Water Leakage in the Forbidden City

"Upon careful re-examination, the originally reported leaks amounted to 152 locations. Among them, 78 places had been repaired in the 34th year of Qianlong’s reign and again in the 36th year, yet are now leaking once more. Another 16 locations were repaired in the 34th year but are now leaking, while 25 places were repaired in the 36th year and are now leaking again. These 119 locations were all repaired in the 34th and 36th years, meaning more than three years have passed. Moreover, this year’s rainfall has not been particularly excessive, so how could the leakage reach such an extent? The supervisors in charge, Yicao and Qi, cannot evade responsibility for this."

—— August 8th, 1773 The Proposal for Repairs of Leaking Areas Within the Palace

"今细加查核，原奏渗漏一百五十二处，内有乾隆三十四年修过，三十六年赔修过，今又渗漏者七十八处；三十四年修过，今渗漏者十六处；三十六年修过，今渗漏者二十五处。以上一百一十九处，俱系三十四、三十六等年动项修理，未满三年，今岁雨水又不过多，何以渗漏至此？各该监怡草齐之咎，实所难辞。"

——乾隆三十七年八月十八日 奏为宫内各处渗漏处今拟动项修理事折

This study examines the historical water leakage issues in Yangxin Hall 养心殿, a prominent structure within China's Forbidden City, through a comprehensive analysis of archival documents and architectural evidence. By reconstructing the history of leaks, analyzing repair methods, and investigating architectural modifications, this project provides insights into the building's long-term water management challenges. Our research employs structural modeling and considers factors such as design modifications, material degradation, thermal stresses, and environmental influences. The findings reveal complex interactions between original design elements and later additions, particularly in the Tiangou area, which significantly impacted the building's water management capabilities.

"On the front slope of roof of Yangxin Hall and the east four sections of the Baosha are experiencing leaks."

—— October, 1764

"In front of the eaves of Yangxin Hall, there is one leak at the east end and three leaks at the west end of the large Tiangou; both the east and west gutters behind the hall are leaking, two leaks at the east end of the front eaves of the rear hall, and three leaks in the large gutter at the west end."

—— August, 1772

"One leak in each of the east and west large Tiangou of the front and rear eaves of Yangxin Hall, one leak in the small sky gutter of the east corridor, one leak in the pillar-hugging of Meiwu, and one leak in the outer eave of the rear hall corridor."

—— August, 1782

……

"养心殿东暖阁前檐渗漏二处，西暖阁天地香亭大天沟渗漏一处，梅坞西抱柱渗漏一处。"

——1785

Yangxin Hall

History and Introduction of the Structure

Yangxin Hall, also known as the Hall of Mental Cultivation, stands as one of the most significant structures within China’s Forbidden City. Spanning over five centuries, it evolved from a minor building to a crucial center of imperial power.

Yangxin Hall served as the residence of emperor for over 300 years during Qing Dynasty.

Yangxin Hall comprises five distinct sections as the configuration before it was converted into a museum: the main central hall served as administrative offices and a space for the emperor's study and leisure activities, the back hall functioned as the residence, two side halls were designated for Buddhist or Ancestor worshipping, the surrounding buildings served as living quarters for concubines, and a perimeter house, situated right outside the Yangxin Gate, was used as the guards’ quarter.

Timeline of Yangxin Hall's Development

Layout of Yangxin Hall

Gable and Hip Roof with Single Eave

Bargeboard

Main Ridge

Diagonal Ridges

Historical Records and Data Analysis

Data are collected and analyzed from historical records from Yangxin Hall. The analysis reveals a complex relationship between structural interventions and leak occurrence. The graph shows periods of higher and lower leak frequencies, often correlating with significant renovations. Notably, the 1847 renovation of the curve connection roof area had a profound impact, effectively eliminating leaks for nearly three decades. This extended period of improved performance strongly suggests that addressing underlying structural issues, rather than just surface repairs, is crucial for long-term leak prevention. The cyclical nature of leak occurrences is evident, with periods of increased frequency followed by decreases, often coinciding with renovations. This pattern indicates that while interventions were effective, they weren't permanent solutions, pointing to ongoing structural or design challenges.

Water Leakage Location

Water Leakage Frequency

An examination of the repair methods employed over time provides valuable insights into the recurring issues faced by Yangxin Hall and the evolving strategies to address them. The analysis of the frequency of water leaks in Yangxin Hall in the aspects of seasons, damages caused by the leaks, as well as the repairment methods applied during past 500 years allow us to narrow down the causes of water leakage and increase the understanding of the impact of various factors, such as the precipitation and temperature difference, on this historical structure.

What are some common repair methods?

● Structural components replacement

● Glue roof tiles

● Apply decorative ironwork

● Adjust ridge tiles

● Damps

SUMMARY

Common Leakage Reasons

What are some common leakage reasons?

● Wood structural component decay

● Roof tile loose

● Wood structure damage

● Gutter dripping

● Full leakage

Structural Analysis

Based on archival analysis an initial hypothesis is that one of the primary factors contributing to the persistent leakage issues was the incompatibility between the original architectural design and subsequent modifications.

Considering the potential incompatibility of the architectural phasing, a key element would be the flying rafters, a component designed for both aesthetic and functional purposes in high-standard Chinese architecture. This component was typically connected directly to the roof sheathing board by three evenly-spaced nails without any further structural support.

The added extension implement additional pressure to the flying rafter, further causing potential movement inside the wood structure.

Extension of Yangxin Hall

Section of Yangxin Hall

Conclusion

This year-long research Reveals a complex interplay of several factors influencing Yangxin Hall's long-term water management challenges. Architectural modifications significantly disrupted the original system. The Tiangou area (1&2), acting as a transition between the original structure and later additions, was identified as a key vulnerability. These modifications changed the original structural load, causing further structural deflection and movement to other components(4). Combined with material degradation over time(5) and Beijing's harsh climate(3), the change of various components created new pathways for water infiltration.​

This research extends beyond the specific context of Yangxin Hall, offering valuable insights for the preservation of various architectural forms in ancient Chinese structures. The findings provide practical guidance for preventing and remedying rain leakage in gable and hip roofs, common features in traditional Chinese architecture. By identifying key vulnerabilities and effective intervention strategies, this study contributes to a broader understanding of water management challenges in historical buildings.

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