What happens when two adjoining design details required in the same property are incompatible?
Cavity Trays of Yeovil has come up with another simple but practical answer:
How to prevent water passing from one dwelling to another via the separating wall as depicted in the NHBC guidelines, when the usual evacuation route for such arrested water is prevented because radon / methane barriers are present in the external walls.
NHBC section 6.6 (external timber framed walls chapter) provides guidance under 6.2(d) on how a party wall should incorporate measures to prevent water that accidentally spills or floods within one dwelling from permeating an adjoining dwelling. The NHBC detail shows a drainage channel into which water can flow and discharge at the open ends of the arrangement.
Extract courtesy of the NHBC
Unfortunately such a shallow channel can be easily breached and/or filled with debris during the course of construction. Furthermore it is only able to accommodate a small volume of water (even if the concrete under is channelled / cast with a fall). Most importantly, if the property in question has radon or methane barriers in its external cavity walls, the inner leaf section of those barriers commonly terminate at a higher level than that of the party wall channel – so the barriers block the ends of the channel. With the barrier in the way, water from the channel has nowhere to discharge.
Given that there is no alternative option for draining the water and the arrangement must maintain gas-tight integrity, merging all design requirements is problematic.
The Type PWRB Rising Party Wall Barrier provides an alternative approach. The base of the barrier acts as a dpc and extends under both skins of the party wall. The upstand acts as a dam against water flooding from within one dwelling contaminating the adjoining dwelling. The upstand is capable of holding back a far greater volume of water than a channel.
Because the profile extends upwards instead of downwards, it can oversail and interface with commonly used perimeter gas barriers. Any water can thus be directed safely into the lower portion of the perimeter wall gas barrier, from which subsequent evacuation is via the already present barrier caviweeps. At all times the gastight integrity can be maintained.
The Party Wall Rising Barrier eliminates the problem of incompatibility, increases the capacity to prevent inter-property water contamination and satisfies the design detail requirement.
It is suitable for use in timber frame and traditional construction.
2400mm x 500mm x 150mm (dimensions easily varied to suit specific site details)
*This cost saving alone is generally sufficient to cover the cost of the Type PWRB – so no additional money is expended on site and the work advances more swiftly. Thus there is a cost and speed advantage in addition to compatibility, greater protection, radon and methane barrier benefits, and the ability of the Type PWRB to provide the dpc element to both skins.
Alternative arrangement to maintain the dry status of the timber skins in the event of flooding is to have them elevated on masonry. The Type PWRB remains at the base of the wall.
The upstand of the barrier extends over and into any gas perimeter cavity barrier that is present. Thus arrested water can always discharge out of the structure. In contrast, a channel outlet is substantially lower and the presence of a perimeter cavity barrier obstructs the usual channel evacuation route. Thus water is trapped and the channel floods into the adjoining property via the party wall. Where masonry courses are present under the timber skins to elevate them as a precaution above floor level, the Type PWRB is positioned between the masonry.