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McGarry and Madsen's home inspection blog for buyers of
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What is a double trap in plumbing?
Monday, March 30, 2020
A double trap is two plumbing traps in tandem, one after the other in line, and this often causes the first trap to get sucked out by the last trap. An example of one sink trap connected to another sink’s trap is shown above. Only one trap is allowed between a plumbing fixture and a trap arm by the International Residential Code (IRC P3201.6). The right sink should not have a trap.
Double-trapping the same sink, like in the two examples below, is also not allowed—although we once listened to a passionate argument by a homeowner that it’s actually twice as good. Oh well. It creates an excessively large trap volume and makes the drain more likely to clog.
Also, see our blog posts Why is it a problem when a trap under a sink is installed backwards? and What are the code requirements for layout of drain piping under sinks? and Can I make a trap under the sink from straight pipe and 90º elbows and Why is there a flexible accordion pipe under the bathroom sink? and What is an "S-Trap" under my sink? Why is it a problem? and What is a P-trap?
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