Moisture from the ground, from minor leaks and fine surface cracks – which are inevitable when a hard brittle material is applied to a soft flexible substrate – make the walls wetter. Attempts to exclude moisture with damp courses and impermeable membranes only exacerbate problems by causing wetness to build up on one side or the other.
Stone and brick usually decay because of the crystallisation of soluble salts, as moisture evaporates from their surfaces. If joints are impermeable, this process is exaggerated and concentrated in the adjacent masonry itself. Hard mortar causes sacrificial damage to the stones; permeable mortar handles the evaporation and reduces damage to the stones and bricks themselves.
In direct contrast to traditional building, modern construction uses materials which are hard, strong, inflexible and impermeable such as steel, concrete, glass, ceramics and plastic. They depend on waterproof and airtight membranes, so that water is excluded in terms of moisture management. Traditional building and modern construction are two entirely different systems: the two should not be confused.
Lime should be the default material for the repair of traditional structures. Portland cement is too hard, strong and impermeable and damp-proof membranes don’t help. The use of lime mortars and lime finishes prevents the trapping of moisture, keeps walls drier and warmer by allowing dampness to evaporate outwards, and facilitates the recycling of materials. All of these things are relevant in the context of the current climate-change crisis.
There are different sorts of lime and different ways of preparing and using them. The selection, specification, preparation and use of lime mortars requires knowledge and skill. For the DIY owner, there are courses where the selection, preparation and use of lime mortars can be learned. List of courses.
The Building Limes Forum welcomes all: no prior knowledge or experience in the use of lime is required. Being a member provides access to a wide body of accumulated experience and expertise, an opportunity to share knowledge and helps to ensure that we are giving our buildings the love and attention they deserve. How to become a member.
Members of the Building Limes Forum form a community of lime enthusiasts and practitioners, most of whom are producers, suppliers, specifiers or users of lime.