Cavity Wall Openings

On the 1st January 2023, the use of conventional roll DPC to provide protection against wet ingress over window and door openings in cavity walls changed.

The NHBC announced within its’ Technical Guidance 6.1/38 that Polythene Damp Proof Course made to BS 6515:1984 will not be accepted on all plots starting or at DPC level on that date.

The NHBC announcement aligns with the usage/restrictions stated within the long-established British Standard, that qualifies such DPC is not suitable where there is downward movement of water. The announcement includes reference to Code of Practice - BS 8215:1991, that provides installation guidance of damp course in masonry construction.

Many will welcome this move, as the downside of any roll DPC is the necessity to cut on site and form on site, with the accompanying variances, errors and inconsistencies that can occur.

The alternative way forward is to use a preformed Cavitray.

When horizontal preformed Type C Cavitrays are used with lintels, one eliminates the variances of a site-fabricated approach. Preformed Type C Cavitrays are ready-shaped to harmonize with the window head construction / lintel choice, so the build detail is always consistent and always correct. Type C Cavitrays are also self-supporting, which means they hold to profile and do not require building into the inner skin. (unless you request otherwise). So, no seeking out a bedding joint at an appropriate height to hold up the protection. Full cavity width arrestment is provided by the preformed tray that is biased to take up the cavity width encountered. Both skins remain independent of each other. Adverse thermal bridging is not introduced.

Importantly, Type C Cavitrays have extended ends that project each side of the wall opening by 300mm. These extended ends serve two purposes: Firstly, they ensure adequate shelter of whatever reveal closing method is deployed. Secondly, the 300mm projection guarantees a brick perp joint beyond the opening is always located to accommodate the stopends in the optimum position. Interestingly, if one looks at BS 8215–fig 15, it illustrates a perp joint naturally occurring to accommodate a stopend, 112.5mm beyond the opening. But if a whole standard 225mm brick occurs (there’s a 50/50 chance it will), the detail depicted is not possible.

Using a preformed DPC tray with extended ends ensures appropriate perps are accessible, thus eliminating the problem.

NHBC 6.1 D6. B refers to window and door openings having an external turn-down drip lip. Whilst difficult to form using roll DPC, where a preformed tray is used, an external lip is one of many options available to the specifier/contractor.

Cavitray scheduling is straightforward – the Company does it for you. Trays are delivered packaged per plot, so stock and cost is controlled. Importantly, a consistent build detail is achievable – every time.

There’s a CPD silent study tutorial that provides more information and some interesting data on the Direct Contact Exhibitions website.

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