PP Boards Used in the Palace Museum

PP Boards Used in the Palace Museum

    After demonstration by the Cultural Heritage Conservation and Science and Technology Department of the Palace Museum, PP boards (polypropylene boards) have become an innovative waterproof material for ancient buildings due to their four core advantages, which perfectly meet the needs of ancient building conservation: First, light weight and low load-bearing pressure – with a density of only 0.91g/cm³ and a weight of less than 2 kg per square meter, they are much lighter than traditional lead backing materials and do not impose additional pressure on the wooden structural beams and frames. Second, excellent waterproof performance – their dense molecular structure results in a water permeability rate of only 0.01%, and they do not deform or degrade even after long-term immersion, solving the problem of "softening when damp" that plagues traditional thatched backing layers. Third, weather resistance and adaptability – they can withstand temperature fluctuations from -20℃ to 120℃, maintaining excellent dimensional stability in the environment of large day-night temperature differences in northern China and avoiding cracks caused by thermal expansion and contraction. Fourth, reversible installation – they can be laid through mechanical fixation without damaging the original thatched backing layer, complying with the "minimum intervention" principle of cultural heritage conservation.​

    As of 2024, the PP board waterproof layer has withstood 16 years of testing and demonstrated outstanding long-term performance: In terms of waterproof performance, annual inspections by the Palace Museum's cultural heritage conservation department show that the leakage rate of the renovated area is zero, and the moisture content of the thatched backing layer has always been controlled within 12% (the safety threshold for wooden structures), completely solving the previous "eaves dripping" problem. In terms of material condition and conservation benefits, the PP boards show no obvious aging or cracking; laser rangefinder tests indicate that the roof flatness error remains within 0.3‰ (comparable to the craftsmanship precision of the mid-Qing Dynasty). The underlying wooden roof boards and beam brackets show no new signs of mildew or insect infestation, and the mid-Qing Dynasty early-stage colored paintings on the inner eaves of the Hall of Supreme Harmony have retained their colors well due to stable humidity. In terms of industry value, this solution has provided a reference for the waterproof renovation of other ancient buildings such as the Summer Palace and Suzhou Gardens, promoting PP boards to become a preferred material for the conservation of historical buildings.