Go Round the Masonry Bend with Assured Detail
Curved cavity walls and spherical corners have in the past posed problems for designers requiring sloping roofs to converge with the shaped masonry that might be concave or convex. There are a number of considerations:
The dpc within the wall and the flashing need to be of a different shape and size at every intersecting course. This is because the actual angle of intersection changes, despite the roof pitch remaining constant.
How does one determine the optimum dimensions so the protection in each lower course integrates with the protection in each higher course in a manner that is watertight and continuous?
How is the upstand in the cavity supported given that the inner skin is curved and conventional dpc cannot expand to compensate?
What happens within the bedding course where the dpc is required to either elongate or contract as the convex or concave masonry turns – roll dpc cannot do so?
If the inner skin is built in block work, what happens when the nearest course you rely upon to provide support for your dpc is too high or too low?
Externally the flashing is required to act as a waterproof medium to link the wall with the roof tiles. But how can one join the flashing and the dpc so the union is watertight?
How is wind-driven rain prevented from entering under the flashing leading edge where it dresses vertically against the masonry? The amount of water-wash traversing a curved wall is considerably more than that flowing over a straight wall because of gravitation and the windscreen deflection effect.
The solution is to use preformed individual components, each of which is designed to exactly match the requirements at every intersecting course. These are called Type X cavitrays for curved walls and are based on the proven design awarded European Technical Approval.
The advantages of using preformed cavitrays are numerous:
Fabrication on site is eliminated – there is no measuring, no cutting and no wastage. Preformed also means the shape is already determined – installer mistakes or oversights need not be a consideration.
Each cavitray has its own adjustable cavity upstand.
There is no requirement to locate and build into an inner skin course as upstands are self-supporting.
Every tray has an integral end upstand that rises within the masonry at the inboard end to provide a vertical link to the tray above. Thus as each tray is laid it integrates with the adjacent tray.
Where trays are supplied with the flashing already attached the traditional need to rake out joints and introduce flashings separately is eliminated. The union is pre-established.
Because each cavitray and flashing overlaps the next cavitray and flashing, any wind-driven rain that seeps under a vertically dressed flashing leading edge does not come into contact with the masonry under (unlike conventionally cut running flashings). The overlap configuration provides a secondary layer to arrest such water, preventing it from permeating towards and under the adjacent sloping roofline.
Cavity Trays Ltd undertakes to calculate concave and convex abutment requirements / schedules for clients. Each tray is numbered and the mason has only to numerically build in trays as the wall is raised. Petheleyne is used for the manufacture of each tray, having been identified as the most accommodating and durable of all dpc materials tested for such purposes. The external flashing may be selected from either lead or Perform – a BBA approved synthetic alternative.
Cavity Trays Ltd is the only cavity tray manufacturer awarded European Technical Approval. Trays benefit a performance undertaking for the benefit of architect, builder and client.
Further information from Cavity Trays Technical Manual number 22.