How to Plumb a Washing Machine Drain (8 Steps)


Whether you’re putting in a new build or moving the location of your washing machine, it needs a drain. That will mean plumbing the drain, often from scratch, with new waste pipes and vent pipes. 

Fortunately, the plumbing on a washing machine drain is relatively simple. Your largest concern will be the distance to the nearest soil pipe and to the vent stack. 

Here, the difficulty of installing a washing machine drain will depend on which room you install it in and the difficulty of accessing existing pipes. 

Plumbing A Washing Machine Drain

In most cases, any new washing machine installation will be in the bathroom or kitchen. If you’re installing your washing machine in a room without existing plumbing, you will have to take some extra steps – namely running your wastepipe and vent further. 

What You’ll Need:

  • 1 ½” PVC pipe for drainage line 
  • 1 ½” PVC P-trap (both halves)
  • Brackets for P-trap 
  • Sanitary Tee converting from 1 ½ to 2” 
  • 2” PVC vent pipe 
  • Either a 2 inch sanitary tee to branch the washing machine vent into the branch vent, an outdoor vent, or an air admittance valve 
  • Any turns or fittings you need to install the vent pipe 
  • UPVC soil pipe fitting (normally four inches) with a sanitary tee to 1 ½ inches 
  • Hacksaw or circular saw
  • Level 
  • Chalk string or string, nails, and a pen/marker 
  • A power drill 
  • Forstner bit (2” and 2 ½”)
  • Measuring tape 
  • Pipe cement 
  • Disposable gloves
  • 2×4 lumber to reinforce the walls 
  • Wood screws 

You’ll likely have to assess both the waste pipe and the vent pipe to determine what corners you need. You’ll also want to buy a wye or a sanitary tee to connect your lines to the vent and waste pipe. Check how your plumbing is fitted before doing so. 

1. Preparing the room 

No matter what room you’re installing your washing machine in, you’ll have to access the walls and the subfloor. Here you have a couple of options. 

For example, if your bathroom is tiled you may want to see if you can access the pipes from the opposite side of the wall and from underneath the floor. Removing a panel of sheetrock from the ceiling or wall is a lot easier than removing tile and putting it back. 

Otherwise, you will want to strip the wall down to the baseboards. You should do so for the entire section of wall you intend to run pipe through. That means figuring out where your vent stack is or where you can vent your washing machine. 

  • Most bathrooms have branch vents between the sink, toilet, and shower. You can connect to this.
  • Most bathrooms have a waste pipe running directly between the toilet, shower, and sink. You can also tap in directly to this.

If you’re installing a new washing machine, you’ll want to figure out where the nearest waste pipe is. You should normally tap into this rather than the main sewer line, even if the main sewer line is closer.

Remove the flooring and the subflooring to the point where you can connect the washing machine drainage to the waste pipe. 

2. Assessing your plumbing 

It’s important to start out plumbing a washing machine by assessing what you need. For example, if you’re installing the washing machine in the bathroom, the job should be relatively simple. You can tap into the existing waste line and the existing vent branch. 

You might also want to run your ventilation directly outside. This makes sense if your washing machine is against a wall and the nearest vent pipe is far away. 

The important thing is that you measure everything, decide what turns you need, and figure out how to lay everything out upfront.

Here, most people find it extremely helpful to use nails and string to visualize the pipe layout. You can then measure the string and buy exactly what you need. 

3. Drilling holes 

Use your chalk line or string to plot the path between where your pipes go into the wall and where they come out. In most cases, you’ll have to drill through the support beams. 

If your boards are load-bearing or flooring, you’ll want to reinforce the beams before you drill through them. This tactic is called sistering.

To do it, you simply cut a 2×4 to size and fit it against the existing board. Attach it with wood screws. Then, drill through both, keeping to the center of the board where you can. 

If the wall isn’t load-bearing you can often skip this step. However, reinforcing your studs will ensure your wall remains stable for as long as possible. 

  • Vent pipes have to be at a 45-degree angle or horizontal. Keep the pipe horizontal as much as possible to avoid having to drill. 
  • Wastepipes need a ¼” fall per foot to ensure they drain properly. So, if you’re moving your drain 4 feet across the bathroom, the connection point to the soil pipe should be 1” lower than the connection point to the washing machine’s P-trap. 

Drill your holes at an angle, using an electric drill and a Forstner bit. Then, clean up the sawdust before proceeding to make sure it doesn’t get in the drain. 

You’ll also want to drill a hole in the floor and the subfloor to put the drainpipe through. 

In most cases, washing machine P-traps are above the floor. However, if the space between your subfloor and the ceiling is wide enough, you may want to install it there. Additionally, you can choose to install the P-trap in the wall. 

Drill your drainage hole according to those preferences. 

4. Laying out waste pipe 

Starting from the waste pipe, cut the pipe and install a fitting. If you’re connecting to an existing wastewater pipe in your home, you can use a PVC sanitary tee or wye depending on the angle you’re running the pipes together at. 

On the other hand, if you’re connecting directly to a soil pipe, such as the one the bathroom line already connects to, you’ll need a uPVC fitting. Soil pipes are normally 10 inches. 

However, chances are, you already have a connector installed, with multiple openings. 

Normally, you’ll just have to take the plastic cap off of an existing boss connector. Make sure you double-check if you have a chemical seal or a push fit seal on the boss connector. 

Then lay your pipes out and cut them to size. You don’t want to glue anything just yet.

5. Laying out vent pipe 

You can legally vent outside, to the roof, or to an existing vent branch. However, if you’re connecting to the sewer, which you are, you need a vent. 

Here, you’ll want to lay your pipes out and loosely fit them together, where possible. This will allow you to double-check measurements and your fittings. Vent pipe must be installed at a minimum of a 45-degree angle. You can also install it vertically. 

The vent pipe should connect to the waste pipe, at a horizontal point. Normally, you’ll want a sanitary tee with a conversion to a 2” fitting for the vent. Connect this to your existing waste pipe you’ve already laid out. 

If you’re connecting your pipe to a branch vent, go ahead and cut the branch and start the pipe there. Insert a connection, either an elbow for an end connection or a sanitary wye or angles connector for a center connection. 

On the other hand, if you’re venting outside, drill a hole in the wall. However, you shouldn’t install the vent cap until the pipe is permanently in place.

6. Fitting pipe 

Once you’re sure everything fits, nothing is too tight, and all angles are good, you can glue everything together. Make sure you clean and debur the edges of the pipe as you go to ensure debris doesn’t become trapped in the pipe. Then, apply your pipe cement following the package instructions. 

Additionally, make sure you use gloves and operate in a ventilated area. 

7. Installing the P-trap 

Connect the P-trap to the drainage pipe at a minimum height of 6” from the floor and a maximum height of 18” from the floor. Use a sanitary tee for this connection. Here, most plumbers use a single sanitary tee to connect the waste stack to the vent stack and the p-trap. 

You can then install a horizontal connecting pipe at a minimum of 3” and a maximum of 6”. 

Fit both sides of your P-trap together and glue everything in. Then, fit your vertical standpipe into the top of the P-trap. This pipe can be a minimum of 18” and a maximum of 30” higher than the p-trap. 

8. Installing the washing machine 

Once everything is dry, you can simply insert the washing machine drain hose into the standpipe. You may choose to use a connector if you prefer. However, it’s often not necessary. 

FAQs

Installing a washing machine drain may be intimidating and chances are, you still have questions. 

Is it safe to run a washing machine drain outside? 

Normally it is safe to run a washing machine outside. However, your local municipality and regulation may prevent it. 

Can you connect your washing machine to the sink? 

You can connect your washing machine drain to the sink. However, for good backflow prevention, you probably shouldn’t. Instead, connect to the waste line the sink connects to, or tap in further down from the P-trap. 

How do you install a washing machine drain on a concrete floor?

If you’re installing a washing machine drain on a concrete floor, you’ll have to drill or chisel a hole for the pipes. That can add considerable difficulty and you should consult with a plumber first. If you have this done professionally, it will also considerably add to the cost of the plumbing

Conclusion 

Plumbing a washing machine drain might be very simple or very complex. If you have to take off the tile, you’re looking at several days of work. On the other hand, if you already have the wall open, you could be done in a few hours. 

Eventually, it’s important to ensure that your plumbing meets code requirements. For example, the slope is important to prevent blockage and air bubbles. Otherwise, your plumbing installation should be relatively straightforward.

Brandy Cross

Brandy is a freelance writer with a background and strong interest in DIY and home repair, stemming from her childhood and days spent helping her dad who worked as a handyman. Today, she's renovating an apartment she shares with 100+ houseplants.

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