Typically, a cavity wall consists of two wythes separated by a gap of at least 2 inches, which may contain insulation while still maintaining a minimum clear space of 1 inch to facilitate adequate drainage. Advanced methods like the insertion of a continuous drainage mat can further reduce this space while ensuring effective moisture expulsion.
Weep holes, strategically placed at the base of the cavity, are critical for draining accumulated water. These openings are created by leaving head joints open, often protected with screens to block pests. Flashings are implemented in two forms: external and internal. External flashings, including metal copings and counter flashings, protect vulnerable areas like parapets and intersections with flat roofs, preventing water ingress. Internal flashings are integrated during wall construction, positioned at potential water entry points like window heads and sills. These flashings catch and direct water toward the exterior via weep holes.
Materials for flashings vary, ranging from durable and expensive metals like copper and stainless steel to flexible, cost-effective options like asphaltic membranes and synthetic rubber. Given the complexity of replacing internal flashings and the relatively low cost of high-quality materials, the use of durable flashing materials is advisable to avoid future expenses related to water damage. Proper installation, including the overlapping and sealing of flashing sections, is vital for ensuring that all moisture is directed outwards.
In cavity walls, a minimum separation is maintained to ensure unobstructed cavity drainage.
Often, a continuous drainage mat within the cavity is used to deflect the mortar droppings and facilitate a clear drainage path.
The water entering this cavity moves out through the weep holes spaced appropriately at the cavity's bottom and placed above the flashings.
Additionally, vents near the cavity's top promote air circulation within the cavity and aid in evaporation of cavity moisture.
Flashings can be installed externally to prevent moisture from penetrating the exterior wall, or internally to divert water that enters the cavity back towards the outside.
Internal flashing is installed at the bottom of the cavity and places where the cavity is interrupted.
One end of the flashing is overlapped by a water-resistive barrier provided on the interior wythe, whereas the other end extends beyond the exterior wythe and bends down to form a drip edge to facilitate free fall of water away from the wall.
Sheet metal flashings, asphaltic membrane flashing, composite flashing, and synthetic rubber are the commonly used flashings.
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