Masonry Structural Stability and Surface Staining
The practice of providing support to cavity-spanning DPC’s by building into both masonry skins came about because there were few alternative options to hold a soft or pliable DPC in position. Unfortunately, the practice can cause structural concerns where the supported DPC occurs at an exposed high level subject to wind pressure. A cavity wall parapet is a typical example where adherence to the textbook approach results in the entire high-level masonry being separated from the supporting masonry under. An earlier information sheet issued by this Company refers to ‘parapet ratcheting’, a term to describe movement of the masonry above DPC level when temperature changes and wetness cause masonry expansion that is not fully matched by the extent of contraction. Over time the parapet above the DPC becomes out of alignment with the masonry under the DPC.
The accompanying problem of isolating a mass of masonry (especially at high level) is the exposure to penetrating rain and where that penetrating rain manifests? The current PD 6697 mistakenly advises a parapet DPC may step inwardly or outwardly, but stepping inwardly permits water to under-track the supported DPC and trickle towards and into the ‘inner’ skin of the cavity parapet below roof level. The result is a damp internal wall.
Stepping outwardly can prevent this happening, but if the DPC travels through the exterior skin the structural weakness is not addressed and the likelihood of surface staining increases. Pending the type of DPC used, the extent of staining can be severe as the example above clearly shows. (Lead is a durable medium but when used uncoated within Portland mortar it promotes rapid carbonation of free lime). The Lead Sheet Association draws attention for the need to protect lead in such situations by applying an appropriate bituminous coating. In practice, this rarely happens so the lead lift will not be as long as it could – or should be.
The structural stability of a cavity parapet can be improved and staining of the front elevation eliminated by replacing a soft DPC or lead acting as a DPC, with a ready-shaped DPC that is self-supporting. This is provided in the form a preformed self-supporting cavitray. The Type P cavitray by Cavity Trays Ltd is built into the inner skin only and has a downward cavity-spanning section that crosses the cavity and terminates against the inside face of the exterior skin. The exterior skin remains unpunctuated as the preformed cavitray does not project through it. Structural stability and bond remains unbroken. Water is not discharged to cause staining on the exterior elevation.
The Type P therefore projects through one skin only (the inner skin). Where it emerges on the roof side it projects downwardly forming a turn-down exterior lip. This lip provides weathering cover to the bedding course of the cavitray and importantly protection where the roof flashing unites under the tray.
We suggest the advice given in PD 6697 advising parapet DPC’s may step inwardly is in error and is not practised.
Further reading: Protecting the Building Envelope, volume 22.
Cavity Trays Limited Administration Centre Yeovil Somerset BA22 8HU
T: 01935 474769
F: 01935 428223