sagging of vault

例句：

1. 1.The old building's walls were starting to sag, revealing the foundation beneath.
   这座旧建筑的墙壁开始下沉，露出了下面的地基。
2. 2. The weight of the snow caused the roof to sag, making it difficult for anyone to climb up.
   积雪的重量使屋顶下沉，使得任何人都难以攀爬。
3. 3.The construction crew had to repair the sag in the ceiling to prevent further damage.
   建筑工人不得不修复天花板的下沉，以防止进一步损坏。