Sewage backflow preventing valve

A sewer lateral line as part of a gravity flow sewage system from a residential or commercial building, including a sewage backflow preventing relief valve, comprising:

• the backflow preventing relief valve having a hollow body with a closed upper end and an open lower end connected to a top opening of an upwardly extending leg of a Tee in the sewer lateral line of the gravity flow sewage system,
• at least one sewage outflow hole, open to atmosphere, in a side of the body,
• a valve seat in the body below the at least one sewage outflow hole,
• a valve closure member within the body, adjacent to the at least one sewage outflow hole, the valve closure member comprising a float movable upwardly and downwardly in the body, to be buoyed and forced upwardly in the event of liquid moving upwardly from the open lower end of the body into the body, and the valve closure member being configured to form a substantially sealing closure against the valve seat in normal configuration with the valve closure member resting by gravity on the valve seat, when liquid has not moved upwardly into the body, and
• the valve closure member being in a height relationship with the at least one sewage outflow hole such that when the valve is resting on the valve seat, the valve closure member cannot be forced upwardly by a pushing force against the valve closure member exerted from the at least one sewage outflow hole, as by a rat or other rodent,
• whereby if a blockage occurs in the sewer lateral line downstream of the Tee, sewage will accumulate in the lateral line and enter the Tee, raising the valve closure member and overflowing out through the at least one sewage outflow hole, preventing backup in the sewer lateral line and into the residential or commercial building.