Your washing machine P-trap must be 6–18 inches above floor level. This distance is measured from the floor to the centerline of the trap arm. The trap arm is the straight pipe that runs from the U-bend of the P-trap to the drain pipe. This P-trap height conforms to the standard building code. However, P-trap height opens up many questions about washing machine standpipes and drains, which each have their own specific guidelines.
Table of Contents
Does P-Trap Height Matter for a Washing Machine?
Washing machine P-trap height is determined by Uniform Plumbing Code (UPC) in the United States. It is illegal to install a washing machine P-trap that is against building codes. Make sure to follow UPC guidelines for your safety. If you install a P-trap improperly, it can release toxic, flammable sewer gas into your home.
- Washing machine P-trap height is determined by the Uniform Plumbing Code (UPC) in the USA.
- UPC mandates that a washing machine P-trap must be between 6–18 inches above floor level.
- In most cases, it is illegal to plumb a washing machine P-trap that does not conform to UPC standards.
- An incorrectly installed P-trap can vent dangerous sewer gas into your home.
- Consult your local building code or consult a professional plumber for additional building code guidelines in your region.
In this article, we will discuss washing machine P-trap height as defined by the UPC. Keep in mind, your local municipality may have additional guidelines. Review your local building code or consult a professional plumber for further information.
How Do You Measure P-Trap Height?
P-trap height is not measured using the U-bend of the P-trap. Instead, it is measured from the floor to the centerline of the trap arm. The trap arm is the straight pipe that runs parallel to the floor, connecting the U-bend of the trap to the main drain line.
- P-trap height is measured by the vertical distance between the floor and the centerline of the trap arm.
- Measure from the floor to the underside of the trap arm. Then, add ½ of the trap arm pipe’s diameter. This will provide the most accurate measurement.
To measure your P-trap’s height, it’s easiest to measure from the floor to the bottom of the trap arm. Then, add half the diameter of the trap arm pipe. If you are using a 2-inch pipe, measure from the floor to the underside of the trap arm, then add 1 inch.
Why Does P-Trap Height Matter?
An incorrectly installed P-trap won’t function as intended. A P-trap installed too low is more likely to experience evaporation of the water that forms a vapor barrier. This will allow sewer gas to flow up the drain line and into your home. A P-trap that is too high is less dangerous but still not advisable.
- If your P-trap is too low, the water in it is more likely to evaporate, allowing sewer gas to flow up the line.
- UPC code mandates P-trap height must be 6–18 inches above floor level.
- This height allows for proper functionality and easy repair of the P-trap.
Another good reason to install a P-trap according to UPC code is that the 6–18-inch distance from the floor allows easy access and disassembly in case the P-trap is clogged and needs to be removed for cleaning. A low P-trap is difficult to service.
Does Your Washing Machine Drain Need a P-Trap?
If your washing machine drains to a sewer line, such as a city sewer system or a septic tank, it must have a P-trap. A P-trap contains water in its bend. This water is an essential vapor barrier that prevents sewer gas from coming up through the drain line and entering your home. If you don’t have a P-trap on a washing machine that drains to a sewer line, toxic gas will flow up through the line and into the building.
- A P-trap must be installed in any washing machine drain line that empties into a sewer line or septic tank.
- You can only install a line without a P-trap if your washing machine drains to a gray water tank.
The only case where you do not need a P-trap on your washing machine drain line is if the washing machine drains to a gray water tank. A gray water tank is specially designed to handle water that contains no sewer or kitchen waste. Washing machines, showers, and tubs can be diverted to greywater tanks, but sinks and toilets cannot.
What is Correct Height for a Washing Machine Standpipe?
The standpipe is the vertical pipe that accepts drain water from your washing machine, which flows down into the bend of the P-trap. According to UPC standards, the top of the standpipe must be no higher than 36 inches from the floor.
- The standpipe accepts water from the washing machine drain hose.
- The top of the standpipe can be no more than 36 inches above floor level.
- Subtract the height of the P-trap from 36 to find the maximum length of your standpipe.
- Example: If your P-trap is 6 inches from the floor, your maximum standpipe length is 30 inches.
Because the standpipe extends upwards from the P-trap, the height of the P-trap affects the length of your standpipe. If your P-trap is 6 inches from the floor, the standpipe can be up to 30 inches long. This will bring the top of the standpipe to 36 inches, total. If your P-trap is 18 inches from the floor, your maximum standpipe length is 18 inches. To make the job easy, subtract the height of your P-trap from 36 to determine the maximum length of your standpipe before installation.
Additional Standpipe Guidelines for Washing Machines
In addition to height, other standpipe dimensions are also strictly controlled. Your standpipe cannot be more than 30 inches from the wall, preventing washing machine installations that will not drain properly.
- The standpipe can be no more than 30 inches from the wall.
- Your standpipe must be at least 2 inches in diameter.
Also, a washing machine standpipe must be at least 2 inches in diameter to allow proper drainage. Any attempt to use drain pipes smaller than 2 inches in diameter will lead to backups and improper drainage.
Do You Need to Vent a Washing Machine Drain?
Your washing machine drain, like all drain lines, must be vented. A vented drain line ensures the line will drain quickly, preventing backups and overflows. However, it’s not necessary to install a new vent for your washing machine drain. Instead, tie your washing machine vent in with a vent for other drains in your home.
- A washing machine drain must be vented in order to ensure it drains properly.
- Tie your washing machine vent into an existing vent for sinks, toilets, etc.
- An unvented drain line will not drain properly.
A vent allows air to escape from the line as water enters. If your drain line does not have a vent, it will drain extremely slowly. This will cause the drain line to back up and flood. Whether your drain line feeds a sewer line or greywater tank, make sure the line is vented.
When Should You Call a Professional?
If you are unsure whether your P-trap is within building code standards, cannot install a P-trap that fits UPC standards, do not know where your washing machine drains to, or cannot vent your washing machine drain line, it’s time to call a professional plumber. A state-licensed plumber will be able to perform work that is up to code.
- You are not sure if your washing machine installation meets UPC standards.
- You are unable to install a washing machine P-trap that meets UPC standards.
- You cannot determine whether your washing machine drains to a sewer line or a gray water tank.
- You cannot vent your washing machine drain line.
Rather than bend the rules of UPC, it’s best to call in a professional. Cutting corners on a washing machine P-trap installation can lead to clogs, sewer gas leaks, backups, and flooding.
Can a Washer P-Trap Be Installed Below Floor Level?
According to UPC standards in the United States, installing a washing machine P-trap below floor level is illegal. Your P-trap must be 6–18 inches above floor level. If you believe your local building code differs from UPC, contact a local plumbing professional for advice and further information on local code. If you are unsure, default to UPC guidelines.
How High Should a Washer P-trap Be?
Universal Plumbing Code (UPC) mandates that a washing machine P-trap in the United States must be 6–18 inches above floor height. Do not install a washing machine P-trap above or below this range. Doing so is against the building code. When performing a plumbing install for a washing machine, it’s also essential to follow additional guidelines for standpipe height, pipe diameter, and proper venting of your drain line.
