Narrowing and Converging Cavities Arches and Pinch Points

100 years ago in the 1920’s, a West-Country family of builders started fabricating ‘damp courses and other devices to allay the fears of the unpredictable and volatile English climate.’ The Company is now called Cavity Trays Ltd and the Company continues to specialise in keeping the building envelope dry.

Whilst cavities today continue to get wider, there are always situations where the opposite can occur, and here we look where cavities narrow and the need to address water evacuation.

Rain blown against a cavity wall will gradually permeate the exterior masonry skin and gravitate. Where the cavity wall incorporates an arched opening, the water is prevented from permeating in and around the arch opening by incorporating 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. Such caviweeps are usually offset both sides away from the opening on the springing line.

However, where a number of arches are linked together or the distance between each arch is minimal, the gravitating water from the higher masonry absorption area can end up gravitating into the converging valley between arches. In such cases the discharge arrangement should be reviewed to reduce water volumes flowing into what is commonly called a pinch point.

Multiple connected arches and arches rising off shared columns, mullions or piers can create pinch points. Pinch points also occur where a cavity width reduces, narrows or ceases because of a structural feature.

Water control can be introduced into a cavity wall to reduce the volume of water reaching a pinch point. This is achieved by incorporating what is termed an Arresting Barrier. The Type Q Arresting Barrier invisibly reduces water-wash in cavity walling above a given feature or construction detail, and evacuates it 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 an 80+% reduction in the downward water impacting the converging pinch points.

Approved Cavitrays are manufactured by Cavity Trays Ltd of Yeovil - the UK cavity tray manufacturer awarded European Technical Approval. Comprehensive Technical Manual sent upon request.

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