Consider the Relationship Between Inner and Outer Skins
Brick and block dimensions and styles are now more varied and consequently it is common for adjacent skin heights to drift out of level when raising a cavity wall. The height of one laid block no longer necessarily equals the height of three courses of brickwork.
Inspection of window openings within a cavity wall will often show tell-tale signs of skin levelling and alignment, as will the top of most cavity walls where the plate is bedded.
Any difference in skin and course levels can be problematic at higher levels where stepped and staggered gable abutments are present. Gable dpc protection needs to run within the external skin masonry and step down from course to course, matching the angle of the abutting roof slope. The arrangement also needs to cross and protect the cavity and it is usual for support of any dpc or cloak to be provided by building into the inside skin. Thus simultaneous working on both masonry skins is necessary. In the absence of harmonizing course levels, effective and uniform three-dimensional protection can be almost impossible to execute.
The alternative approach is to use preformed cavitrays with adjustable cavity upstands. The Type X Cavitray is specifically designed to protect stepped and staggered gable abutments and requires no cutting, fabrication or site work prior to building-in. It differs from all other gable abutment trays by incorporating a number of design advantages and its manufacturer Cavity Trays Ltd is the only tray specialist awarded European Technical Approval.
Preformed cavitrays do not need to be built into the inside skin. The moulded Type X dpc tray is robust and self-supporting, and requires building into the outer skin only. Each tray has a clear cavity compartment and a hinged adjustable cavity upstand that takes up the cavity width encountered.
Compliance and protection of the ‘as built’ cavity width is assured and deviances in skin levels do not affect the tray or its performance. Accordingly the Type X Cavitray can be used in all styles of cavity wall, regardless of the style or type of inner skin construction. To the front of each tray is attached a ready-shaped flashing.
Several flashing material options are available including lead and synthetic (no scrap value attraction). Once the tray has been bedded into position, no further work is required until the roof finish is in place following which the flashings require dressing only.
Trays are now available to service cavity widths up to 200mm and all dimensions of masonry module. A performance warranty accompanies Type X cavitrays for the benefit of Architect Builder and Client.
Further details within the following new publications available to industry disciplines upon request.
Protecting the Building Envelope. vol 22.
The Book of Wise Decisions – Best Practice Guide. vol 40.