Hong Kong's stock of old buildings is rising, as those built during the construction boom from the 1950s to the '70s age. Those managed privately or under government stewardship are inspected and repaired. Of urgent concern are housing blocks with no owners' committee or management supervision. William Xu reports from Hong Kong.

About one in five of Hong Kong's residential high-rise buildings risk collapse if not repaired. They have reached the critical half-century age alert. Buildings have a lifespan. There will be 500 more buildings joining these ranks every year from now, growing the "50+" stock to more than half the city by 2050.

Hong Kong's high humidity, summer temperatures, typhoons and rainstorms, corrode pipes and metal structures, crack cement surfaces, spall concrete and peel facades over time.

The Hong Kong Institution of Engineers has warned that the number of buildings at risk will jump from about 5,000 now, to 20,000 by 2030. It urges prompt action to keep pace with the rising stock of "at-risk" buildings. The Business and Professionals Alliance for Hong Kong (BPA) has suggested the city redevelop 750 of these potentially dangerous buildings every five years.