Various British Standards have been withdrawn in the past few years but one that remains is BS 8215. If you pick up the latest PD 6697 it currently refers back to BS 8215 as a source for competent construction. Unfortunately, BS 8215 contains errors, and adherence to it will result in a structure suffering dampness. British Standards has in the past confirmed such errors exist which prompts the question of how and why this situation has arisen? A good example concerns stepped and staggered gable abutments and the manner in which abutting sloping roofs should integrate.
At this point it is probably a good time to suggest in your imagination you consider standing at the foot of a staircase with a bucket of water in your hand! Throw the entire bucket of water into the staircase and watch it cascade down from one tread to the next. Now imagine repeating the exercise but this time throwing the water into an open plan staircase. The water hits the treads but continues inwardly between them. With no connecting risers, it travels through, beyond and under the staircase slope.
Protecting a sloping abutment correctly requires a DPC staircase with connecting DPC treads and connecting DPC risers. BS 8215 does the opposite, directing construction every three courses so the arrangement is discontinuous, with a gap between the DPC treads.
Regardless of whether one is building in brick, block or stone, the failure to provide a continuous DPC staircase to arrest damp permits penetrating water to soak inwardly, via the masonry and mortar. Dampness does not discriminate. For instance, omitting a vertical DPC to a window opening within a cavity wall will result in inward absorption and damp reveals. Gable abutments are no different.
Exposed to the elements they require thorough attention. Adherence to the BS 8215 stepped arrangement will not prevent damp ingress.
The NHBC suggests the use of cavity trays to protect gable abutments as a preformed way of achieving regulation compliance.
Type X cavitrays manufactured by Cavity Trays of Yeovil are specifically designed for use on stepped and staggered intersections. Trays are available to suit all masonry styles, dimensions and thicknesses. Each tray has anti-capillary features and two integral upstands, one at the tray end and one that rises in the cavity. The rigid end upstand features a top edge flex-link to provide positive integration with the tray above it.
The upstand in the cavity differs in having an integral hinge so it can adjust to suit the as-built cavity width. Compatibility with the cavity width is maintained regardless of whether the construction is traditional, timber or metal framed. Type X cavitrays also have attached flashings already shaped to suit the roof pitch. Once the trays have been built-in, the flashing requires dressing only when the roof finish is in place.
Gable Abutments Product Guide volume 21.
Protecting the Building Envelope volume 22.
(Both publications available upon request)