Converting tub to shower.

Asked By: luisfr on Wednesday, September 29, 2010
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Answer Currently have a tub with drain at end of the tub. Converting the tub to a shower. I have removed the tub and need to move drain from end of the tub area (where tub was) to the center for the new shower. There is a 1 1/2" P-trap (from the tub drain) that ties onto an existing 2" sewage drain (from toilet next to old bath tub). Installing the new shower drain in the middle is closer to the 2" sewage drain than from the original 1 1/2 drain fro the tub. Question: Is a P-trap needed for the new shower drain? Is so what size P-trap? Can I tie directly onto the 2" sewage drain that is closer to the new shower drain location or do I have to run the piping back to the original tub drain? Also what is the correct height of the shower drain above the bathroom floor? I will be using mortar and shower pan technique, with a tile finish. I greatly appreciate your help and guidance in this matter. Thanks in advance for your help. Luis Rodriguez

Thanks for your question Luis. Well, there is a lot here to answer and I am going to show you several photos and drawings so that you get the correct idea on how to proceed with this project.

First of all I do want to recommend that you at least ask a competent plumber in your area to verify the codes etc. and the installation procedure. You can also go to your local plumbing supplier or home improvement center and ask for this information as well.

To get started we need to talk about the P-Trap. Showers are required to use a 2" P-Trap, sometimes called an S-Trap, in order to properly handle the amount of water that is being drained. For this reason, by code, you must always connect this trap to a 2" drain system and run it to a larger pipe somewhere down the line.

You cannont reduce the 2" to a 1 1/2" drain pipe. This will not only be insufficient but will not meet the plumbing code. You can check with your local building department to verify this. That being the case, you will need to remove the old 1 1/2" trap and pipe from the old tub drain location, and cap it off. You will then need to connect the P-Trap from the new drain location and connect to the 2" drain that you have mentioned.

Take a look at the following photo that will show the proper way plumbing drains are typically installed in a home.


You will notice that every plumbing item has a trap and every thing not only drains into the same size followed by a larger drain, but also everything is vented as well. this is the proper way the plumbing should be done in a home.

Based on this then, it is very important that you install a P-Trap, also know as an S-Trap, on this drain, otherwise there are risk factors that you need to know about. The important thing about the P-Trap is that it is has a water seal along the curve of the trap. The seal prevents noxious air or gases to back-flow from the sewer line, but the original waste can still exit into the sewage system. If the gases were allowed back into the home, not only would they smell, but they could cause illnesses and have even been known to explode.

Plus, installing a P-Trap is required by code on all drains in the home. for these reasons, it is a must. 

Now let's talk about the height of the drain. The photo below will show how the finished drain in the tile should look.

Notice that the drain cap is flush with the finished tile floor and that the tile is cut to allow for the grout to go all around the perimeter of the drain. The next photo shows you a cross section of how is should look and be installed. In it, you can see the open floor space where the floor joists are located, the liner and the mortar plus the tile and the entire drain including the drain cap that is flush with the tile.

At first it appears that there is no P-Trap in this drawing, but sometimes the trap installed further down the line as in the photo shown below, again showing how important it is to make sure you install one in the shower drain system.

There are two types of drains that can be used in a shower. One has a square drain cap, as shown in the examples above, and the other has a round drain cap. The square one is much easier to work with when cutting the tile to fit. However some people prefer to use a round one, and usually someone who a very good tile installer, has no problem fitting the tile properly. 


I hope this helps. Rick

Rick Maselli is Founder and Editor of and Ask Rick.




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