Arches, Cavity Walls & Pinch Points
Rain blown against a cavity wall will gradually permeate the exterior masonry skin and gravitate downwardly. Where the cavity wall incorporates an arched opening, the downward flow is interrupted and water is prevented from permeating in and around the arch opening by a preformed DPC cavitray known as the Type BA (Barrier Arch). The cavitray is ready-shaped to match the opening design, and arrests the water that is then evacuated out of the structure via caviweeps, normally located both sides of the arch on the springing line - away from the opening itself.
However, where a number of arches are linked or the distance between each arch is minimal, the discharge arrangement should be reviewed to prevent water flowing into a converging point – commonly called a restricted point or pinch.
A typical example where review is necessary is where arches rise off columns, mullions, piers or cavity walling where the cavity width reduces. The water needs to be managed so flow to pinch points is minimised.
This is achieved by incorporating what is termed an Arresting Barrier. The Type Q Arresting Barrier invisibly reduces water-wash from cavity walling above a given feature or construction detail, and channels it to discharge away from its usual gravitational path. By so doing the residual onward downward water flow is minimised before any feature or construction detail is reached. In the case of multiple arches, the shelter and evacuation benefit of a higher level barrier can commonly result in less than 15% of the original water volume actually reaching converging pinch points.
Cavity Trays Ltd of Yeovil is the only UK cavity tray manufacturer awarded European Technical Approval. Water penetration and management advice is available free of charge under its Protecting the Building Envelope service.