The first thing a flat owner should do if he or she becomes aware of a leak is report the case to the property's management office as well as the Food and Environmental Hygiene Department. The department will inspect your flat to check whether there is leaking and establish the source.

The department's service was free of charge, but had limitations, Wong warned. 'Owners should understand that the department will spend less than half an hour to inspect your flat. That may not be enough time to conduct a full inspection and identify the cause of the leak,' he said.

To ensure a speedy resolution and keep liabilities to a minimum, flat owners suffering from a leak in a flat above could appoint a loss adjuster to handle the issue, said Wong.

The adjuster will thoroughly investigate the source of the leak on his own or hire a waterproofing company. He would then issue a report on the outcome and assess the damages, which would help an owner seek compensation.

The service would cost HK$30,000 to HK$40,000 for a 500 to 600 sq ft public housing or mass residential flat, while a survey conducted in mid- to high-end estates could cost HK$50,000 to HK$60,000.

Alternatively, an owner could hire a waterproofing firm to investigate.

Edward Wong Kwok-cheung, project manager at Mayer Waterproofing, said his company would issue a report identifying the sources of the leaks and advise flat owners of their rights. The service would cost between HK$2,000 and HK$3,000.

The firm will use what is known as 'pond tests' or a microwave moist detector from Germany to identify the sources of a leak.

'We can use the detector to measure the humidity of the affected area and then draw a graphical visualisation by using the figures,' he said. 'This helps us to find out the reasons for the leakage, even if the flat owner upstairs is not willing to let us visit the flat,' Wong said.

Stewart Wong of Professional Surveyors said there were four basic causes of leaks in residential buildings, namely leaking water pipes, insufficient waterproofing in bathroom floors, leaking drains, and leaking water closets in toilets.

'Sometimes it may be due to the design of the housing estates, and Mei Foo Sun Chuen in Kowloon and South Horizons in Ap Lei Chau are among the examples,' he said.

'Mei Foo Sun Chuen is an old housing estate and all bathrooms in the flats are directly above one another in the same locations,' he said.

'The drainage and water supply pipes at South Horizons are placed in the same areas on each floor as well, and if there is a leakage in any flat upstairs the water pours down into the flats on the lower floors and it is not easy to fix the problem.'

Edward Wong of Mayer Waterproofing said that after a waterproofing company identified the source of a leak, a report was issued to the flat owners.

'They could use the report to pursue the party responsible for the leak and get them to fix the problem. If the party refuses, we suggest the client fixes the problem on their own initially and institute a claim later because it could take two to five years to seek compensation if you take legal action and in the meantime the damage bill grows.

Stewart Wong echoed this view. 'Flat owners should try to settle the case before going to court,' he said. 'We came across a case of a flat owner who asked for compensation arising from a water leak in the flat above of about HK$40,000.

'But the flat owner on the upper floor would not admit responsibility and the matter went to court. It ended up with the two parties having to pay total legal fees of about HK$800,000.'

Flat owners could take disputes to the Small Claims Tribunal where compensation sought is less than HK$50,000. For compensation claims ranging from HK$50,000 to HK$1 million they have to go to the District Court, and if they are looking for compensation of more than HK$1 million they have to argue the matter before the High Court.

'But they could also appoint a mediator to settle the case before a trial starts to avoid paying expensive legal fees,' said Wong, who pointed out that such a service typically cost HK$500 to HK$1,000 an hour. But there are mediators who work for no fee.

For flat owners who want to avoid costly legal fees during a dispute over a leak, Edward Wong suggested using a waterproofing injection method to temporarily fix the problem.

'It costs between HK$4,000 to HK$5,000 for an area of 10 sq ft, but may have to be repeated a second time. Also, it could last only a half a year or a few years and if the owner in the flat above doesn't act to stop the leakage, the problem could arise again,' he said.

New-home buyers should take care when inspecting a flat to check whether there are water stains on the ceiling and on the outside walls of bathrooms, he said.

'If you find the skirting outside the wall of a bathroom and the paint on a wall is new, this may be because the flat owner has tried to cover up a leakage problem,' he said.