Cavity Trays’ technical manual provides best practice advice for terraces and balconies
Specifiers should always consider to what extent water might penetrate a balcony or terrace outer cavity wall, and where that penetrating water might end up.
In the example shown, the exposed outer parapet wall to the high level terrace incorporates an inward-stepping DPC with a flashing under to dress over the terrace surface upstand. Whilst acceptable in theory, the arrangement is flawed. When the exposed outer skin saturates, water can track on the underside of the inward stepping DPC, feeding wet directly into the room(s) below.
In situations where there is living accommodation below the terrace, it is essential that the protective DPC arrangement steps outwardly – despite it being shown otherwise in some technical manuals.
A similar scenario exists with doorways providing access onto an upper-level terrace with rooms below. The protective arrangement in the cavity wall must step outwardly. It should also be remembered that the Building Regulation requirement of ‘fins’ on reveal cavity closers guides penetrating water downwardly, so any adjacent DPC horizontal arrestment detail must interface to receive and evacuate water at that level. Failure to do so can allow water to continue downwardly, discharging from closer fins into the area under - that is supposed to be kept dry.
Masonry skins of all thicknesses can saturate, including exterior skins of 225mm (two-brick ) thickness. Beware of potential under-tracking with any inward stepping DPC in a cavity wall.
The 200-page Cavity Trays Best Practice Technical Manual demonstrates how such weakness can be eliminated using preformed outward-stepping approved Cavitrays and compatible interfacings.
Cavity Trays of Yeovil is the longest-established UK company in its specialised field and the only UK tray manufacturer awarded European Technical Approval.
T: 01935 474769
(this is an edited version of an article that appeared in Achitecture Today 2021)