I'm working on improvising a shower to add to our existing clawfoot tub. It's just going to be the piping supported to the wall for now as we're going to redo this bathroom a year or so down the road.

Tub faucet
(source: vintagetub.com)

Our tub faucet looks something like this and has no way to add on a shower to it, and the faucets with the showers cost a ton to add. So, since it's temporary, I'm going to T off of the supply lines after the valve. However, I don't know much of why the tub would be piped how it is.

The supply line comes out of the floor at a 1/2" and then into the valve and out at 3/8". I know this is done for sinks and such, but I don't know why or why they didn't use a 1/2" to 1/2" valve instead?

It comes out at 3/8" and goes through a short 3/8" copper line and then into the back of the tub. The 3/8" copper line is screwed into a (reversed?) 3/8" to 1/2" reducer. Then this was screwed into a (reversed?) 1/2" to 3/4" reducer which is screwed directly into the tub's faucet's pipe. I understand that the 1/2" had to be converted up to fit the tub, but why does all of the pipe downsize before it when the tub can fit a 1/2" line?

To retain the most piping and fittings, I'm following the same route with my shower valve, although it also accepts 1/2" lines. Is this the wrong approach, should I resize it all to 1/2"? And why would it downsize when it didn't have to?

I think the downsize can lead to faster water or increased pressure, but I couldn't find anything directly answering this.

Tub piping currently

  • So the idea is to tee off the supply lines (after the shutoffs), to feed a mixing valve for the shower?
    – Tester101
    Commented Feb 16, 2016 at 18:11
  • @Tester101 Correct.
    – TFK
    Commented Feb 16, 2016 at 18:12
  • NOTE: If you wanted to, you could probably replace the existing shutoff valves with 1/2" x 1/2" valves. Then you wouldn't have to reduce to 3/8", then increase back up to 1/2". Though, you'll have to use an adapter since I'm not sure if shutoffs exist with 1/2" compression outlets. That's likely why they used 3/8" instead.
    – Tester101
    Commented Feb 16, 2016 at 18:15
  • @Tester101 Why not use a different valve? And is there a benefit to downsizing?
    – TFK
    Commented Feb 16, 2016 at 18:43
  • 1
    Looks like the connection to the tub is threaded, so you'll either need compression fitting, or you'll have to install a union.
    – Tester101
    Commented Feb 16, 2016 at 18:57

1 Answer 1


There is no benefit to "downsizing" the supply size; however, with tub fillers it usually is not a deal breaker because it just takes a bit longer to fill up the tub. If you look at the size of the tub filler spout, it may be no more of a restriction than you already have.

Both the stop valves and the threaded adapters that are screwed into your cast brass tub valve arms are available for 1/2" O.D. tubing (1/2" compression). Stop valves for 1/2" compression may be difficult (not impossible) to find in a "straight" configuration (as opposed to an angled 90° configuration), I would suggest that is the reason the plumber used 3/8" tubing and fittings.

How will an aesthetically pleasing shower mixer arrangement be created for this? I recommend that you consider changing the filler valve to a faucet set that has a shower diverter incorporated, there are many available styles:

enter image description here

  • It won't necessarily be pleasing to view, but it's only temporary. It's a secondary bathroom that has never had a shower and so it's never really used other than truly as a half-bath. So our goal is to get a working shower in now for cheap and redo the whole layout in a year or so. Although they do sale the sets specifically for this, they aren't cheap. I'm going to skip the compression fittings all together and jump straight to the 1/2" line for an easy hookup.
    – TFK
    Commented Feb 17, 2016 at 4:12

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