Concave and Convex Cavity Walls - 9th September 2011
Concave and Convex Cavity Walls
Caution with Window Openings and Abutments
As well as affecting the extent of localised water-wash passing over the surface of the masonry, curved cavity walls can also change the behaviour of water and its permeation of the outer skin.
In a concave situation, any window opening within the central arch of a wall predominately at 90 degrees to the wind-driven rain direction will experience a greater volume of permeating water.
The wind blows into and along the curve suspending immediate gravitation of water wash.
Whenever a window opening is encountered, it provides masonry edges into which the wash of water can cascade.
Thus, every opening acts as a recess into which water will initially flood and saturate the masonry.
Openings within curved walls require measures to cope with such volumes of water that can far exceed those commonly experienced within a flat surfaced wall.
The effect with a convex wall is far less severe, although water is usually blown around and away from the prevailing wind direction and will predominately be concentrated where gravitation takes hold and the water commences to run downwardly.
The reveal closer must integrate with the lintel arrangement whilst at sill level a sill closure tray can ensure any water that congregates externally at the reveal sill cheeks and permeates inwardly is arrested.
This is particularly relevant where masonry or tile sills are incorporated or pinch points exist between inner and outer skins.
Curved walls meeting horizontal and sloping abutments must incorporate appropriately dimensioned dpc trays that match the cavity width and provide unobstructed cavity compartment areas into which any volume of water tracking along a tie can discharge.
Curved trays within parapet walls and associated measures providing coping dpc support must harmonise with any dpc/ tray provision at lower levels.
In the case of tall buildings, arresting barriers at intermediate level(s) can lessen demands through aggregate gravitational accumulation and control water volumes.
Pending the wall type, structure and finish, evacuation out of a cavity wall can be via a variety of caviweeps.
Where wind-driven rain is of concern, the designer has the option of choosing models that use positive and negative air pressure to advantage.
There are also options that minimise the exit route aperture but require greater frequency.
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