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152 Barry’s Advanced Construction of Buildings
to the vulnerable angle joints, these overlapping cloaks and gussets must be carefully
shaped and applied. The effectiveness of these membranes as waterproof tanking depends
on dry, clean surfaces free from protrusions or cavities, and careful workmanship in spreading
and lapping the sheets, cloaks and gussets.
Cavity tanking
Cavity drain structures make allowances for the small amount of water that may pass
through the external wall. The basement is constructed with two walls forming a void
between the external and internal leaf, and a cavity is formed in the walls around the basement
and below the floor (Figure 3.73). Traditionally cavities were formed with floor tiles,
which created a void, and two separate leafs of masonry walling. Nowadays rolls of studded
1 mm thick polyethylene are used, with raised studs (domes), which stand between 5 and
20 mm from the surface, depending on the manufacturer and purpose. The sheets of polyethylene,
which come in rolls 20 m long by 1.4 m wide, are applied to the structural walls
and floors to form a very small but effective void. The sheets are fixed to the wall using an
effective plug and screw system, and then permanently held and protected by a concrete
screed floor and in situ concrete wall. Any water that penetrates through the external wall
or floor is guided to drainage channels where it is then pumped out of the building.
Internal concrete wall
provides a smooth vertical
internal finish and holds the
cavity drain system firmly in
place
Secant (interlocking) pilled
retaining wall
High-density polyethylene with
20 mm studs mechanically fixed
for plugs and screws to
concrete piles
Where water does penetrate
through the retaining wall, it
drips down the face of, or piles
to, the drainage channel
50–70 mm
concrete screed
The drainage channel removes
the water to a sump hole
where the water is pumped
out of the building
Cavity drain system
Structural
floor
Figure 3.73 Drained cavity system.