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Cladding and Curtain Wall Construction 427
soft limestones suffer a gradual change of colour over the course of years, and this weathering
is said to be an attractive feature of these stones. Limestone facing slabs are 75 mm thick
for work more than 3.7 m above ground and 50 mm thick for work less than 3.7 mm above
ground. Hard limestones are used as facings for the hardness and durability of the materials.
This type of stone is generally used as flat, level finished, facing slabs in thicknesses of
40 mm for work more than 3.7 m above ground and 30 mm for work less than 3.7 m above
ground.
Sandstones are used as facing slabs. Some care and experience are necessary in the selection
of these native sandstones as the quality, and therefore the durability, of the stone may
vary between stones taken from the same quarry. This type of stone is chosen for the colour
and grain of the natural material whose colour will gradually change over some years of
exposure. Because of the coarse grain of the material, it may stain due to irregular run-off
of water down the face. Sandstone facing slabs are usually 75 mm thick for work 3.7 m
above ground and 50 mm thick for work less than 3.7 m above ground.
Marble is less used for external facings in northern European climates, as polished
marble finishes soon lose their shine. Coarser surfaces, such as honed or eggshell finishes,
will generally maintain their finish, provided white or travertine marble is used. Marble
facing slabs are 40 mm thick for work 3.7 m above ground and 30 mm thick for work below
that level. Reconstructed stone made with an aggregate of crushed natural stone is used
as facing slabs as if it were the natural material, in thicknesses the same as those for the
natural stone.
Fixing natural and reconstructed stone facings
The size of stone facing slabs is generally limited to about 1.5 m in any one or both face
dimensions or to such a size as is practical to win from the quarry. Stone facing slabs are
fixed so that there is a cavity between the back of the slabs and the background wall or
frame to allow room for fixings, tolerances in the sawn thickness of slabs and variations in
background surfaces, and also to accommodate some little flexibility to allow for differential
structural, thermal and moisture movements between the structure and the facing. The
cavity or airspace between the back of the facing slabs and the background walling or
structure is usually from 10 to 20 mm and free from anything other than fixings so that
the facing may suffer small movements without restraint by the background. Small differential
movements are accommodated through the many joints between slabs and, more
specifically, through vertical and horizontal control (movement) joints. The types of fixings
used to support and secure facing slabs in position are:
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Loadbearing fixings
Restraint fixings
Combined loadbearing and restraint fixings
Face fixings
Soffit fixings
These fixings are made from one of the corrosion-resistant metals such as stainless steel,
aluminium bronze or phosphor bronze. Stainless steel is the general description for a group