Wenatchee Home Inspections

“S” trap or not. This seems to come up quite often on home inspection forums. Then there seems to much confusion on what is an “S” trap and what is actually allowed in a trap arm.

First let start with some basics. What is a trap in regards to plumbing, or more specifically what is a P-trap.

                                

A P-trap is a plumbing device which prevents odorous gas in plumbing system, drains and sewers,  from rising up through a toilet, sink or floor drain or any other drain into a home or business.

                            
         
The simple answer is if your trap from the fixture cannot see the vent then it is installed wrong. We must always keep the vent connection above the trap weir to prevent siphoning and keep proper pressures on both side of the trap.

So let’s look at the codes-

2015 UPC-

1002.2 Fixture Traps. Each fixture trap shall have a protecting vent so located that the developed length of the trap arm from the trap weir to the inner edge of the vent shall be

within the distance given in Table 10 - 1, but in no case less than two times the diameter of the trap arm.



Based on our pipe diameter we will get a maximum length to our sanitary tee at the proper height (based on the elevation of a fixtures drain outlet).  Using Table 1002.2. if the fixture requires a 1 ¼ -in. trap, the distance from the weir of the trap to the vent pipe opening “shall not” exceed 30 inches.

Now if we look at the IPC we have have a different set of numbers but the concept is the same.

2015 IPC

909.1 Distance of trap from vent.

Each fixture trap shall have a protecting vent located so that the slope and the developed length in the fixture drain from the trap weir to the vent fitting are within the requirements set forth in Table 909.1.

Exception: The developed length of the fixture drain from the trap weir to the vent fitting for self-siphoning fixtures, such as water closets, shall not be limited.

The moral here is that the vent connection must always be above the trap weir!!  No matter what code you are using.


Now let hit a pet peeve of mine… bad info on the internet. This is why we need to use the codes as the a way to define what is correct.

Below is a image that floats around showing how to correct an “S-Trap” which is still wrong and will still siphon the trap.  Again, this is not allowed by code… do you see the vent?

But there are more,  here is another image that is incorrect.

The maximum distance between a trap and its protecting vent has been presented in the form of the plumbing  tables for decades, but it really is very simple. if your trap cannot see the vent it just is not correct.

So here are a few bad trap configurations from various home inspections.

                                                                    Classic “S-Traps”

                                                              Still wrong

                                                                 Close but no cigar

                                                          Again, very close but still not correct

Remember, the trap has to see the vent.

“It's not what you look at that matters, it's what you see.”

Henry David Thoreau

If you find any errors or have additional information that would expand on any code, building standards or manufacturer requirements please let me know.

NCW Home Inspections, LLC  is a Licensed Washington State Home Inspection service located in Wenatchee Washington serving Chelan County, Douglas County, Kittitas County, Okanogan County and Grant County Washington and the cities of Wenatchee, Leavenworth, Cashmere, Oroville, Cle Elum, East Wenatchee, Quincy and many more…  

Your Wenatchee and Chelan Professional Real Estate, Home and Structural Pest Inspection Service

Instructor- Fundamentals of Home Inspection-  Bellingham Technical College

WA Home Inspector Advisory Licensing Board

www.ncwhomeinspections.com                                                   509-670-9572

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