Technical advice guide Introduction to lime in traditional buildings


HISTORIC BACKGROUND Lime has been used in the construction and finishing of buildings for at least 10,000 years. Before the mid-19th century local sources of lime were almost invariably used, although some lime was transported over greater distances by sea. Geology varies and, historically, local limestones (the raw material burned to make lime) produced a range of different types of lime, from relatively pure to eminently hydraulic. Many different types of lime were available, with different strengths and weathering properties.


WHY USE LIME? Why should we continue to use lime in the 21st century? As a construction mortar, from mass foundations to walls, arches and vaults, lime is resilient and flexible, adjusting to minor settlement and accommodating thermal movement. It is ideal for structural


104 Listed Heritage Magazine January/February 2020


repairs to lime-built masonry buildings. For repairs and repointing of masonry work, lime mortar can be matched to the density of the stone (or brick), to minimise the risks of salt damage and stone decay. The use of lime mortar in wall cores, in joints and on the face of the wall assists in the exclusion of driving rain by ‘mopping up’ free water before it can penetrate to the building interior and encouraging re-evaporation to the outside air. As an external weathering coat, lime mortars in the form of harling or rendering, can provide a more effective coating than cement- based materials. Good lime coatings do not suffer from the micro-cracking almost always present in cement-based coatings and, being resilient and flexible, they are also much less likely to develop cracks in response to minor movements of the underlying masonry. The porous nature of the material means that any minor cracks which do occur are less likely


to admit water, and may even heal over a period of time by the slow action of dissolution and recrystallisation of lime. Lime mortars are also used at roof level for bedding stone or tile ridges, for forming fillets and for parging slates and pantiles. Internally, lime mortar finishes (lime plaster) are breathable and non-toxic. They should be used without vapour barriers and will minimise condensation and promote a comfortable, healthy internal environment. Lime can also be used for floors in the form of lime concrete or lime-ash flooring and for deafening between floor joists to reduce sound penetration and increase fire resistance. Limewash finishes, internally and externally, complete the picture and complement the breathability of the construction as a whole.


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