Whether a reconstructed stone assembly is new or an existing one being recycled, all joins and junctions between sections and adjacent masonry must be united in a manner that maintains integrity against water ingress. Up to 2.25 litres of water per hour can penetrate one square metre of masonry. If the absorption area (the extent of masonry) above and around a reconstructed stone opening is extensive, the pro-rata flow and demands for water control are extensive. The reconstructed stone mullion window has a requirement for a total of three cavitrays to maintain damp-course integrity across the window head.
At the highest level bedded on the decorative label-mould (string-course) is the first cavitray, the purpose of which is to act as an arresting barrier. It receives the wash of cavity water from the masonry above and discharges it via weep-vents positioned at the extended tray ends, - well clear of the side mullions. A range of caviweeps including discreet models that have minimal visual impact means the water exit points need not be noticeable.
The second cavitray is shaped to rest and bed into the supporting metal angle that provides the load-bearing qualities to the assembly. There is very little cavity water at this level, the majority having been stopped at the higher level.
The third cavitray provides a vertical barrier between the lintel on the inside skin and the reconstructed stone head. All trays are self-supporting and do not require building-in to both skins. Self-supporting trays eliminate the problems encountered when the nearest support course is not at a convenient height. Additionally, thermal break insulation can be supplied bonded to the tray situated at the lowest level, to provide thermal separation.
Reducing the volume of water descending onto and penetrating around a reconstructed stone mullion should always be the objective, together with providing defined exit routes for that water that enters the cavity. Type Y Mullion Assembly Cavitrays can maximise the protection standard and integrity of installation.
A multi-tray approach is also beneficial when a timber lintel is incorporated to provide a traditional cottage style appearance. An innovative approach is to build in the timber lintel so it is purely a visual element, and provide support of the external cavity wall via a steel lintel immediately above the timber. The steel lintel is of a longer length than the timber under it, so its ends project past those of the timber and bear onto masonry. This configuration means no structural demands are made on the timber itself as it adopts a visual presence only.
This arrangement also provides opportunity to introduce enhanced thermal insulation (using sleeved compressible insulation barriers) immediately behind the timber and adjacent to the window head. The window structural opening benefits higher overall thermal integrity along this often less-considered level.
Protection against damp ingress is provided to the opening in two areas, in the form of Type C cavitrays.
A Type C cavitray bedded on the steel lintel arrests most of the gravitating rain permeating the external skin. Supplied complete with stopends, this cavitray discharges the arrested water via discreet caviweeps incorporated towards either end the tray within the masonry areas. At a lower level under the timber is located a second smaller cavitray. This safeguards against wind-driven rain entering where timber and masonry abut, acknowledging that eventually shrinking and settlement will occur.
A Company spokesman commented: ‘We recommend the two level – two cavitray approach. No structural demands are made on the timber, and in future years should the timber require attention or replacement, it can be done without affecting any load bearing aspects of the structure.
Type C Cavitrays are available in profiles and dimensions to suit all opening styles. Whether the requirement is traditional, modern or classical, there are preformed trays to protect the detail.
Compatible compressible insulation barriers are also manufactured by the Company.
Further information may be found within the latest Book of Wise Decisions, volume 44, available from Cavity Trays of Yeovil.