Papers obtained by Weekend Herald show concerns over extent of problem
Leaky homes are probably still going undetected outside the main cities, advisers have warned the Government.
Papers obtained by the Weekend Herald under the Official Information Act show the Building Research Association highlighted the issue last year to consultants, who were asked to estimate the size and cost of the problem for the Government.
Strategic business development manager Dr Wayne Sharman said it took building surveyors in Auckland years to learn how to spot the danger signs.
"When you get outside the main centres the industry at large and the building surveyors are less experienced and will pick up fewer leaky houses," he told consultants PricewaterhouseCoopers and the Department of Building and Housing.
"This is one of the factors which has delayed detections in the South Island."
The report Weathertightness: Estimating the Cost gave a range of possible failure rates, from 22,000 homes (costing about $6 billion to fix) to 89,000 homes ($22 billion).
It described the $22 billion prediction from invited building experts - based on failure rates of up to 100 per cent for houses built with untreated timber and monolithic cladding - as the "extreme view".
The report said government officials recommended grading these homes by risk factors, which produced a "consensus forecast" of 42,000 homes and a price tag of $11 billion.
Working papers show a clash between the consultants' statistical approach and the experts' sweeping predictions that nearly all homes built with untreated timber and monolithic cladding would fail regardless of risk factors.
Department of Building and Housing senior adviser Sian Smith told colleagues that the suggested failure rate of up to 100 per cent would "significantly impact" on the work done so far.
Dr Sharman replied that the high failure rate would not necessarily cause major problems in every house.
Ms Smith also warned the consultants not to stray outside their brief by delving into the causes of leaky buildings.
"The re-estimate is to be done using the agreed methodology and the data collected."
Building and Construction Minister Maurice Williamson said some critics claimed the Government did not want to face the experts' $22 billion estimate, but the papers showed there was no improper influence in reaching the $11 billion figure based on 42,000 homes.
"The Government has accepted PwC's findings, including that there could potentially be up to around 89,000 leaky homes in New Zealand," he wrote in an accompanying letter.
"No one can say exactly what the number of leaky homes is, and the size of the problem is big no matter what way you look at it."
