How to Install a Bathtub Drain & Trap: 6 Steps (Do This!)

If you’re putting in a new bathtub, you’ll have to install the drain and the trap. Normally, this will also mean connecting a vent. It may also mean running a new connection to the waste pipe. In either case, the installation should be relatively simple depending on the drain you’ve purchased. 

Here, the most important part about installing a bathtub drain is that you buy fittings that work for your tub. Make sure the overflow drain fits your tub. This is especially important if you have an overflow on the side rather than under the faucets. Otherwise, you should just be able to fit the diagram together from the package you’ve purchased. 

How to Install a Bathtub Drain and Trap

Installing a bathtub drain and trap is relatively simple, providing you have the tub and can measure what you need. Here, the installation is a multi-part process consisting of roughing in the shower drain, connecting the vent, and then connecting the trap. From there, you can install the tub at your leisure. In most cases, you should stop at roughing in the shower drain if you’re not ready to install the tub. 

Things You’ll Need: 

  • Drainpipe in PVC or ABC. If you just have a bathtub, a 1.5” drainpipe is all you’ll need to meet code. However, if you have a shower in the tub as well, you’ll want a 2” drainpipe. 
  • P-trap 
  • U bed for a water seal 
  • T fitting 
  • 1.5” or 2” T or Y converting to your waste pipe size (usually 40mm)
  • 1 ½” threaded female adapter (PVC)
  • Corners and angles to get you to the main sewage line 
  • Y fitting  
  • Fittings and connectors as needed 
  • PVC pipe cement
  • Gloves
  • Saw or pipe cutting tool 
  • Marker 
  • Sandpaper 
  • Box knife 
  • Measuring tape
  • Bathtub drain kit 
  • Jigsaw
  • Power drill 
  • Forstner bits 
  • Plumbing tape 

1. Cut a Drainage Hole 

Determine where the bathtub will be and mark the exact location of the drain exit onto the floor. Then, use a Forstner bit to start a hole. Use a jigsaw to cut out an opening large enough to access the crawlspace between floors. If you’re installing a tub drain on a concrete floor, you’ll have to drill out the concrete to create a hole. Importantly, the hole should go through the floor and the sub-floor – unless you’ve already roughed in the drain. 

If you haven’t laid your waste pipe, you’ll want to do that in advance. Normally, wastepipe must be laid before you put in the subfloor, but you may be able to slide it into place otherwise. This guide assumes the waste pipe is already in place. 

2. Assemble the Bathtub Drain Kit 

Check your bathtub drain kit and assemble it. Depending on your kit, it may route spill off directly into the main drain. It may also connect just behind the P-trap. In the first instance, you can simply proceed with the next steps. In the latter, you’ll want to run a PVC line from the drain to the P-trap. The first type of drain is the most common. Here, your drain should look like an L, with a raised spillover drain connecting to a standard drain under the tub. Both will drain through the same trap. Here, you should check your bathtub drain plumbing diagram (this comes with the kit) for more details. 

3. Place the P-Trap 

Wrap plumbing tape around the 1 ½” female threaded adapter and screw it into the drain on the bottom of the tub. Cut a 4” length of 1 ½” PVC pipe as a nipple. Debur it and fit it inside of the female threaded adapter. Slide the P-trap onto the nipple. 

4. Place the Vent 

Cut a length of PVC pipe a minimum of 4” but no more than 5 feet and fit it into the P-trap. Here, you’ll want to assess where your vent stack is. If you can connect to the bathroom vent stack or to the vent branch, you should. Cut your first length of pipe where it will most conveniently allow you to angle off to that stack. 

Install a 1 ½” to 1 ½” angled T. Fit your vent pipe into that and branch it off to the vent stack. This should be inside the wall, so make sure you cut your pipe to that length. You can read more about how to vent a bathroom drain here. 

5. Place the Drainage Pipe 

From the new drainage hole, measure the distance between the waste pipe and the drain pipe you’ve just cut. Cut your waste pipe and install an angled fitting with a connection to the size of drain pipe you are using. 

Cut the waste pipe and install a sanitary tee with a 1 ½” outlet facing the trap. If the waste line is cast iron, you’ll want to use a cast iron threaded inlet and screw in a PVC male adaptor. You can then attach your already cut PVC pipe to that. 

Use a level to make sure that your waste pipe maintains a ¼” per foot slope. You may want to use a board and a bubble level to ensure you maintain this slope. 

6. Glue Everything 

Once you’re certain that everything fits and is the right size, you can use pipe glue to secure your plumbing. If you’re not yet ready to install your tub, you can also cap off the drainage pipe just before it connects to the P-trap. You can then install the P-trap and tub as a final process. In addition, you shouldn’t install the tub drain lines connecting to the P-trap and the overflow until the tub is in place. 

It’s important that you only use pipe cement in a well-ventilated area. Use gloves to prevent the glue from burning your hands. 

Replacing The Drain Pipe Under The Bathtub

If you want to replace a drain pipe under your bathtub, you’ll have to either move the tub or cut a hole in the sub floor to access it from below. Otherwise, you should just be able to cut the pieces out and replace them using the same process detailed above. 

Tub Drain in Concrete Floor Installation

If you’re installing a tub drain in a concrete floor, you’ll have to break the concrete floor. Here, you can try to either use a concrete drill or a combination of a drill and a hammer and chisel to make a large enough hole. Unfortunately, the sewage will always have to be underneath the concrete, unless you lay it before pouring the concrete. This can mean putting significant effort into making a hole large enough to install a waste pipe if there isn’t one already. 

You’ll want to keep holes as small as possible to avoid stressing your concrete foundation. Here, drain pipes are 1 ½ inches. However, the trap will need about 4” of space. It’s also a good idea to trace the fastest line to the nearest soil pipe and see if you can tunnel it under the foundation, so you don’t have to keep breaking up your foundation. Here, you may also want to hire an architect to assess if your floor is capable of being broken without damaging the integrity of your home. 

Do you need a trap on a bathtub drain? 

Yes. You need a trap on any drain that connects to the sewer. That’s important to prevent air and sewage gas bubbles. Here, air bubbles can cause your drain to stop draining. The air can push water up, causing overflow and flooding in your bathroom. Sewage gas can cause your bathroom to smell and can be a hazard. 

Therefore, it’s crucial to have a vent on your bathtub. However, you can connect your bathtub vent directly to the vent branch connecting to the toilet, sink, or washing machine. This vent branch can directly vent outside or can go to the main vent stack. 

How far can I install a P-Trap from a bathtub drain? 

The P-trap should screw directly into the bathtub drain. However, the vertical distance between the drain fixture and the P-trap may be a maximum of 24 inches vertically and 30 inches horizontally. This can allow you to drop the P-trap down below the floor to avoid making larger holes in your floor. However, it’s important to note that your pipe between the P-trap and the drain must be straight and maintain the same diameter as the drain.

Related Questions 

If you still have questions about installing a bathtub drain, these answers should help. 

How do you open up a bathtub drain trap?

Normally you can simply unscrew the drain trap from the bathtub. Here, you’ll want to remove the knob from the stopper on the top of the drain. Unscrew it by turning it counterclockwise. Then, go under the tub to unscrew the trap. You may have to cut the trap away from the drain pipe before you can turn it. 

Where do I find my Bathtub Drain Diagram? 

Bathtub drain diagrams refer to how the bathtub drain fittings go together. This varies per manufacturer, with dozens of options available. Unfortunately, you can’t find a diagram without knowing the brand of the drain kit you used. Here, most will fit together by screwing together to form an elongated L shape between the overflow drain and the drain under the tub. In most cases, you’ll have a single connection to the P-trap.

How do you cement around a bathtub drain? 

In most cases, you can easily cement around a bathtub drain by using a larger pipe to protect the drainage pipes. In most cases, you can fit a 12” pipe around the existing drainage, secure that, and pour cement around it. This is a common step during a bathtub drain rough-in

What size pipe for bathtub? 

Bathtub drains use 1 ½-inch pipe. However, if your tub also has a shower, you probably want a 2” drain pipe. That’s because tubs can drain more slowly without causing issues. 


Installing a bathtub drain and trap is relatively simple and you should be able to do so without much effort. However, it’s important to ensure your drain kit fits your bathtub. In addition, you’ll want to ensure you maintain the proper slope on both the drain and the vent. Finally, you always want to check local regulations, if you’re allowed to do your own plumbing in your area, and if there are any custom codes or regulations in your area.

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