Residual transference in cavity wall masonry
Minimal residual transference can occur in cavity walls where some styles of dpc deployed to provide damp / water arrestment fail to arrest all of it, with the consequence some finds its way beyond the arrestment point. It is often witnessed in old chimney stacks that are not totally protected at high and low levels where the stack penetrates the roofline. In such instances damp patching below the roofline occurs. If this is minimal and restricted to just below the roofline only within an open attic space, whilst not ideal its presence presents no immediate threat to the integrity of the structure.
Minimal residual damp transference at other levels in a building however, are of major concern.
In the case of stepped and staggered gable abutments the arrestment level is required to step within the outer skin masonry and cavity, above the level of the abutting sloping roof. Any damp that bypasses is in a masonry skin that whilst ‘external’ above the roofline, becomes ‘internal’ below it. The same skin becomes an inside wall so it is important total arrestment is achieved and no residual transference can occur.
This is achieved using preformed gable abutment cavitrays that interconnect to create a ‘staircase’ dpc presence. Each cavitray incorporates features to capture the penetrating dampness and control its evacuation, but importantly to address potential residual transference also.
If one considers a dpc tray installed over a window or door opening, its placement within the mortar bed across the opening is relatively level. One end cannot be much higher or lower than the other end. The bedding course thickness limits deviation. But stepped arrangements consist of short lengths, and the steeper the pitch the shorter each stepping length becomes. There is opportunity to build in short lengths with a tilt/back-fall, and so doing can promote residual transference via back-tracking.
The preformed cavitray avoids this potential weakness. The installer is prevented from building in a short cavitray out of level as the base of each tray has a number of gauging drip bars. These provide two functions:
1) They permit the installer to bed the cavitray into mortar until the bars make contact with the lower masonry course. This self-gauging eliminates bedding out of level.
2) The self-gauge bars extend where the tray base projects into the cavity, where they can act as drip bars against discharging water from back-tracking.
These features together with more than 17 accompanying ones have contributed to the Type X cavitray manufactured by Cavity Trays of Yeovil being awarded European Technical Approval – a standard that cannot be claimed by any other tray manufacturer. The Type X cavitray is accompanied with a product performance undertaking for the benefit of Architect, Builder and Client.
‘Protecting the Building Envelope’ © Cavity Trays Ltd 2011.