[Photo provided to China Daily]

Sun, who was in charge of the restoration, says restoring the roof was the most difficult part of the process as there are very few similar structures in China, and thus there was a need for continuous experimentation.

"Drainage was another key aspect of the project," says Sun.

"Many structures from the Qing Dynasty have not been repaired for a long time, and many of them have flat roofs built in the Tibetan style. So, if the drainage system does not work well, it can be a potential hazard."

Still, he says, as long as the original drainage works and needs only partial restoration, no modern drainage will be introduced.

Improvements in the security and fire-alarm systems have also been included in the renovation.

Though the main restoration of the structures has been completed, he says, some follow-up auxiliary projects like the indoor decorative paintings will continue until the end of this year.

Tan Pingchuan, who leads the painting restoration, says: "We don't aim to give them a new look.

"The vestiges of previous restorations is also a part of history and need to be maintained. We will reinforce and clean the paintings, but we won't draw something new."

The restoration also means that a lot of data and files have been created on these World Heritage sites, says Lu Qiong, the deputy head of the cultural heritage protection office under the State Administration of Cultural Heritage.

"These files will form the basis for further protection," she says.

"A long-term mechanism of preventive protection is to be set up to enhance daily maintenance under more rigid supervision."