Essential Control and Evacuation
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 paths 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 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 can affect a wall visually and structurally.
Failure to provide adequate control and evacuation measures can cause structural and visual concerns.
Where an extensive absorption area above an angled or curved opening exists, consideration should be given to reducing the extent of water wash gravitating to opening level. This is achievable by introducing an Arresting Barrier (Cavity Trays Ltd) with offset Caviweeps (Cavity Trays Ltd) 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, physically drawing off and discharging it within an area subjected to less demands.
An Arresting Barrier at a level prior to a feature opening(s) will minimise water wash demands – an essential consideration where curved and circular openings are adjacent.
In this example of natural ham stone, the presence of integrating ham surrounds (or quoins) necessitates the opening protective dpc Cavitray extends into the surrounding random coursing where caviweep water evacuation can take place – away from the feature surround. Discharge locations need not be in the immediate vicinity of the feature or opening. Complete protection of the opening head is achievable using a preformed Type BA Cavitray (Cavity Trays Ltd) that extends upwardly and outwardly.
Cavity wall water control assessment is recommended to avoid penetrating water from adversely affecting masonry structurally and aesthetically. Important: Approved Cavitrays and Caviweeps are accompanied with a performance undertaking for the benefit of Architect, Builder and Client.
The above subjects are tackled in detail within a Cavity Trays Ltd CPD presentation.
Suggested reading: Protecting the Building Envelope by Cavity Trays Ltd of Yeovil.
Products to reference:
Type BA Barrier Arches
Type Q Arresting Barriers
Caviweeps (5) Types W, Beak, Telescopic Extendable, Euroweep, Pyramid
