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Convex  Concave and Flat Walls on plan

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Curved walls present the designer with many additional design considerations, some of which may not be immediately apparent.

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 predominately horizontal, but as the wall turns the angle changes and those changes continue until the top of the roof.

The question arises of how to best protect the structure against wet penetration and externally flash the intersection? Fabricating on site is really a non-starter. Every external masonry course up the slope requires DPC protection. The protection is required to be three-dimensional. It also usually requires support from the inner skin and at the same time it must integrate with the protection at the higher course level, whilst turning to accommodate the concave or convex masonry skins. And because the roof intersection creates a different pitch as the wall turns, every configuration can be different!  Such variances apply also to the external flashing of the masonry/roof junction.

Preformed curved Cavitrays provide the solution.  Every 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. Each tray cavity upstands are self-supporting and hinged. This means there is no need to build trays into the inner leaf, as they support themselves. The hinge allows the upstand to take-up the as-built cavity width – so it always suits.

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 across 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. The wash of water traversing a square metre of curved surface is far greater than that experienced with a vertical flat surface where wind-assisted rain moves predominately downwardly by gravitation.

Intersections abutting concave walls are likely to be subjected to more extreme demands compared with convex walls. Water can focus within a dished (concave) surface whereas it will discharge off (around) a convex surface providing the opposite deflection.

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. (Trays with flashings already attached are recommended wherever possible, as they benefit flashing retention within a bosem jaw that ensures positive anchoring and 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 an 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.

Cavity Trays Ltd  Yeovil BA22 8HU

enquiries@cavitytrays.co.uk

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