Parapet Problems

Both skins of a cavity parapet walls are exposed to the weather, and the resultant wetting and drying of the masonry present thermal expansion and contraction considerations that can result in masonry ratcheting and associated water penetration problems.

Location

Manchester. Factory premises constructed with cavity walls. Brickwork external skin. Horizontal parapets to all sides of the block, rising to a height of approximately 800mm. Capped with pre-cast concrete copings.

Problem

Dampness was being experienced in the internal walls immediately below the parapet. Not all walls were affected, and the occupier reported the dampness occurred immediately following heavy rainstorms. The occupier also qualified the dampness had not been noticed during the first two years following completion of the property.

Investigation

An inspection was carried out of both sides of the parapets. The inside (roof) face was not brickwork but instead was finished in rendered blockwork, possibly as a cost-saving measure as the elevations were not visible from the ground. The rendered finish provided the first clue, as fine horizontal cracks were visible, extending the entire length of each wall. On the external face, similar crack lines were detected along the brickwork, but several courses higher. Removal of the pre-cast coping revealed a correctly laid DPC, fully supported on an under-coping closer. Thus this aspect appeared adequately detailed, and the investigation continued in the area of the cracks, which necessitated taking down a small section of wall.

Inside the wall the cause of the water penetration was quite clear. The cracking of the parapet had occurred along the courses in which the lower level parapet DPC was bedded. The DPC arrangement passed through both skins, effectively isolating structurally the 800mm parapet from the lower cavity walling. Expansion and contraction of the exposed parapet masonry had resulted in the telltale horizontal cracks. The conventional roll DPC used within the parapet had been built into the brickwork skin and stepped downwards 150mm towards the 'inner' skin, where it passed right through the bedding course and linked with a weathering flashing. The inward stepping profile had long been a textbook detail, - but one with a serious weakness.

Conclusion

In the first two years the building was not susceptible to a damp problem, but initially there were also no cracks in the parapet. Damp started to appear after the cracks opened up in the twin faces of the raised parapet. The crack in the external brickwork face was along the course in which the roll DPC (for support) had been built-in. Driving rain aided by high wind penetrated the brickwork crack, and because the DPC stepped inwardly, the penetrating water could run on the underside of the DPC until it reached the inside skin at lower level. It is prudent to record another deficiency was discovered when further opening-up of the parapet was executed. Several laps of the conventional roll DPC were not watertight, where they spanned the cavity. Therefore water had another opportunity to enter, albeit this time via the upper surface of the DPC.

Recommendation

The recommendation was for Type P parapet cavitrays to be installed. Type P cavitrays aid the structural stability of the masonry. This is achieved by being built into one skin only, rather than two. Thus the parapet is not separated from the lower masonry, as bonding on the external face is uninterrupted. The Type P also steps outwardly rather than inwardly, preventing the problem of water under-tracking towards the inside. Use and linking of Type P cavitrays is straightforward, as the product is preformed and does not rely on site fabrication.

Section shows the Type P cavitray which importantly steps down outwardly across the cavity rather than stepping inwardly.

Important footnote:

When a traditionally constructed parapet wall becomes saturated with rain and in extremes of heat and cold, expansion of the masonry can prejudice the masonry structural continuity. When this occurs the DPC bedding course in a parapet offers the least line of resistance and acts as a slip-plane, so movement at DPC level in the outer and inner skins is commonly witnessed. Where the amount of expansion is not fully compensated by contraction, progressive movement over the years may conceivably occur mainly as a result of ratcheting. This is easily identified initially by witness lines and eventually by horizontal cracking along the courses in which the parapet DPC has been supported. The alternative approach using Type P parapet trays permits the outside facing skin to rise uninterrupted to coping level. Thus the solidity of the masonry and the bonding is maximized.

Further information:

Type P cavitrays by Cavity Trays of Yeovil. Telephone

01935 474769.

Information provided by and extracted from ongoing survey being conducted within the UK by Cavity Trays Ltd. Yeovil Somerset BA22 8HU