Directory of Traditional Building Skills - Mourne Heritage Trust

6 PLASTERWORK AND RENDERSUntil the 1930s lime based plasters were standard and so most old buildings are plasteredinternally with three coat lime plaster, either directly onto the brick or stone (on the hard) or onhorizontal laths on timber framing or joists. The backing coats were generally reinforced withhair or vegetable fibres (fibrous plaster) especially the fine, run (made in situ) cornices whichwere common even in modest buildings. Decorative plasterwork of the eighteenth andnineteenth century can be quite stunning.Excessive movement in weakening ceiling joists causes cracks, and damp penetration cancrumble the surface. Both problems cause the plaster to detach from the laths and eventuallyfall off. Lath and plaster ceilings and walls are difficult to patch and much fine plasterwork hasbeen lost through lack of timely maintenance. Rot in supporting timbers has long been theexcuse for wholesale stripping out of plaster but non-destructive methods are possiblecombined with the removal of the source of the problem and the insertion, where necessary, ofventilating voids. At the very least cornices and ceiling roses can be securely fixed and repairedin situ.Cornices, roses and other features are all available as pre-formed mouldings and goodplasterers can make up and repair run mouldings. Most fibrous plaster companies can replicatethe patterns of cornices and ceiling roses from damaged work or make models fromcontemporary properties.For repairs to ordinary, as well as more important, historic buildings plaster made from limeputty and fine sand should be used. In certain circumstances, for example where a wall is verydamp, it may, however, be expedient to use a bagged, lime based plaster, such as Limelite fromTilcon which retains some degree of ‘breathability’ and flexibility after setting.External renders vary from simple work lined out to resemble stonework, to elaborate stuccopilaster cornices and mouldings. Early examples are in lime based render but by the midnineteenthcentury ‘Roman Cement’, a naturally occurring clay and lime mix, was the mostcommon material; it is not available today.Renders are subject to thermal stresses and movements in the building fabric. In order toaccommodate such movement without cracking and to allow water which is drawn into thefabric to evaporate off, renders, like plasters, should always be lime based. The addition ofcement as a gauging material, although common place, is not generally regarded as goodconservation practice. In any event, the proportion of cement to lime should not exceed 1:2. Ifthe mix is too cement strong, it will crack, let in water which is unable to escape and eventuallydetach from the wall.Many renders contain a stone aggregate. Dry dash, or pebble dash, where the stones are flungon to the wet base coat, is an inappropriate finish for any historic building. A traditional wetdash, where the aggregate is mixed into the render before it is applied to the wall is moresuitable. The exact size and type of stone and the texture and colour of the sand used is criticaland should be matched to the original work where possible.Traditional smooth rendered walls, sometimes with ‘ashlar’ lines marking the ‘courses’ toresemble stone, were intended to be painted, unless it was already self-coloured by the additionof coloured sands in the render mix.101