When an existing house is constructed with cavity walls, extending the building should always follow the ‘parallel rule’ – the term applied to both skins maintaining an equal distance apart. It is also important to consider how the existing house walls will interface with the new walls:
- At roof intersection level the original outer skin of the existing property becomes an inside wall below the abutting roofline. The exposed wall will saturate during wet weather. Measures should be in place to prevent dampness gravitating below the roofline from outside to inside.
- Where the cavity skins attach to the original house wall, both skins of the extension should follow the parallel rule and the cavity link with the cavity of the existing building.
Extensions with flat and mono-pitch roofs are usually horizontal where they abut the existing house wall. The insertion of Type E Cavitrays into the wall provide a swift way to Building Regulation compliance as they arrest wet from continuing its gravitational path and channel it out of the wall via Caviweeps. Type E trays require only one course of brickwork to be removed and each tray is interlocking, enabling a continuous protective run of trays to be formed with ease and without even entering the building- all work is carried out from the outside.
Where the cavity skins of the extension link with the original house wall and traditional bonding is rejected in favour of using metal profiles to secure the extension skins, the need to maintain the parallel rule (or an equivalent version of it) comes into play. This approach involves cutting of a small chase vertically between profiles into which is inserted an insulated Type B vertical DPC. The Type B introduces discontinuity of the masonry skin and vertically blocks the permeation path penetrating water could otherwise take.
Both the Type E and the Type B are manufactured by Cavity Trays of Yeovil which is not only the longest-established specialist in this field, but also the only cavity tray manufacturer awarded European Technical Approval. There are over 200 specialist products manufactured by Cavity Trays, that offers a free scheduling / take-off service.
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