BCD 2019

96 T H E B U I L D I N G CO N S E R VAT I O N D I R E C TO R Y 2 0 1 9 C AT H E D R A L COMMU N I C AT I ON S where there is little chance of exposure to frost. Where bricks such as handmade or soft rubbing bricks are being used these may come with a frost resistance rating lower than F2. In such situations it is likely that this will match the properties of the original and will be acceptable. SOLUBLE SALT CONTENT Active soluble salts content is a further technical consideration when matching bricks for repair work. It is important to consider soluble salts in any new replacement bricks being inserted into a wall. The water-soluble salts content of a brick is defined in the relevant technical data for new bricks as follows: ACTIVE SOLUBLE SALTS RATING OF NEW BRICKS Rating Exposure S0 not exposed S1 normal exposure S2 prolonged saturation For the repair of traditional construction an S2 rated brick is likely to be the most appropriate. Given the long lifespan of traditional brickwork and the greater exposure to rain that climate change will bring, bricks meeting this rating will give a greater margin of safety for the future and reduce the likelihood of soluble salts from bricks used in repair work migrating into adjacent original work. Again, exceptions to this may arise in particular circumstances but an S2 rated brick is likely to be the most durable in the long term. WATER ABSORPTION RATE The water absorption rate of a brick is an integral indicator of how that brick behaves in a wall. It is important to match, as far as possible, the water absorption rate of new bricks that are being used for repair work to those which are already present in a wall. If the water absorption rate of a new section of brickwork is significantly lower than that of its neighbours, there is a risk that rainwater which hits the wall will run off from the new section and saturate the original brickwork. Likewise, if the water absorption rate of the repaired section is significantly higher than the original brickwork then the new work may suffer from excess saturation. This is not to say that water absorption rates must match exactly, but rather that it is good practice to broadly have the absorption rates of original and repair work in alignment. Tests on the water absorption rates of samples of traditional bricks carried out for Historic Environment Scotland show broad patterns in the absorption rates for traditional bricks of different kinds. All bricks manufactured today provide a water absorption rate as part of their technical data. Broadly, if bricks of the same type are sourced for a repair project this will mean water absorption rates are in alignment. WATER ABSORPTION RATES OF TRADITIONAL BRICKS Brick type Likely water absorption rate Common 7–9% (lower absorption class) 12.5–14.5% (higher absorption class) Handmade 13–17% Pressed facing 6–9% Extruded 8–9% (lower absorption class) 11–18% higher absorption class) Glazed 0.5% (fully glazed) 7–11% (partially glazed) Finding the right brick for a repair project may require an investment in both time and cost – time in terms of sourcing a suitable match and cost in terms of paying for a higher quality product which matches the original. The investment will, however, ensure a better match which will give an improved technical performance and an authentic repair, true to the original aesthetics of the building. In this way our brick built heritage can be repaired and maintained in a way which will last long into the future. Further Reading G Lynch, Tudor Brickwork, The Building Conservation Directory , Cathedral Communications, 2012 G Lynch, Brickwork History, Technology and Practice , Rutledge, 1994 G Lynch, The History of Gauged Brickwork , Elsevier, 2007 Historic England, Practical Building Conservation Earth, Brick and Terracotta , Ashgate 2015 M Jenkins, Traditional Scottish Brickwork , Historic Environment Scotland, 2014 All photos were taken by Moses Jenkins unless otherwise stated. MOSES JENKINS PhD is senior technical officer at Historic Environment Scotland. He has written various guidance notes for HES, most notably on brick and energy efficiency, and his current research is on insulation for traditional buildings and the properties of traditional materials. In 2009 he edited the book Building Scotland and in 2018 authored the book The Scottish Brick Industry . Special bricks of many different shapes are found throughout Britain. In this case a moulded convex brick has been used. Pressed facing bricks generally exhibit a smooth, uniform surface appearance. EarlyGeorgianbrickworkatSherman'sHall,Dedham,Essexwithfinelycarvedbrickwork(PhotobyJonathanTaylor)