The department concluded that overloading caused the collapse of the roof of the Hu Fa Kuang Sports Centre, which was filled with hundreds of students taking an exam just a few days earlier.

The report said three factors were involved – a levelled layer of material applied to the surface of the roof structure being thicker than the original design, the laying of greenery on the roof and large puddles of water.

The report said the overall dead load – permanent, built-in fixtures such as drainage systems – imposed on the rooftop structure rose to 5.75 kilopascals (kPa), which is about 35 per cent more than the approved dead load of 4.25 kPa.

The loading from the greenery cover, about 1.32 kPa under fully saturated conditions, also doubled the measurement recorded by City University.

But the report said in this case, the green roof did not constitute building works under the Buildings Ordinance, which meant that prior approval was not needed.

Whether adding a green cover on the roof constituted building works depended on factors like the intended use, its permanence and the extent of the cover.

The university’s spokeswoman said the institution would study the report carefully and seek professional advice to determine necessary follow-up action.

Dr Edward Yiu Chung-yim, the lawmaker representing the architectural, surveying, planning and landscape sector, was not surprised that there would be no prosecutions.

“The Buildings Ordinance simply does not define a green roof as a buildings work ... I think that’s why they are only having the professionals face an internal hearing,” he said.

Yiu said such hearings usually led to a fine or reprimand, which were not as strong a deterrent as a criminal prosecution.

“I understand that the government will issue guidelines regarding the installation of green roofs, but it must also consider amending the ordinance to plug the loophole,” he said.

But former Hong Kong Institute of Surveyors president Vincent Ho Kui-yip disagreed. “Building a green roof is as simple as laying a carpet of vegetation directly on top of a roof, not requiring any permanent, intrusive fixtures to the existing structure,” he said.

But Ho said it was important for organisations to first seek independent professional advice to conduct feasibility studies and designs before tendering out to consultants or contractors to carry out the project. City University did not seek separate advice from an independent surveyor beforehand.

He also said it would be difficult for prosecutors to collect evidence that spanned over 20 years to determine whether the levelled layer of material was thicker than what was approved when it was built in 1990, or whether it was done later.

Shortly after the accident last year, the Buildings Department issued guidelines for green roofs in school buildings, such as seeking professional advice from an authorised person or a registered structural engineer before starting a project in an existing facility.