A zero-carbon school by White Design, demolished last year, was damaged by water ingress ‘largely due to defects in the design’, a damning report has found
The 2012 report by the Building Research Establishment (BRE), obtained by the AJ under the Freedom of Information Act, sheds light into the demolition of Dartington Primary School in Devon just seven years after its completion. The school, built for £6.5 million, had previously been lauded for its groundbreaking use of sustainable architecture.
The report criticised White Design for not specifying adequate waterproofing materials and failing to include a vapour control layer on the warm side of the building insulation in its design – needed to prevent condensation damage.
‘White Design Associates fell below the level of competence expected of the architect’s profession,’ said the report, ‘in failing to comprehend the need to control the flow of water vapour through the envelope of the building and in failing to provide a roof design that was fully watertight.’
The report, Investigation of roof leaks at Dartington Primary School, Devon, was commissioned by Devon County Council just three years after the school was completed. It provided a snapshot into the design failings of the school at the time, and called for ‘urgent’ and ‘major remedial works’ to be carried out.
However, in its planning application to demolish the school buildings last year, the council concluded that the buildings were ‘beyond economic repair’.
The council settled out of court with White Design and contractor Interserve for more than £8 million.
Dartington primary school foi response page 073
The school consisted of four clusters of buildings, each consisting of single-storey pitched roof classrooms connected by flat roof areas.
The 2012 report concluded that the leaks in the school buildings with pitched roofs were primarily caused by the use of feather-edge sweet chestnut weatherboarding without a waterproof underlay. It said the leaks to the flat roofs were primarily caused by the absence of a vapour control layer.
The report found the feather-edge weather boards, which ranged from a 25mm to 80mm overlap depending on the steepness of the pitched roofs, did not overlap enough to keep water out. As a result, water passed through the gaps. This situation was exacerbated when some of the boards warped in ‘driving rain conditions’, allowing water to pass above and below the boards’ edges.
Dartington primary school foi response page 059
It noted that, in the specification, these boards are described as ‘rainscreen cladding’. However, the BRE could not find guidance or examples of the use of feather-edge boards as ‘primary waterproofing on roofs’ in a search of best practice publications, including the Timber Research and Development Association, BRE and the Timber Cladding and Decking Association.
White Design used Pavatherm Plus boards, made using waste timber permeated with latex, beneath the roofing. However, the report said these boards are claimed by the manufacturer Natural Building Technologies (NBT) to be ‘water resistant’ for a period of just three months, after which the boards will break down allowing leaks to occur.
The report added that, although ‘poor workmanship’ was evident in the roof construction, this was ‘not the cause of the water ingress’. It stated: ‘The basic problem with the design of the roof is the absence of any effective waterproof layer.’
The report also found that the leaks in the flat roofs were primarily caused by condensation forming beneath an impermeable membrane and that this was the result of a failure to install a vapour control layer beneath the insulation.
It pointed out that, according to NBT guidance, a vapour control layer is needed on flat roofs.
In addition, the report stated that ‘a similar problem of water vapour causing condensation’ appeared to be affecting the timber floor panels, which the walls rested on, and photographs showed that condensation was causing fungal growth to take place, causing these panels to decay.
White Design Associates told the AJ that it was unable to comment on the report
The report said the BRE had not been requested to look at the damage to the floor panels, but that this was ‘potentially more serious than the roof leaks’ and urgent action was needed to control the decay ‘before it causes irreparable damage’.
White Design Associates told the AJ that it was unable to comment on the report, as the firm is bound by a non-disclosure agreement following the out of court settlement. Interserve declined to comment.
Dartington primary school foi response page 056
The report did acknowledge attempts by White Design to rectify the design of the buildings during the construction process.
For example, it noted that White Design raised the issue of using plain-shanked nails instead of ring-shanked nails to fix the weather boards, which the report said ‘did not provide sufficient resistance to the moisture movement’, resulting in the ‘gaps between the courses to open further’.
However, White Design accepted the use of the plain-shanked nails following the response from Interserve, which said that ring-shanked nails were not available for the nail gun in use.
White Design should have understood that the design they were proposing was stretching the boundaries of the systems they were specifying
Towards the end of the BRE’s report, it concluded: ‘White Design as architects should have understood that the design they were proposing was stretching the boundaries of the systems they were specifying and that it fell to them to understand exactly how each material was to be used and the conditions under which each material would have to perform in the completed building.’
The report also found: ‘The damage that is taking place is largely due to defects in the design carried out by White Design, which failed to take proper consideration of the condensation risk within the flat roofs and beneath the floors, and also caused a roof to be constructed which relies on wood fibre panels treated with latex to exclude moisture.’
A Devon County Council spokesman said: ‘Devon County Council has settled its claims in relation to Dartington Primary School in the aggregate amount of £8,015,000.
‘As is common in claims of this nature, no admission of liability was made by any party.’
Children at Dartington Primary School have been taught in temporary classrooms on the site since 2014. Construction has started on a replacement school, designed by Atkins, which gained planning permission in October. It will provide 315 primary places and a 30-place nursery school.