The Building Conservation Directory 2023

P R O T E C T I O N & R E M E D I A L T R E AT M E N T 4.1 129 C AT H E D R A L C O M M U N I C AT I O N S 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 A coat of sand/cement/lime is trowelled flat onto a wall and then while still wet, clean small sized aggregate is thrown onto the wall using a machine or hand held scoop. This style of render was developed in the late 19th century and it became particularly common in suburban architecture of the 1930s. THE APPLICATION OF RENDERS The preparation work carried out prior to commencement on site is just as important as the practical work itself. Render work must be accurately specified so it can be correctly costed and, as with any trade or craft, to achieve good quality work it is imperative to employ good craftspeople with relevant experience and knowledge. Once chosen it is always worthwhile consulting with the contractor at an early stage to add their experience to the process. It also must be borne in mind that every project will differ due to variations in the type and condition of the surface being rendered, as well as its relative exposure: each elevation may be exposed to different environmental conditions. The type and thickness of the render to be applied must also be taken into account, as must the time of the year. For instance, a render applied in the spring may require a different type of care and aftercare than one applied in the autumn. The surface to be rendered must be surveyed to check that a good bond can be achieved. It must be understood that surfaces to be rendered will vary in their porosity: some may absorb a lot of water so will need repeatedly wetting, while others may need to be left to dry out. In all situations any loose material will need to be removed and, when ready, the wall surface must be dampened down to control suction. Excessive suction will draw water from the new render, resulting in weakened areas and possibly detachment between substrate and render. Lime coatings should be applied in relatively thin layers. The number and thickness of coats will depend on whether the work is required to match historic work and its original thickness. Generally, two coat work is used on vernacular buildings, while three coat work is best where flat surfaces are required. SURVEY AND ASSESSMENT The initial purpose of a building survey of a rendered surface would be to describe its condition, identify any defects, and assess other adjacent building elements that may have an influence on the render’s condition and performance. The aim is to diagnose apparent causes, assess the urgency of any repairs and estimate costs. Surveys can range from a basic visual inspection of the render and its immediate environment to a more detailed survey that may be directed at a specific problem. A basic survey would generally entail a visual inspection of the rendered area and the preparation of a report. This should include a basic description of the building noting its age and general form, photographs of the relevant areas and information on the following: • the underlying building fabric – brick, stone or lath • the probable composition of the render – lime or cement based • the type of surface finish • any repaired areas that may be of historical significance • evidence of deterioration of the render such as cracks, areas of delamination and friable surfaces • symptoms and possible causes of deterioration, together with options for repairs • other sources of potential problems, such as tree’s too close to the building. A more detailed survey would carry out the requirements of the basic survey but would be aimed at a specific problem. This might include opening up to expose the main body of the structure underneath, taking render samples for analysis, and extended monitoring of the deterioration. Where a survey requires opening up the fabric of a listed building, the local authority should be notified as these types of investigation might require listed building consent even where like-for-like repairs are proposed. Successive reports can be used to create a record for future reference, so it may be possible to assess whether a problems is progressive. Cracking and detachment are the two most frequent symptoms of problems with renders on solid backgrounds, but delamination and detachment together with unsuitable repairs are also common. Failures are often attributable to water penetration, sometimes from lack of maintenance. Cracks Cracking can vary from fine intermittent small cracks and fissures to cases where the cracks cover more of the surface of the render and are generally connected. The more obvious ones may well cut deeper into the surface of the render and be connected with areas of detachment. For both newly applied and historic renders there can be a wide range of causes: Incorrect specification of materials can result in poorly graded aggregates; too many fine particles in the aggregate can encourage overworking and lead to shrinkage. Similarly, incorrect lime to aggregate ratios can result in excess water being used in the mix to assist with workability, which can result in surface cracking and detachment when the render is drying. Poor workmanship and the lack of general understanding of lime renders and their application and curing can lead to various issues including cracking. In particular, failing to dampen down the substrate and each intermediate coat can result in shrinkage, and time for aftercare should always be built into the contract as otherwise the surface may crack and become friable due to lack of carbonation. Structural movement can result in surface cracking to render and differential movement between the main structure and hidden features such as timber lintels or later additions to the structure. Some cracks may indicate movement of the main structure and may require monitoring to establish whether there is an ongoing structural issue. Detachment Hollow and detached areas of render may be easily identifiable where highlighted by sections of bulges and cracks. These areas can be further confirmed by gently tapping with a chisel or the back of the hand. Tapping will also help to establish the perimeter of the detached areas, the severity of the hollowness and whether the detachment is between coats or between render and substrate. But in some cases renders may have been detached and hollow for centuries, so a period of monitoring may be the first approach. As with the problem of cracking, incorrect specification, poor application and inadequate curing can lead to the render separating from the main body of the wall. Incorrect specification : a high strength finishing coat applied onto a weaker backing coat will result in differential thermal movement and can lead on to delamination between coats and cracking. Poor practice : over working each coat with steel or wooden floats can cause separation from the underlying masonry or render coat. Other poor practice such as the lack of key between coats or a final render coat being too thick will invariably lead to long term concerns. Areas of detached stucco on a Regency house under repair in Cheltenham A detail showing a fine crack in pebbledash: because it is relatively less porous than a lime render, water is liable to be trapped and freeze. (Photo: Elizabeth Heywood)