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Rain Amplification with Curved Cavity Walls

Author/Issuer: Cavity Tray Standards    Added:

Additional Considerations for Designers and Contractors

Cavity Trays of Yeovil has announced an increase in demand for trays to suit curved (on plan) cavity walls. The Company currently manufactures curved cavitrays for use over windows, horizontal and sloping intersections and changes of level.

Curved trays for curved walls present the designer with many additional design considerations compared with standard flat surfaced / straight walls.

A good example is where a pitched roof abuts a concave or convex cavity wall. The pitch of the abutting roof may be constant, but where the roof actually strikes the wall the angle of intersection changes throughout the slope. For a brief moment at the commencement of the slope the intersection may be horizontal, but as the wall turns the angle changes and those changes continue until the top of the roof.

The question of how to best protect the structure against wet penetration and externally flash the intersection is answered by using preformed curved cavitrays. Each tray is individually and compositely computer calculated to take into account the arc of the wall, the masonry coursing and its position up the slope. Each tray is then numbered so the builder can literally start laying by numbers – with assured interfacing and compatibility.

Cavity Trays Ltd also takes into account how the wall (and its flashings) will behave during storm conditions. Compared with a straight wall, the effect of a concave or convex arc is to accentuate the water-wash being blownacross the curved surface. Provision must be made for weathering functionality into the direction of each flashing lap as well as with the direction of the lap. This is because the concentration (build-up) of water-wash traversing a sq metre of curved surface can (subject to the radius of the arc) increase by up to 200%. This is when compared with a flat surface where the plane does not promote deflection (windscreen principle) in any direction other than downwardly by gravitation.

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Intersections abutting concave walls are likely to be subjected to more extreme demands compared with convex walls. This is because water can accumulate and be ‘focussed’ within a dished surface whereas it will discharge off (around) a surface providing the opposite deflection.

Cavity Trays calculates this status, and each flashing supplied on each individual Cavitray provides appropriate cover and appropriate lap. Conventional site-applied running flashings cannot match the performance of the shaped lapped flashings fitted to Cavitrays. The Cavitray lapping arrangement provides a second layer of water arrestment and the flashing anchoring within a bosem jaw guards against capillary ingress.

The use of recessed pointing on any curved masonry is best avoided as it can act as a conduit along the wall surface. Flush pointing is favoured as it offers a satisfactory uninterrupted finish and promotes a solidity of masonry and tray bond. In terms of cost effectiveness, flush pointing also eliminates the need for further attention from the mason. Only the tray flashings require dressing later in the build programme.

Finally, the water penetration capacity of a wall is directly influenced by the type and composition of masonry used. Bricks or similar masonry units of low absorbency will act like a raincoat initially causing water to run off / wash over the adjoining masonry. In identical weather conditions, bricks or similar masonry units with high water absorption will soak up a considerable volume of water like a sponge, initially reducing the amount being passed on as a ‘wash’ onto adjacent masonry. Once saturation of masonry has occurred, both types will behave in the same manner. This behaviour underlines the need to design all intersections and junctions to accommodate the most extreme scenario – the eventual extent of exterior water-wash as well as the maximum volume of water capable of permeating through the masonry.

 

Cavity Trays of Yeovil operates concept design and advisory services for which no charges are applied. The Company is the only cavity tray manufacturer awarded European Technical Approval.

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