Cavity Trays discusses curved cavity walls
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preformed curved cavitrays
Cavity Trays announces an increase in demand for trays to suit curved cavity walls, which offer many additional design considerations when compared with standard flat surfaced, straight walls.
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 start laying by numbers.
Cavity Trays also takes into account how the wall and its flashings will behave during storm conditions.
The effect of a concave or convex arc is to accentuate the water-wash being blown 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 because the build-up of water-wash traversing a m2 of curved surface can increase by up to 200 per cent.
Intersections abutting concave walls are likely to be subjected to more extreme demands compared with convex walls.
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.
Flush pointing also eliminates the need for further attention from the mason.
Only the tray flashings require dressing later in the build programme.
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 highlights the need to design all intersections and junctions to be able to accommodate the eventual extent of exterior water-wash, as well as the maximum volume of water capable of permeating through the masonry.
Cavity Trays operates concept design and advisory services for which no charge is made.
The Company is the only cavity tray manufacturer awarded European Technical Approval.
Type BA Cavity Barrier Arch cavitrays from Cavity Trays have been designed for use in shaped rather than horizontal cavity walls to provide opening protection. Type BA cavitrays offer total protection to the angled head, extending upwardly and outwardly to prevent the build up of concentrated water wash.
New Pyramid Weep for cavity masonry download
The new Pyramid Weep for cavity masonry from Cavity Trays replaces traditional reed straw systems. Triangular in profile, the new weep provides an aesthetically neutral means of removing water that may permeate a structure. The possible flow width of the weep is maximised due to its flat base which also allows it to bed as flat and low as possible to service water evacuation from lintels, dpc’s and trays. The Pyramid Weep is manufactured using PVCu and, as a result, will not rot.
Cavity Trays is reporting that cavity walls that support an opening using separate lintels will improve the overall thermal performance of the structural opening as the lintels do not connect and therefore do not conduct heat loss. Using this method alongside a self-supporting Cavitray and an infilling profile Cavistock enhances performances levels even further. This arrangement can promote a lintel value of up to 0.05W/mk.
Cavity Trays movement challenges the lastest British Standard PD 6697: 2010 detail that results in the entire parapet masonry above the DPC being separated from the masonry below the DPC and is not the ideal structural arrangement for a highly exposed wall at the top of a building. They outline why the use of preformed self-supporting Type P parapet cavitray is the preferred option.
Cavity Trays, already the only UK manufacturer of preformed cavitrays to hold European Technical Approval, has now secured additional approvals from LABC Registered Details (Local Authority Building Control) for preformed curved and straight cavitrays plus its extensive range of Cavicloaks.