How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
Why is it a problem when a trap under a sink is installed backwards?
Monday, June 18, 2018
The U-shaped pipe configuration under a sink is called a “trap” because it traps and holds enough water after the sink has drained to create a liquid plug that keeps any noxious sewer gas from rising up out of the drain into your home. The pipe from the trap that extends into the wall is called the “trap arm”, and the whole assembly is named a “P-trap”, likely due to its sideways P-shape.
The height of the water column that keeps the sewer gas out is called the “trap seal”. The bottom is at the “trap dip” and the top is the “weir”, which is the point on a trap where the water begins to flow down the trap arm, as shown above.
The building codes set the minimum trap seal at 2” and the max at 4”. Less than 2” does not provide adequate protection against any gas pressure penetrating the seal and more than 4” will make the trap more likely to clog.
The outlet end of the trap pipe should be lower than the inlet when installed properly. The example shown has it backward, which creates a trap seal more than 4” and makes the drain more likely to clog—and that’s a long explanation for why a backwards trap is a bad thing. Also, no professional plumber would make that mistake, so it indicates that the plumbing was done by a weekend warrior or handyman.
But a backwards trap is still better than no trap at all, like in the photo below.
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