Can teflon tape be used on a threaded joint behind a wall?

Question

I'm remodeling my bathroom. Last night I took the cement board off the wall where the pipes run for the shower. One of the pipes that carries water to the shower was corroded. Upon further inspection, I noticed a small leak at a threaded joint (more or less where the hot water knob is). Can I simply take the joint apart, wrap teflon tape around it, and put it back together? Remember, when I get the bathroom rebuilt, this will be behind the tile shower, so I do not want to have to get back there again. How long should the teflon tape last?

Would you advise doing the cold water side too? I figure it is only a matter of time before that is leaking too. Might as well get it now, right?

Answer 1

I would examine why the pipe was corroded. Was it a galvanic failure caused by joining copper/brass to galvanized steel?

Remove the fitting entirely and inspect/clean the threading. If the threading is damaged then replace the valve... In fact, you could replace the valves anyway if they're old and you've got the wall off. Many modern valves have replaceable cartridges that can be swapped when they wear out rather than having to replace the entire valve.

But onto your actual question: Teflon tape will be fine if used properly (wrapped in the right direction, not torn, etc). You may consider using high-density tape. For in-wall pipes I like to use pipe dope instead. Depending on the nature of the joint (and especially if it's between plastic and metal or two dissimilar but non-reactive! metals) you can use both:
Tape first, then put dope on top of the tape. For fittings, both plastic and brass can change shape easily....the threading/tightening can change the shape so the tape fills voids in the threads and the dope seals it.

Answer 2

I wonder if the corrosion that you mention might be mineral buildup on the outside of the pipe due to the small leak. If this is copper pipe, give the area a sand and inspect the pipe. A photo would help assist further.

As for the use of teflon tape on joints in concealed in walls: this is totally acceptable and is the standard practice.

Most plumbers tend to us both teflon tape and pipe dope for threaded connections. The thing to remember is that it's the threads that make the seal (they are tapered and the more they are tightened the more they seal) and not the teflon tape or dope. The purpose of the teflon tape and/or pipe dope is to lubricate the threads so that they can be sufficiently tightened. Obviously the use of teflon tape and dope will assist in sealing as you're adding something in-between the threads and filling in any imperfections in the threads, but it should not be relied on as the seal.

Answer 3

I'm guessing that your shower valve requires a male adapter. If so, these threads definitely require tape and/or pipe dope. If there is not currently any tape or pipe dope on them, then this is likely why it is leaking. I do wonder though if is it leaking because it is corroded, or if it corroded because it leaked. Pinhole leaks that develop overtime can indicate a grounding issue in your house, though if its localized to this one area, that is likely not the case.

In any event, yes, do disassemble both the hot and cold to add teflon tape and/or pipe dope. There are plenty of opinions out there as to whether to use tape, dope or both. I personally use both. The pipe dope adds an extra layer of protection because it expands to fill in small cracks that tape alone might not seal against.