Surveyors warned on Monday that half of the 50 old buildings they inspected on Hong Kong Island pose immediate safety risks, and called on officials to speed up inspection works.

The Hong Kong Institution of Building Inspectors, in collaboration with the DAB party, recently examined several dozen buildings that are at least 40 years old and have yet to be inspected by registered professionals.

Their move comes amid a recent spate of incidents involving concrete falling from buildings.

A former president of the Hong Kong Institute of Architects, Marvin Chen, said after analysing the data that some of the buildings inspected were in a very poor condition. 20 percent of them, for example, had structural cracks.

"For this one, what we normally call concrete spalling, the steel reinforcement comes into contact with water and gets rusted. In the process, it expands and pushes the concrete off," he explained.

"The process will continue, and if the rust becomes so severe that the diameter of the steel reinforcement is reduced, the load-bearing element cannot meet the designed requirements anymore. It becomes a structural problem that needs to be addressed as soon as possible.”

DAB lawmaker Edward Leung said the party will pass on the findings to the Buildings Department for follow-up action.

"[The government] has told us that 4,800 buildings have already received [mandatory building inspection notices], and out of 4,800, 1,800 orders have already expired … so what we have to do is to check up on these 1,800 buildings."