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Cladding and Curtain Wall Construction 435
Facing with angle
support and cramps
Bronze channel cast
into concrete soffit
carries hanger and
plate set into edges
of stone soffit
Figure 7.17 Soffit fixing for stone facing (insulation omitted for clarity).
slabs it will be trapped in the cavity between the slabs and the background wall, will not
evaporate to air during dry spells and may cause conditions of persistent damp. Open or
butt joints between slabs should be avoided in external facework.
The joints between sedimentary stone slabs, such as limestone and sandstone, may be
filled with a mortar of cement, lime and sand (or crushed natural stone) mix 1 : 1 : 6 and
finished with either flush or slightly recessed pointing to a minimum depth of 5 mm. Joints
between granite and hard limestone slabs are filled with a mortar of 1 : 2 : 8 cement, lime
and sand (or stone dust) or 4 : 1 cement and sand to a minimum thickness of 3 mm. As an
alternative to mortar filling the joints between stones, a sealant may be used. Sealants such
as one part polysulphide, one part polyurethane/two parts polysulphide and two parts
polyurethane are recommended for the majority of stones. These sealant joints should be
not less than 5 mm wide. The jointing sealants will accommodate a degree of movement
between stones without failing as a water seal for up to 15–20 years, when they may well
need to be reformed. Mortar joints will take up some slight movement between stones but
may in time not serve as an effective water seal as wind-driven rain may penetrate the fine
cracks that open up. Some penetration of rainwater through joints between stones may well
occur as sealants age and mortar cracks. Thus it may be necessary to hack out and reform
joints to prevent moisture penetration to the inside face of the building.
Movement joints
Much of the early elastic shortening of the columns of a structure will have taken place
before a wall cladding is fixed. The long-term shortening of reinforced concrete columns,
through creep, has to be allowed for in horizontal movement joints. Differential temperature
and moisture movements of a wall facing relative to the supporting structure will