Concave and Convex Cavity Walls – Roof Intersections
Author/Issuer: Cavity Tray Standards Added:
Be Aware of Deflection Behaviour
When a roofline intersects a concave or convex cavity wall, it presents the designer with an unusual problem. The actual angle at which the roof strikes the masonry varies throughout the intersection – it does not remain constant despite the roof itself being at a constant angle.
The above example illustrates the principle. Because the roof wraps around the wall, the angle changes and becomes for a brief moment horizontal at the lowest point of intersection. Thus the question arises of how best to externally flash and protect the structure against wet penetration?
Cavity Trays of Yeovil has experience in this specialised field and offers curved cavitrays to provide a ready-to-use warranted approach.
The dimension of each tray is individually and compositely computer calculated to take into account the arc of the wall and the masonry coursing. Curved cavitrays can be supplied with the lead flashing already attached.¹
Appropriate flashing dimensions are calculated to provide optimum weathering, deflection and overlap protection. This is an important consideration given the behaviour when wind driven rains are directed against a curved wall (unlike a flat surface). The effect of the 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 lap as well as with the direction of the lap.²
Wind and rain will not discriminate and both will be experienced.
Within the masonry wall the cavity requires protection and this is achieved with self-adjusting cavity upstand flaps that can take up the cavity width encountered whilst lapping to create a continuous barrier.
Curved Cavitrays as illustrated are manufactured only by Cavity Trays of Yeovil and are accompanied with a performance warranty for the protection and benefit of Architect, Builder and Client. The Company will take-off and schedule requirements.³
¹As long as the radius of masonry arc permits attached flashings to be lifted and handled during installation. Intense curves can require varied approach.
²Example: the concentration of water-wash traversing a sq metre of curved surface can (subject to the radius of the arc) increase by 200%, compared with a flat surface where the plane does not promote deflection (windscreen principle) in any direction other than flowing downwardly by gravitation.
³. Recessed pointing should not be employed as it acts as a conduit on the wall surface. Designers are recommended to finish with flush pointing.