Multi-storey car parks

The premises comprised a roof top car park located above a provincial shopping centre. The deck provided access for servicing the shop units as well as visitor car parking. The development dated from the 1960s.

In 1996, due to progressive defects in the asphalt surfacing of the car park, the then managing agents concluded that out of a number of options, the most suitable would be the provision of a liquid-applied, polyurethane resin-based, waterproofing wearing surface incorporating a layer of quartz granules. The selected system had an Agrément Certificate, was supplied by a European manufacturer and laid by licensed operators.

Work started in July 1997 on the first of what was intended to be 4 separate contracts, the intention being that the majority of the works would be completed in the first 3 phases, with the car park ramps forming the final fourth phase. The works involved the retention of the original asphalt wearing course save for any defective areas, skimming the asphalt to achieve a level surface and then spraying a number of layers of various resins, which when cured would provide an impervious barrier to water, while permitting moisture to escape from below the surface.

Defects in the new surfacing soon began to appear in the first phase works and these were repaired while the contractor was still on site. By the end of the final phase, other similar defects had begun to appear in a number of previously treated areas (mainly in the running aisles), by which time it was apparent that the scale of defects was greater than first thought.

A dispute then arose between the employer and the contractor over payments and retentions. Further expert evidence revealed the problem to be with the asphalt substrate over which the membrane had been laid rather than with the liquid-applied membrane itself. The cause of the asphalt failure was due to water held within the concrete slab under the asphalt which, due to the effects of heat (from solar gain), had caused water vapour to create blisters within the asphalt. Over time the asphalt above the blister thinned and collapsed under traffic load.

After a protracted dispute over liability, the contractor returned to site to execute remedial works, but before they were complete went into liquidation (coincidentally along with the managing agent). Given uncertainties in the performance of the new coatings, the employer took the decision to replace the waterproofing in its entirety with new asphalt.

Note the final outcome. The cold-applied membrane was a highly-engineered product and capable of working under extreme conditions. However, it was no substitute for a poor substrate. At the time of its repair, only one UK-based operator was licensed to use the product and this left the employer vulnerable to the effects of poor trading conditions.