Lewis Proudfoot of Cliveden Conservation looks at moisture management in historic buildings, and why he believes lime should be the default material for repairing traditional structures
The challenge of maintaining and repairing historic or traditional buildings often comes down to how well we manage water ingress. Poor moisture management in a building is often the main cause of deterioration to masonry or timber, which is why water is known as a key ‘Agent of Decay’ in old buildings. The presence of damp in walls and floors is the chief headache for many historic building owners. The problem is often exacerbated by inappropriate intervention following poor advice, or a misunderstanding of the nature of how traditional buildings are constructed.
Large or small, any historic building constructed of masonry – be it brickwork or stone – will have been built with local materials using lime mortar. These materials were relatively soft, weak, flexible and permeable. A building’s ability to manage water ingress was down to the architectural detailing of drips, overhangs and rainwater goods that keep moisture away from walls in the first place, and secondly, by the natural abilities of lime mortar to make masonry resilient to the two forms of water-related decay – namely frost, and the less well known, but more common, salt attack.
The issues of freeze-thaw damage are well known – when moisture becomes trapped within a masonry unit or joint and then freezes, it expands – causing damage to the surrounding material. But the damaging crystallisation of soluble salts onto the surface of materials after evaporation is less known. This is the reason that lime mortars and renders are so effective at managing moisture in buildings: softer than the surrounding masonry units, with high porosity and a good free lime content, an appropriate lime mortar will act as a poultice, drawing moisture away from masonry units and out onto the surface of the wall to evaporate.
The damaging salt crystallisation (which will always occur) will do so at the surface of the joint rather than on the stone or brick, and the erosion will be to the sacrificial mortar. Repointing, re-rendering and shelter coating are therefore essential and normal aspects of good building maintenance, safeguarding the important masonry elements and allowing moisture to move away from the building.
The use of cements, hard NHL mortars, waterproofers and membranes can stop moisture movement – either trapping it in a building, thereby increasing the window of frost susceptibility, or causing damp walls to absorb more salts from the atmosphere.
Case study: St Michael’s
Generally, water always finds a way out, and it will choose the route of least resistance. We recently worked with Caroe Architecture to complete a small scheme at St Mary & St Michael’s Parish Church, Trumpington, Cambridge. The internal stonework in a side chapel was in a poor state, especially some lovely carved corbels below a monument. The cause was moisture in the wall – entering from outside and below, but because the lower section of the wall had been previously rendered in a hard cement, all the evaporation occurred on the surface of the soft clunch stonework, causing serious damage. This cement render came off in sheets, exposing the historic masonry construction, with previous lime renders still evident in some areas.
The damaged string course and window sills around the chapel were replaced or repaired with a hot mixed lime mortar, and the walls were lime washed. The four carvings were consolidated with lime mortars and sheltercoats. The stone that was replaced was crushed to form the aggregate used in making the hot-mixed repair mortars and limewashes, ensuring a perfect colour match with the historic fabric.
The damp walls were visibly drying out during our time on site, and the permeable render that was applied further aided a far more effective form of moisture movement rather than impeding it like the solid cement had. In doing so, the ongoing erosion to these important carvings has been halted, ensuring their longevity and appreciation by future generations.
Conclusion
Building limes have been used as the principal binder for mortars and plasters for thousands of years and should be the default material for the repair of traditional structures. 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.
To raise awareness of building limes, generate discussion and share knowledge, The Building Limes Forum (BLF) was set up in 1992 to bring together a community of lime enthusiasts, practitioners, and professionals. Being a member provides access to a wide body of accumulated experience and expertise and an opportunity to share knowledge.
Lewis Proudfoot is managing director of Cliveden Conservation and committee member of the Building Limes Forum