a sagging roof的用法和样例：
- There was a sag in the chair seat.
- If a brownout lasts less than about a second, it is called a SAG.
- I'd been provided with a sagging army cot and a folding cardtable chair.
- Given time enough, however, the balance becomes a sagging to the grave side.
- The stone made a dent in the roof of my car.