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.
source https://specifierreview.com/2018/11/09/cavitray-compliant-cavity-walls/
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