That Pesky P Trap

Home Inspector with Hergert Inspection LLC

That Pesky P trap | Home Inspector Seattle Everett Bellevue

During my years in the construction and home inspection industries, I have found that most people do not understand the purpose of the P trap.

I have heard comments that that is a place to keep large stuff from going down the drains, the place where the pipes leak, poor design causing clogs in pipes, or a way to accelerate the flow of the waste water.

The purpose of the P trap is to create a barrier to prevent sewer gasses from rising out of the plumbing fixtures. With the thermal (infrared) image below, you can see the cold water that is sitting at the base of the P trap once the waste water from this sink has stopped flowing (sink turned off). The sewer gasses cannot pass thru this water barrier and travel up thru the sink drain.

Drains that are intended for emergency overflows, such as a floor drain, may have a P trap primer. The domestic water pipe will drip very slowly into the drain to keep water in the P trap below as the water may evaporate over time.

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Comments (30)

Team Honeycutt
Allen Tate - Concord, NC

That is a good explanation.  I  knew it was there but now I know how to explain it .

Oct 12, 2011 10:47 AM
Joy Daniels
Joy Daniels Real Estate Group, Ltd. - Harrisburg, PA

After I read your posts I always feel much smarter! :)  Thank you.

Oct 12, 2011 11:07 AM
David Burrows
Classic Realty - Fairfax, VA
No Pressure, Just Seriously Devoted to Real Estate

Very important information. I've understood this for a long time but you explained it so well.

Oct 12, 2011 11:15 AM
Eric Michael
Remerica Integrity, Realtors®, Northville, MI - Livonia, MI
Metro Detroit Real Estate Professional 734.564.1519

And all this time I thought it was a safety catch for women's wedding rings when they take them off to wash the dishes :)

Oct 12, 2011 11:42 AM
Marge Piwowarski
Phoenix AZ Horse Property - Phoenix, AZ
Phoenix AZ Horse Property, LLC

This is why in the desert, we need to flush toilets and run water in sinks in unoccupied homes and apartments about monthly.  Otherwise the water in the traps evaporates and the resulting fragrance lacks appeal to buyers. 

Oct 12, 2011 02:16 PM
Joan Whitebook
BHG The Masiello Group - Nashua, NH
Consumer Focused Real Estate Services

Interesting. I had no idea what a P trap did.  Definately want to make sure that the gases don't get back into the home.  Thanks for the post.

Oct 12, 2011 03:34 PM
John Juarez
The Medford Real Estate Team - Fremont, CA

I always thought the p-trap was to create a water barrier to keep rats and snakes and werewolves from sneaking into the house.

Oct 12, 2011 04:28 PM
Dolores "Dee" Mauriello
Keller Willaims - Wayne, NJ
Realtor, Homes For Sale Wayne NJ

Hergert, amazing how a little curve in a pipe can keep harful gasses from escaping. Thanks for the explanation.

Oct 12, 2011 05:20 PM
Gita Bantwal
RE/MAX Centre Realtors - Warwick, PA
REALTOR,ABR,CRS,SRES,GRI - Bucks County & Philadel

Thank you for the post. I did not know the purpose of the P trap either.

Oct 13, 2011 12:14 AM
Aaron Seekford
Arlington Realty, Inc. - Arlington, VA
Ranked Top 1% Nationwide 703-836-6116

A client of mine asked me about this. Now I have a blog post to direct them to! Thanks for the great info.

Oct 13, 2011 01:56 AM
Team Honeycutt
Allen Tate - Concord, NC

I think the use of your thermal image just but 2 and 2 together for alot of people and I am going to use the same should I ever have to explain . Thanks! ( J )

Oct 13, 2011 02:23 AM
Scott Seaton Jr. Bourbonnais Kankakee IL Home Inspector
SLS Home Inspections-Bradley Bourbonnais Kankakee Manteno - Bourbonnais, IL
The Home Inspector With a Heart!

Nothing worse than a dried out P trap in a vacant house. That kind of starts the inspection off on a rotten note. Nice picture BTW. 

Oct 13, 2011 02:37 AM
David Gibson CNE, 719-304-4684 ~ Colorado Springs Relocation
Colorado Real Estate Advisers LLC - Colorado Springs, CO
Relocation, Luxury & Lifestyle residential
I actually carry a P trap in my car so when we encounter a house where the water has evaporated I can explain to buyers about the P trap and why the house smells. I also carry a jug of water to refill them. Sometimes buyers can get a bargain others have passed on just because of the smell.
Oct 13, 2011 03:02 AM
Tim Maitski
Atlanta Communities Real Estate Brokerage - Atlanta, GA
Truth, Excellence and a Good Deal

It's great to know about the P trap. If you recognize that this is the cause of an awful smell in a vacant house, you might be able to get a good deal. Others fear the small and avoid it. You can recognize it and know that just a little water down the drains will solve the "problem".

Oct 13, 2011 03:55 AM
Warren Edwards
Tucson, AZ
Professional Photographer - 360 Virtual Tours

     Brilliant!  I once went to photograph a multi-million dollar house that had a foul sewer type odor through out.  I remember my dad explaining this to me and running the water, shower, and flushing toilets weekly when I went on vacation.  I used a screwdriver in my car to open the water valve near the street and we started running water.  I received a thank you card and few other nice things from the realtor in exchange for additional business.  It was another one of those ghee whiz moments. 

Oct 13, 2011 04:23 AM
Rockwell Institute Real Estate Education
Prelicense and Continuing Education - Bellevue, WA

Excellent post! It's amazing how many parts of our homes we don't understand--thanks for shedding light on an important one.

Oct 13, 2011 04:39 AM
Michael Setunsky
Woodbridge, VA
Your Commercial Real Estate Link to Northern VA

Kevin, good explanation defining the purpose of the P trap. I always called it a J trap though.

Oct 13, 2011 05:05 AM
Evelyn Kennedy
Alain Pinel Realtors - Alameda, CA
Alameda, Real Estate, Alameda, CA


Although I like John Juarez'(#10) explanation as well, yours is amazing.  I have to tell this to everyone I know.  I know most people don't even know what a ptrap is.  Thanks for the info.

Oct 13, 2011 06:09 AM
Sharon Parisi
United Real Estate Dallas - Dallas, TX
Dallas Homes

You are making us so smart!  I did not know the purpose of a P trap before your post.

Oct 13, 2011 07:53 AM
Steven Cook
No Longer Processing Mortgages. - Tacoma, WA

Kevin -- You have done an excellent job of explaining the purpose, working of the P trap - and using the thermal image just cemented the explanation.

Oct 13, 2011 08:33 AM