Elevation Appraisal: Potential Penetration Path Problem

Cavity Trays Ltd was asked to visit an attractive church that had been erected ten years earlier. It was built of cavity wall construction, punctuated with some large traditional stone mullions. A sympathetic blend of new with old.

We had been contacted because despite being in a relatively sheltered area, the church was showing signs of internal dampness in and around a double gothic arch situated within an external wall. The side-by-side arches shared a centre stone pier and just above the dual-arches was a large circular window that was also rather damp, predominately around its’ lower half. Some DPC measures had been originally incorporated, but were inadequate or for some reason were not functioning correctly.

Investigations revealed some of the problem was as a result of blending two construction methods. Rainwater penetrating the masonry above the circular window during heavy rainstorms was then being channelled around the perimeter of the circular window. The accumulating water was then discharging at the bottom of the circle, at a position of approximately six o’clock. This discharge point in the cavity was directly above the centre mullion/pier of the two gothic arches. From the drawing you will observe the volume of water had nowhere else to go – it was within a restricted area with the only outlet possible directing it onto the solid mullion / pier.

The mixing of cavity walls (within which water is able and intended to flow) and solid traditional features requires careful detailing. If construction features exist that channel concentrations of water, those concentrations must be minimised and discharged before any converging masonry features occur. In this particular case the demands were too much and the result was dampness in the building.

It was decided to adopt a two-fold approach;

1.As openings were at the bottom of a relatively high tower, the masonry rain-absorption area above the openings was extensive. It made sense to reduce and minimise the potential water volume gravitating to that lower level.

2.Both circular window and gothic arches were to receive new damp protection, that additionally took into account the presence of a central solid pier.

The introduction of a modified circular Type K cavitray incorporating horizontal arresting barriers at three o’clock and six o’clock solved the water volume problem.

Each arresting barrier enabled the penetrating water to be discharged at the far outside end of each barrier - well away from the arches and mullion features. Caviweeps serviced the water evacuation path.

Type BA curved cavitrays were then introduced to protect the gothic arches. Each arch tray had extended sides to act in the same way as the higher arresting barriers. Again, Caviweeps serviced the evacuation path.

The twofold approach relocated concentrations of water away from the protected openings. Following installation of our recommended products, the problems were banished.

Cavity walls are designed to function with water gravitating within the masonry itself and upon the masonry inner face. Elevation appraisals can identify where penetrating water will accumulate and make provision to control and evacuate it.

Cavity Trays Ltd is based in Yeovil, Somerset, and is the longest-established cavity tray company. The only UK tray manufacturer awarded European Technical Approval, it has just celebrated 100 years of specialism. Cavity Trays manufactures bespoke solutions in addition to the widest range of preformed approved products.

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