In January 2024, revisions within NHBC chapter 6.1 came into force requiring preformed cavity trays to be used for complicated junctions. Using preformed trays is advocated by both NHBC and LABC. It is still permitted to use roll DPC to create long, straight horizontal tray runs, but the internal and external angles, steps, changes of level etc, must be preformed. But how long, flexible roll DPC tray runs integrate with the preformed components has been identified as ‘widely variable’ in a recent R&D programme by Cavity Trays – the longest established tray specialist.
Unacceptable onsite fabrication.
A watertight long-lasting relationship is essential if the arrangement is to remain functional for the projected life of the building. Where the ready-shaped tray is deformable and/or manufactured of a soft/flexible material, the installer is faced with sticking two cavity-spanning flexible surfaces together, that are unsupported across the cavity. Consistently achieving this on site can be difficult. Some manufacturers and BBA certificates state additional support must be provided under joins where the preformed component is soft/flexible.
Failure to correctly integrate converging damp protection measures.
Components from Cavity Trays, including corners, reveal trays, integrated balcony door openings, etc, are different. They are not soft but rigidly preformed with inbuilt shape support. The Yeovil-based company’s manufacturing process and material thickness produces a firm component into which flexible DPC can positively integrate, without the addition of any additional support.
Example of preformed cavity tray from Cavity Trays’ portfolio of more than 3,000 designs.
Rigid trays, such as these, can either have upstands that return into the inner skin or terminate against it – such is the self-supporting capability. This enables specifiers to use roll DPC between tray corners or use preformed tray lengths between tray corners. The benefits of shape support cavity trays include profile compliancy and consistency. Both NHBC and LABC advise integration must incorporate rigid support and be lapped at least 100mm.
Trays manufactured by Cavity Trays provide shape support and shape strength – SSSS – satisfying NHBC 6.1 requirements:
- Have sealed or welded joints which are provided with rigid support.
- The upstand part of the cavity tray should be returned into the inner leaf unless it is stiff enough to stand against the inner leaf without support.
