Balnacraig House is a Category B Listed building located in rural Aberdeenshire. It was originally an early laird’s house dating from the 17th century; the central 5-bay farmhouse forms the oldest part of the structure, with later 18th century wings to either side forming a distinctive U-plan open courtyard. Other distinctive features include the rubble base course, chamfered reveals, regular fenestration with 6-pane sash and case windows and the central attic gablehead.
Scope of Work
Pictured: Balnacraig House: front elevation during works.
LTM were contracted by Ballogie Estates to provide advice on the conservation works, working in conjunction with the client and Historic Scotland inspectors. Works included: reinstatement of traditional lime harling to original building and new extension, lime washing to all newly applied harling, extensive deep void packing and pointing; selected masonry repairs and construction of slate vents; and internal lime plasterwork.
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Challenges and Considerations
Pictured: Gable wall: note condition of masonry and extent of repointing works.
The original lime finishes had been replaced with a hard, dense cementitious mortar. Over time, this impermeable coating had resulted in a catalogue of defects due to moisture entrapment within the mass masonry walls; key considerations were internal dampness and decay of timber joist ends and internal ‘safe’ lintels.
Assessing the building’s performance is essential to understanding material behaviour and therefore the specification of replacement lime mortars. LTM conducted a comprehensive building survey and designed a repair methodology based on meticulous documentation and recording of the building fabric. The building survey and materials analysis established areas of ‘high risk’: specific concerns were the degree of climatic exposure and deterioration due poor architectural design or lack of regular maintenance.
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Solutions to Problems
Pictured: Front elevation: reinstated lime harling.
The cement based mortar was identified as the primary cause of the problems associated with internal dampness. LTM believed that the most appropriate solution was to reinstate the traditional lime harling to compliment the masonry substrate: this was the most aesthetic, technically sound and historically accurate approach. The decision to reinstate the harling was also guided by the Scottish Lime Centre Trust, who were commissioned to conduct mortar analysis and provide advice on the materials and limewash finishes.
The primary objective was to help the building perform like a mass masonry structure, to replace the impermeable cement coating and allow the structure to ‘breathe’. Lime based materials permit the movement of moisture vapour through their physical mass and they are relatively flexible and durable, accommodating thermal movement due to changing temperature and moisture levels. They act as a sacrificial layer to the underlying masonry, thereby ensuring the longevity of the fabric.
The technical benefits of lime mortars are, however, dependent on good site practice. The project was characterised by the painstaking substrate preparation - deep void packing, pinning and mortar ‘flushing out’. This levelled background then allowed the traditional hand cast application of the two-coat harling, complete with six coats of pigmented limewash. LTM specified all lime mortars using the most appropriate binders and well graded aggregates; extant pinning stones were retained and were replaced with matching stones where required.
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