The Building Conservation Directory 2023

97 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 M A S O N R Y 3.2 COPINGS and CAPPINGS on FREE-STANDING BRICK WALLS GERARD LYNCH F REE-STANDING BRICK walls have been used for centuries to demarcate boundaries, provide security, keep out noise and screen landscapes. Properly detailed and constructed, these have proved aesthetically pleasing and highly durable. Traditionally they were no less than one-brick-thick, constructed to accepted craft practices and time-proven rule-of- thumb, largely using handmade bricks set in flexible lime-rich mortars. By modern standards they may appear over engineered but their mass and deadweight created stability and resistance to lateral forces, particularly from high winds. This changed after the second world war with economy of materials and the move to thin-walled construction largely using machine-made bricks set in hard and rigid cement-based mortars, typically up to 1.8m (six feet) high. Neither properly designed nor covered by statutory control, these cheap garden walls sprang-up across the country on new housing estates. Most soon exhibited deterioration and some blew over, particularly where built on plastic DPCs which were unsuited to such construction and acted as a hinge. EXPOSURE Unlike the outside walls of a building, free-standing walls are invariably subject to exposure from the elements on both faces and from above, making them vulnerable to saturation, which is often the main cause of deterioration. The ingress of moisture is particularly severe during the months of late autumn and winter when they are exposed to long spells of wind-driven rain and from melting snow on wall heads. Once saturated, masonry is vulnerable to the potentially damaging effects of freeze/thaw cycles. Although local knowledge of typical weather patterns is important, reference should also be made to exposure maps of the UK that illustrate zones of differing exposure to wind-driven rain when specifying repairs. Most historic bricks were made from topmost clays and, by modern standards, were low-fired. Yet these have proved durable, regardless of strength, because these have an interconnected, open- pored structure that can accommodate and dissipate the hydrostatic pressure arising from an eight per cent expansion of frozen water. The open-pored structure also helps bricks to dry out more quickly once dry weather returns. Many modern bricks, no matter how hard, have pore structures that cannot accommodate this, resulting in spalled faces. It is well known that the air lime or hydraulic lime binders of the mortar joints on traditionally constructed brickwork act as the conduit through which the wall ‘ breathes’, allowing moisture that has saturated the masonry to then rapidly dry-out. So, the wall is said to behave with an ‘ overcoat effect’ (as opposed to a raincoat effect which sheds water). Such relatively low-strength mortars also act as a flexible gasket that, having the same coefficient of expansion as clay bricks, can accommodate thermal changes without the need for movement joints. On the other hand, the strong, rigid and impermeable A beautifully detailed free-standing brick wall at Chicheley Hall, Buckinghamshire (1719–21): its purpose- moulded capping consists of double-ovolo headers over cavetto stretchers (Photo: Gerard Lynch) modern OPC-based mortars are intended to act more as a gap-filling glue and are generally used to bind more highly-fired bricks. As a result the surface of the brickwork sheds rainwater like a raincoat and, being relatively rigid, these walls require movement joints to accommodate the cumulative stresses induced. WORKMANSHIP Of critical importance in the construction and repair of a traditional free-standing wall is that all bricks must be laid frog uppermost upon full beds of lime mortar, not deeply furrowed, and all cross joints fully filled. The bad craft practice of tip-jointing just the brick edges must be