When the top of an opening within a cavity wall is shaped rather than horizontal, it is necessary to review to what extent cavity water-wash might concentrate, and importantly where it might end up. In some instances, the usual water evacuation path might not be possible.

Brick, block, natural and reconstructed stone absorb water to varying degrees, as does the mortar in which such masonry is set. It is reasonable to anticipate that every hour during rainfall, up to 2.25 litres of water will penetrate every square metre of wall area. This will occur during continuous rainfall, once the masonry skin has reached saturation point. Any large absorption area above a shaped opening will feed water onto it and into it. Failure to remove concentration points and provide evacuation routes will affect a wall visually and structurally.

In the case of natural ham stone the presence of integrating ham surrounds (or quoins) necessitates the protective dpc cavitray extends into the surrounding random coursing where caviweep water evacuation can take place - away from the feature surround. Opening protection is achieved using a preformed Type BA cavitray.

Bespoke Type BA cavitrays provide complete protection to the angled head, extending upwardly and outwardly. Caviweep evacuation is therefore possible located in the random stonework, clear of the surround assembly.

Where an extensive absorption area above a curved or angled opening exists, consideration should be given to reducing the extent of water wash gravitating to the opening level. This is achieved by introducing an arresting barrier with offset caviweeps at a higher level. Arresting barriers are not visible externally. They minimise structural water movement concentrations reaching a feature at lower level within the same elevation.

Cavity wall water control assessment should always take place. Controlling penetrating water behaviour prevents features and openings within cavity walls being adversely affected structurally and aesthetically.

Recommended for further reading within ‘Protecting the Building Envelope’:

Type BA Barrier Arches
Type Q Arresting Barriers
Caviweeps (5) Types W, Beak, Telescopic Extendable, Euroweep, Pyramid

Water control assessment essential

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