What characteristic should the manhole covers serving a building drain have?

Manhole covers serving a building drain should be gas tight to prevent the escape of harmful gases from the sewage system into the building or surrounding area. This characteristic is crucial for maintaining a safe environment, as building drains can transport waste and various gases that may be toxic or unpleasant. Ensuring that the manhole covers are gas tight helps in controlling odors and prevents potential health hazards associated with gas exposure.

While other characteristics, such as being metal or removable, may seem beneficial, they do not address the critical safety concerns associated with gas leakage. Lightweight plastic covers may be easier to handle but fail to provide the necessary durability and gas-tight seal. Heavy and open covers, by their nature, would not serve the purpose of effectively containing gases. Therefore, the emphasis on gas tightness encapsulates the primary objective of manhole covers in safeguarding both individuals and the environment from hazardous conditions.