Masonry facades

In contemporary construction (and since the middle of the 20th century) external masonry has taken the form of cavity work, with a large air gap serving to break capillary action and movements of water due to gravity. The design of damp-proof courses or cavity trays had to evolve to deal with the sometimes surprising amount of water that can penetrate a cavity - not just a small amount of leakage but in an exposed area many litres.

Cavity tray detailing is often taken for granted, it being assumed that the contractor is familiar with the essential design requirements. Requirements such as cavity insulation and structural wind posts all serve to make careful detailing and workmanship essential. Construction drawings are often represented in 2 dimensions, which makes an appreciation of the complex needs of a difficult joint hard to anticipate at design stage. The inevitable consequence is that the contractor muddles through during construction.

Most people would accept that a roof needs to be watertight, and an effective gutter must have watertight joints and a means of draining water. Matters such as missing upstands, improperly formed joints and missing sections of roof membrane are all self-evident and would not usually be tolerated by a layman.

However, when it comes to cavity tray detailing, the construction industry often fails miserably. Too often, you find gaps, open laps or missing trays in what should be common sense construction. Water penetrating the cavity will move down the face of the outer leaf until it reaches an obstruction of some kind - a window opening, a floor or a shelf angle, for example. To prevent that water from travelling into the building the cavity tray has to serve as a watertight gutter and must have an effective means of disposing of that water. Cavity tray detailing is straightforward if you treats the tray as a watertight gutter.

BS 8215:1991, Code of practice for design and installation of DPCs in masonry construction, provides guidance on cavity tray detailing, but there are so many permutations in building that it would be impossible to provide a library of standard details to suit every eventuality. Common sense has to apply - forget the outer leaf, if this was a roof how would you detail it?

Possible defects in cavity tray detailing include:

• Sheet materials used instead of pre-formed components at corners: folding a sheet material to suit a cavity tray profile is easy when dealing with a straight run but forming a stop end or a corner is virtually impossible unless pre-formed trays are used. Often, stop ends are simply turned up from the same sheet material, but this causes the tray to buckle with the risk that water can tip off the end of the tray.
• Trays perforated by internal features such as windposts: if detailing a roof, you would expect to see proper upstands around any features that passed through the covering - cavity tray detailing is exactly the same.
• Lap joints unsealed or even unlapped: if making a roof watertight it would be usual to ensure that joints in the membrane are properly lapped and sealed to make them watertight, yet a failure to seal a cavity tray joint is a frequent occurrence.
• No stop ends to cavity trays: A gutter without a stop end allows water to cascade down the face of a building in an uncontrolled yet concentrated manner. A cavity tray without a stop end is exactly the same, yet water is free to pass into the cavity in an uncontrolled manner. Stop ends should always be provided at the end of a tray - leaving water to cascade down inside the cavity until it meets a new tray is not a good idea.
• Gable abutment trays incorrectly detailed; risk of water draining back into the roof void: the detailing of trays at abutments is straightforward in conceptual terms but can be difficult to visualise properly on site. At a gable abutment the cavity tray needs to comprise a series of individual gutters each with a stop end and each overlapping the tray beneath. The stop ends must be arranged so as to occur at a point outside of the roof line. Alternatively, a fully jointed and sealed stepped arrangement could be constructed.
• Cavity trays not bedded in mortar: if the DPC material is laid dry onto brick or stone, the chances are it will create a myriad of capillary routes for water penetration, or larger passages if the material is not absolutely flat. However, failing to bed is a common fault, principally because it is difficult to provide a bed of fresh mortar, DPC and further layer of mortar within the confines of a standard 10mm joint.
• Vertical DPCs and horizontal trays not lapped properly: Vertical DPCs must lap into horizontal trays not behind them so that water can discharge into the tray.
• Incorrect location of cover flashings at mono-pitched roof abutments: The cover flashing should be positioned below the lip of the cavity tray or DPC and not above it, to avoid water being discharged beneath the flashing.

Case study

The building is a purpose built office constructed during the 1980s. At top floor level there is a projecting balcony gutter and a masonry spandrel panel beneath half height ribbon glazing. The spandrel wall faces south-west in a moderately exposed location. A typical section is shown in the drawing below.

Serious water penetration had occurred over the years since construction. The wall plaster had perished and the timber skirtings decayed. Given the proximity of the affected wall to the external gutter, the property manager had assumed that the asphalt lining to the gutter was defective and had arranged for the gutter to be lined with bituminous felt - the application of which was difficult owing to various projections and balustrade details. Attention was also given to the lead flashing and weepholes, the latter being sealed with mastic following the assumption that water was being driven into the cavity during high wind. The joint between the flashing and the brickwork was sealed with copious amounts of mastic sealant.

None of the remedial measures had been effective. This suggested that it was the cavity tray and not the gutter that was at fault. The wall was therefore wetted over a period of 3 hours and then opened up so that the arrangement of the cavity tray could be determined.

The photograph below illustrates the findings of the investigation.

The well-intentioned remedial works had probably exacerbated the problem by restricting drainage, while the main problem was a poorly-installed cavity tray. The joints between lengths of tray were not exposed, but given the standard of workmanship they must be highly suspect.

Several remedial options were considered including rebuilding the spandrel brickwork and installing a new tray or alternatively providing a rainscreen cladding system over the brickwork.

The bottom edge of the cavity tray stops short of the outer face of brickwork. The sealed weepholes and flashing prevent or at least restrict water drainage from the cavity. The water is then free to pass under the tray to the inside.

The lead flashing has been removed to expose the top of the upstand. Water trapped behind the flashing can only run down the face of the brick to be held by the poorly bonded felt against the bedding joint. The flashing had also been stuck to the upstand, further restricting drainage.