If you are a plumbing contractor or just a seasoned homeowner, it may seem silly, but the size of the trap in your toilet could make the difference between a trouble-free bathroom and having to keep a plunger nearby forever.
Toilet traps are not the same size. They range in size from 1 3/4″ for some older toilets, which is large enough for most household needs, to over 4″ in hospital toilets, allowing the disposal of larger waste products.
This answer is rather broad, I know, but covers the scope of the question. Even though it covers the ‘scope,’ this answer didn’t satisfy me enough to just ‘let-it-lie,’ so I dug a little deeper to see why there is such a difference in sizes.
What Does a Toilet Trap Do?
The trap is a basic plumbing need for anything flushed through drainpipes to a central sewer system. The most common name for this bit of plumbing is the “P trap,” which allows water to form a barrier so that sewer gasses and vermin do not enter back into the dwelling.
The trap system was invented in 1775 when it became obvious that sewer gasses were sickening home dwellers and killing some others. Before then, the “S bend” did not adequately vent these sewer gasses and usually just built up pressure until the trapped water was pushed out of the bend releasing the gasses back into the home. Homeowners who had these problems would have to let the gas vent by opening windows and doors and then pour water down the drain to re-seal the trap.
Today, we have the “P trap.” (Some call it a “U bend,” which allows a gas vent to be placed in the plumbing and vents the gasses up to the roof of most houses.)
Most P traps are found in the piping, which takes wastewater from sinks and drains to the sewer, but toilet traps are built right into the toilet design. They are that curved S-shaped tube along the base of your toilet that separates your toilet from the drain line. This placement allows a toilet to be installed to a standard drainpipe on the floor or wall.
Is There an Industry Standard For Trap Sizes?
Sure, but the standard varies for the intended use of the toilet. For example, the residential standard is just 1 3/4″ minimum and has been for almost 50 years. Older porcelain toilets, first used in the late 1800s, could have had “flush traps” as little as 1″ in diameter. This made it quite difficult to flush, especially for solid waste.
Today, toilet traps just need to be “built-in,” with the toilet connected to a vented drain system. The trap’s size is largely up to the builder/homeowner, depending on what style of toilet and the utility they want. From round bowls to elongated, elevated to wall-mounted, tankless to dual flush, the different types of toilets fill many pages of catalogs and are worried over by contractors and homeowners alike.
If you are replacing a toilet in your home, be aware of the different building and plumbing standards and codes. There may have been changes in the building codes regarding toilets and toilet plumbing that must be considered. There may be other changes to the existing plumbing needed because the toilet style will be radically different. Take the time to consult a plumbing professional if what you want is different than what you have.
Does the Toilet Trap Size Matter?
Yes, and for several reasons. The size of the trap primarily depends on what is going to be flushed into the sewer. As we mentioned before, for most household uses, a 2″ trap is more than enough to handle a reasonable amount of solid matter.
But some toilet manufacturers tout their larger trap toilets of 2 3/4″ will flush an amazing amount of solids. In one video online, 7 pounds of mixed salad, including carrots, grapes, celery stalks, and radishes, are flushed without a clog. Or this one that effortlessly flushes down a bowl full of water balloons. The drawback to these types of claims is that the down-stream plumbing has to be able to accommodate the extra waste as well.
Hospitals, clinics, and other medical institutions will use toilets with up to 4″ of clearance in the trap. As you can imagine, they may need to dispose of larger volumes of waste, and with the specialized separation plumbing found in hospitals, the 4″ toilet traps are ideal.
Can a Toilet Trap Fail Due to Size?
Well, first, a toilet trap can definitely become clogged, especially if it’s a smaller size. (That is why we mentioned the plunger!) However, it’s not terribly hard to clear a simply clogged trap. It’s usually a matter of dislodging whatever is blocking it.
If the clog is visible and no other drains (sink or tub) are backing up, it’s just a simple fix: plunge that toilet and free the trap. You may need a toilet auger or drain snake if the clog is pretty severe.
Another less common issue with a trap is when it becomes dry. If the water held in the trap evaporates, nothing is blocking sewer gas from escaping up through your toilet. If there is a sewer odor emanating from your toilet, pour a few cups of water down there. Wait an hour or so and see if the water is still visible. If it is, that odor should go away soon.
If the trap dries out again, call a plumber. You may have a crack or leak that is allowing that trap water to escape.
Final Thoughts
So now you’re off to get that toilet of your dreams—one that is comfortable, fits the décor of your bathroom, and has the right size toilet trap!
I can already imagine the conversation you’re going to have with your plumber, who will be astonished that you know so much about the inner workings of a toilet! Just remember, the whole reason for knowing about the sizes of toilet traps and all that they can do is to keep that hideous-looking plunger out of sight AND out of mind!