John Potter Architect | Architectural services | Design:refurbishment

Single Joint Expert: Investigation and Report

Project: Hipperholme Grammer School.

Client: Ford & Warren, Solicitors, and Messrs Cayton and Co, Solicitors.

Above: photographs of an existing elevation incorporating the external cladding system.

We were instructed as a Single Joint Expert (SJE) by Messrs Ford & Warren, Solicitors, on behalf of Hipperholme Grammar School, and by Messrs Cayton and Co, Solicitors, on behalf of Pickles Architects, to investigate the external rainscreen cladding to The Armitage Wing of the School.

The building itself is a portable structure, relocated to the School some years earlier under a separate contract. Architects were invited to develop and implement a scheme of overcladding to enhance the external appearance of the block. The material chosen by the architects was a cellulose and resin based external cladding system, manufactured using cellulose fibres, pressure bonded into a resin matrix.

The manufacturer's recommendations for the fixing of the panels required a system of vertical counterbattens to allow a free current of air to circulate behind the panels and to allow any penetrating rain to drain safely out of the system. These in turn were intended to be fixed to the existing structure after insulation, and a breather membrane, had been installed.

The architect prepared drawings requiring the panels to be installed in accordance with the manufacturer's instructions. A tender was issued and the successful Contractor entered into a JCT Contract with the School for the intended works. The Contract Sum was some £96,000 + VAT. The Works proceeded, with the architect undertaking periodic Inspections, and Certifying the Works for payment by the School.

Some time after the Works were completed the School noted part of the first floor structure becoming detached from the external wall. Consultants were instructed, and considerable funds were expended investigating the problem and then providing additional fixings between the floor structure and the external walls. The problem was attributed to poor workmanship when the units had been initially prefabricated.

Sometime thereafter, the external cladding panels were noted to be bowing to a considerable degree. At that point we were asked to investigate the problems. We arranged for a typical section of the cladding to be scaffolded and, with the original architect and contractor on site, we began to remove panels.

It immediately became apparent that the panels had not been installed as required: there were no counterbattens, and panels were sealed at each joint. Moreover, the breather membrane was in fact a layer of bituminised paper with little or no vapour permeability, making the entire outer skin virtually vapour impermeable.

The entire outer skin behind the panels was saturated with condensation, and the timber bearers providing fixings for the sealed panels were in an advanced stage of decay, after only two years in-situ.

Not only had the contractor failed to execute the Works as required, but the architect had failed to inspect the Works with sufficient care to note the incorrect application. Furthermore, it became apparent that the structural failure of the floor, corrected by the earlier remedial works, had in fact been caused by the high levels of condensation. This had saturated the external face of the timber floor framing beam to such a degree that it deflected outwards. This had caused the joints between that beam, and the floor joists, to be pulled apart and the floor to dip as a result.

Our Report reflected the failure of the Contractor to execute the Works in accordance with the Contract, and the failure of the architect to inspect the Works with sufficient care.

The moral of the story? For architects: be extremely careful when specifying vapour impermeable external cladding. And, always inspect the Works with great care. If your Specification requires the Contractor to install a product in accordance with the manufacturer's recommendations, make sure you have read and understood those recommendations. And, accept no alternatives! For Contractors the moral of the story is: always follow the manufacturer's recommendations. And, if instructed to depart from those recommendations ensure that instruction is clearly and unambiguously confirmed in writing, by the architect. For added security Contractors might ensure they advise the architect, clearly and unambiguously, that the specified method of installation is incorrect and is contrary to the manufacturer's recommendations. And, that the Contractor will not be responsible for the consequent failure of product, or installation, or both.

We understand the parties negotiated a settlement based upon our SJE Report.

Above: photographs of the saturated timber bearers behind the external cladding panels. Note the condensation on the inner face of the bituminised paper!