Nothing short of continuous use of the fixtures will prevent evaporation of the water in traps. A large dip is recommended for traps on waste pipes to guard against a rapid loss of the seal. When a house will be left unoccupied for a long time, but especially during the hot summer months special precautions should be taken to prevent sewer gas from entering the rooms and saturating carpets, wall-paper and furniture. Replacing the water in traps with oil or glycerine may be recommended, or else the use of common rock salt which attracts sufficient moisture from the atmosphere to make up for the loss by evaporation.

Traps Forced By Back Pressure

It has already been explained how traps under fixtures may be forced by back pressure. This cannot, however, occur with traps under fixtures, if all soil and waste pipes are properly extended through the roof, and provided with a fresh air opening at their foot.