Plumbing drain traps and venting.

One goal of the home inspection is to identify non professional work. With plumbing work, much of the drain piping is not visible once the work is completed. The trap arrangement visible under sinks is often the only visible drain plumbing. If the trap is incorrectly done, this is an indicator of non professional work, and brings in to question all of the plumbing work. For a basement bathroom non professional work is a big deal because most of the drain plumbing is embedded in the concrete and cost to rework can be very high.

After the trap, drain pipes should flow at a low slope, typically about 1:50 slope, and a vent pipe should be attached within a set distance before the pipe drops sharply. On this drain, immediately after the trap, the pipe drops sharply. This can cause the trap water to be siphoned out, allowing sewer gasses in to the house. Risk is a bit lower because this drain is for a small sink in a powder room, typically not filled to the brim and then drained. The chrome pipe is probably about 60 years old and will basically crumble in your hand if you try to work with it. Time for some ABS work to make it OK.

Vent pipes are pipes connected to the drain system that allow air to enter the pipes. The pipes are routed through the house, and on Canadian homes they typically pass through the roof. Providing this connection to atmospheric air prevents siphoning empty of the traps. The traps (loop in the pipe) hold water that provides a seal against sewer gasses. This trap seal is essential to keeping sewer gasses out of the house.

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Drain pipe slopes too sharply after the trap.