The Building Conservation Directory 2023

130 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 3 | C E L E B R AT I N G 3 0 Y E A R S C AT H E D R A L C O M M U N I C AT I O N S Water penetration : the sudden separation and bulging of historic renders can be caused by water penetration resulting from lack of care and maintenance. In some cases this water penetration may not be immediately obvious as it may have been taking place for several months or years. Structural movement : even relatively minor structural changes may result in small cracks and fissures which could allow water to percolate between the render and the substrate, causing hollow areas and separation of the render from the structure. Crumbling and powdering of the surface All renders are considered sacrificial in that they provide a protective covering for the structure which is easier to renew than the structure itself, so an aging render should be expected to become more friable as it ages. However, in a relatively new render a crumbling surface is likely to indicate the incorrect specification, mixing and placing of the render: Insufficient lime in the final coat may arise as a result of poor specification or due to a lower than expected amount of active lime in the binder, perhaps as a result of poor storage. Insufficient carbonation can occur if the final coat of render is allowed to dry too quickly. Salt crystallisation can occur within the pores of the render where soluble salts are mobilised and brought to the surface by the continuous cycle of wetting and drying. As moisture evaporates, the crystals grow, destroying the bonds holding the render particles together and leading to crumbling of the surface. This can be caused by rainwater penetrating the render through the face, through the wall head or from roof level features such as chimneys, or by groundwater rising up through the walls behind the render. In some cases the problem will only rectified by the removal of the render, but in all cases the source of the damp must be addressed. REPAIR OPTIONS Where repairs to any historic building are proposed the first step is to assess the importance of the building and its condition. A complete understanding of the fabric and its structure is required to understand why the render has deteriorated, and the cause of the problem must be addressed before repairs are carried out otherwise the problem may reappear. The proposals must also be guided by conservation principles such as minimum intervention and the use of like for like repairs. The first option to be considered should always be to do nothing and just monitor the problem. If the issue is superficial and the render is stable, then any repair risks destroying historic fabric and creating a scar on the render unnecessarily, as well as wasting money. If repairs are considered necessary then to some extent the age and historic importance of the building and its fabric might influence the type and extent of the repairs carried out. Cracks Areas of cracked render not generally showing any signs of detachment could be filled with either several coats of colour- matched limewash or a fine lime mortar depending on the width of the cracks. Wide cracks (1–3mm) should be V-ed out and thoroughly cleaned to provide a key. As with all lime repairs, keeping the area damp throughout the process will control the setting of the render, prevent the cracks from reappearing and aid carbonation. The surface of the repaired cracks could be finished and cleaned with a damp sponge which will help it to blend in with the existing render. Detached and delaminated areas Any remedial work to consolidate detached render should be designed by an experienced conservator as not only will every render and every building differ, but every detached and hollow area of render can vary in its severity and complexity. Historically, detached and hollow areas of render have been grouted with lime-based grouts, but each case must be assessed on its merits as a heavy liquid grout can force loose render off the wall even if gravity-fed. Another option is to use stainless steel screws, washers and perhaps a stainless-steel mesh to hold back the surface: a 26mm core drill is used to remove just the top layer of render, and a 25mm washer is then fixed into the cored-out hole and held in place with a screw fixed into the masonry. A matching topcoat render is then applied over the area. The render has to be approximately 25mm thick for this repair to work and trials should be carried out first. In some cases the repair can be complicated by cracks and delamination in the same location. If this is the case then a combination of the above methods may have to be considered. Crumbling and powdering surfaces The cause of deterioration must first be understood. If the cause is thought to be a problem with the quality of the existing render or the way it was applied, then consider replacing the defective areas with a suitable material applied correctly. If the problem is thought to be linked with the structure or its environment, then those issues must be identified and dealt with before the area is re-rendered. Missing areas of render Patch or partial repairs to historic render have to take into consideration the cause of failure and the extent and type of render. It would usually be preferable to match historic renders like for like, using the same methods and materials to ensure it matches the original in colour and texture as well as strength. However, if it is discovered that the render has failed because it was inappropriate, then it will be necessary to consider an alternative. For example, if the render was too strong and inflexible for the situation in which it was used, then the replacement render should be designed to be more porous and have a less flexural strength than the existing, while matching it in colour and texture. A range of sample panels using different ratios of binders and various aggregates should be made for the approval of the supervising officer or client. Recommended Reading TAN 15 – External Lime Coatings on Traditional Buildings , Historic Scotland, 2001 bc-url.com/issuu-external-lime- coatings Practical Building Conservation: Mortars, Renders & Plasters , English Heritage, London 2011 John Ashurst, Mortars, Plasters & Renders in Conservation, Ecclesiastical Architects’ & Surveyors’ Association, 2nd Edition 2002 DAVID SLEIGHT IHBC is a self-employed conservation contractor and trainer. He worked for English Heritage from 1979 to 1997, latterly at their Conservation Skills Centre. In 1997 he joined British Waterways as a Heritage Officer training waterway staff and conservation professionals, before becoming self-employed in 2004. Removing the hard cement render from a cottage in Clipston, Northamptonshire and re-rendering with a porous, lime-based two-coat render (Photos: David Sleight)