A main drain trap (Fig. 12) should be provided in every house drain connecting with a cesspool, septic tank or public sewer, to cut off sewer air from the house drainage system. The trap should be located inside of the foundation wall in an accessible position and as close to the wall as practicable. The only fitting intervening between the main drain trap and the foundation wall should be a clean-out Y (Fig. 8). When there is no cellar under a building, the trap should be placed outside of the foundation wall below frost level and a brick manhole built around it to make it accessible.

A main drain trap should have two clean-out hubs in which to calk clean-out ferrules, and these hubs never should be used for other purposes. Main drain traps should be set perfectly level with regard to their water seals. If the heel of a trap is tipped up, the dip of the trap will not retain sufficient water to form an effective seal; and if the outlet to the trap is tipped up, too much water will be retained in the trap and backed up into the house drain. Mason traps, full S traps, three-quarter S traps, half S traps, traps without clean-outs or with clean-outs that are made tight with putty or gasket joints, never should be used.