Threads and what goes on them

Why. Wrap it backward and the fitting unwinds it as it tightens. Tape on the first thread shreds into the line and can clog valves.

The tell. Snug it and look at the joint. Tape bunched or peeled back at the face of the fitting went on backward.

Oatey's guide says wrap two to four times, overlapping each pass about half the tape's width; its FAQ says three to five straight wraps on 1/2 to 1 inch threads when the tape is as wide as the pipe. Either way, more is not better: Oatey says too much tape can crack a fitting. And the tape is not for flare or compression fittings at all.

Why. Dope inside the fitting gets shoved ahead of the pipe into the bore. Then it is in your water.

The tell. Look through the fitting after it is made up. A ring of dope squeezed out inside means somebody doped the female.

The California Plumbing Code says it for threaded galvanized steel joints (605.5.2): tape or compound goes only on male threads, and it has to be an approved type, insoluble in water and nontoxic. RectorSeal's instructions for its PTFE pipe dope say male threads only up to 1-1/4 inch, and both male and female on larger sizes. Where your code says male only, the code wins: the inspector goes by the code book, not the can.

Why. Tapered threads wedge the female hub wider with every turn. Spears calls overtightening the most common cause of threaded joint failure.

The tell. The mistake is one more turn to be sure. The tell is a hairline split along the female hub. Small fittings go first.

Spears says it in capital letters: the single most common cause of threaded joint failure is overtightening. It says position it right on the way in, so you don't have to back it up after it's wrenched. Boshart adds that smaller fittings are more prone to stress cracking, and says skip PTFE tape on PVC threads: it adds bulk and lubricates, both of which push you to overtighten. Wavin's CPVC guide says the opposite, tape CPVC threads every time, and agrees two turns past finger tight is all it takes. Follow the sheet for the pipe in your hand. Spears also warns some pipe dopes can stress crack plastic, so use one listed for it.

Why. Oatey says these fittings are built to seal metal to metal, and tape can cause leaks. The nut's only job is to drive the sleeve onto the tube.

The tell. Tape showing under a compression nut, or a weep somebody chased with more turns. BrassCraft warns overtightening can lead to leaks long after installation.

There's one exception in print. Wavin's CPVC guide says if you use a brass compression ferrule on CPVC, tape goes over the ferrule, because the brass and the plastic grow at different rates; it would rather you use an O-ring transition. That's the sheet for that pipe, so it wins there. Boshart notes straight NPSM threads can't seal on the thread at all; they need an O-ring, and tape or dope won't do it.