Written by James Long.

Lack of care on site during construction, is the primary reason given for faults in house-building. A report and major survey by the Building Research Establishment some years ago concluded the estimated additional cost of rectifying such faults to be at least five times the cost of getting it right in the first place.

Cavity walls are excellent at keeping properties dry and insulated, as long as they are properly constructed. The external leaf of a cavity wall is designed to accept penetrating rainwater that runs down its' inner face. Every wall-tie within the cavity must be positioned to shed water tracking across it, before such water reaches the inner leaf. The principal is simple and well established. Similarly, all lintels that provide connection between external and internal leaf must have a dampcourse to arrest penetrating water and prevent it from using the lintel body as a bridge. The principal is the same.

Why then is so much money spent every year carrying out remedial works to cavity walls? The answer lies in the failure to appreciate the cavity wall operates three-dimensionally, and there is commonly a need to arrest penetrating rainwater within a wall that changes status. The same external leaf can become an internal leaf, and the requirement is to prevent water in an already-wet skin from travelling downwards to where the skin becomes 'internal'. (1)

The wet external skin becomes an internal wall below the roofline.

An everyday example of this detail is at the intersection of a roof abutting a cavity wall. The same principal applies whether the roof is horizontal or sloping. Such intersections require correct detailing if problems in the long-term are to be avoided. If the intersection is 'flashed only' (see 2), the arrangement is insufficient to ensure a watertight and damp-free structure. Wind-driven rain can quickly saturate the external skin above the intersection and by-pass the flashing. The wall below soon becomes damp.

Use of conventional dampcourse material can be employed within the cavity, but such an approach is wholly dependent on site labour maintaining a constant and functional approach. Illustrations 3a and 3b demonstrate just two of the accompanying problems. How do you support the DPC at all times if the coursing of the blockwork internal skin does not present itself at a convenient height? How can you ensure the union between DPC and flashing is positive and sealed? - difficult given that the flashing will have to be introduced at a later date in the build programme by another tradesman. If an attempt is made to use this method on a sloping abutment, conventional DPC is not the easiest material to use to create the three-dimensional format necessary to adequately protect the junction.

The use of pre-shaped cavity trays provide the answer, and there are many from which to choose. Cavity Trays of Yeovil provide a safe approach, with a tray incorporating a cavity upstand that is adjustable. In simple terms it means the tray always fits the wall and protection is always across the whole cavity. Bricklayers find this feature invaluable, as it ensures compliance at all times of NHBC, Zurich Municipal and Building Regulations. Because each upstand is adjustable, the tray is equally at home in the traditional build or timber frame construction. At all times the status of the cavity is protected. A protective barrier is established between the external and internal skin, as well as at roofline level, protecting the important area of status change. The lead flashing is also ready-shaped and bonded to each unit. (4a and 4b)

The tray used was the type X cavitray which is supplied with its own pre-shaped lead flashing already attached. The back of the tray is hinged, so that it automatically adjusts to suit the cavity width encountered. It ensures compatibility and 100% fit every time. Given the cavity width variations witnessed on site, this feature was considered invaluable.

When rain penetrates an external skin, dampness permeates in all directions. Every cavity tray must therefore also arrest any dampness being absorbed and tracking sideways, - into the roof area. The arrangement of trays must follow the roofline, creating in profile a staircase with connecting treads and connecting risers. (5) Arrested water cascades down the 'tray staircase'. Imagine the arrangement without the risers, and water arrested horizontally can freely backtrack into the area that is meant to be protected. It sounds simple and it is if one uses an approved cavity tray with a harmonising link upstand such as the type X. Unfortunately not every manufacturer has addressed this aspect to the same extent and thoroughness.

 The essential accompanying requirement of any tray system is that the installation procedure must be easy to follow and put into practice. Cavity Trays of Yeovil overcome installation problems by doing two things. Firstly they provide an instruction requiring just one measurement, which the mason observes to achieve the optimum layout. Secondly, the company provides back-up with staff who are delighted to provide 'hands-on' help to ensure the masons feel confident and have no queries. Given that many of the staff are ex-bricklayers themselves, this service for which there is no charge is clearly beneficial and sensible. It is worthy to mention that the Yeovil company also build-in a commendable installation tolerance margin to accommodate build irregularities, - acknowledging the mason may have a bad day! The tray mentioned in this feature is accompanied with product liability at no additional charge, providing cover for the architect, builder and client.

 CONCLUSION.

Clear and concise specification of preformed DPC components can raise standards on site and eliminate site mistakes. With simple and realistic installation procedures, the site operatives also benefit in terms of speed of build as well as quality of build. This advantage is directly conveyed to the building owner, with the virtual elimination of in-built problems. Ultimately, the focus of getting it right first time is always initially dependent on specification. (6)

I have been asked to clarify that this company is the only authorised manufacturer of approved type X cavitrays, and they should not be confused with any other company.