Why to avoid a flexible p-trap

Last Updated on November 25, 2018 by Dave Farquhar

I guess it was inevitable that someone would make a flexible p-trap out of expandable, accordion-style pipe. I’ve written about that stuff before, and you definitely don’t want your p trap to be made of it.

The argument for them usually goes like this: they’re easy to install, and smooth pipes can build up gunk too.

Yes they can, but not nearly as quickly as the accordion-style type does. The flexible accordion pipes clog in a short period of time. I had one last about a year in a kitchen and about two years in a bathroom, which are the hardest sinks to clog up. And when you go to plunge these pipes to clear the gunk out, the force is more likely to cause the flexible pipe to pop and spring a leak. Or if you snake it, you’ll snag, bust through it, and cause a leak.

It’s very rare for conventional, smooth PVC pipe to leak anywhere other than the joints. I’m sure it can happen. I haven’t seen it though.

But in the best case scenario, where you don’t get any leaks, the pipe will clog up more frequently than a conventional one and you’ll have to clean it out somehow. If the clog happens in the flexible part of the pipe, that might not be so bad. But the slower water flow through the flexible p-trap can cause clogs beyond it, which will lead to a costlier repair in the future. Slower moving water is more apt to deposit what it’s carrying in the pipe. And slower moving water means less pressure to break up what’s in the pipe.

There’s a second problem. P-traps take on and maintain the shape of a letter p to keep sewer gases out of your house. When you use a flexible p trap, you’re more likely to get the shape wrong, and you’re more likely to accidentally misshape it when you bump the pipe. Or when you bump it, the force might cause it to spring a leak.

When you go to sell the house, the building inspector will take a dim view of these. They’re against code. Then you’ll have to replace it anyway, assuming it hasn’t already failed on you.

Replacing a P trap correctly isn’t as hard as it looks. Use the existing one as a pattern. Take it to the hardware store with you, even. Buy pieces that match it as closely as possible. Ask for advice at the hardware store. It will be easier to get advice at the hardware store, but even at a big box store there’s usually someone there in the plumbing section who can answer questions.

Take it home.  Put it together loosely. Wiggle all the parts until everything fits correctly. Then, and only then, tighten everything. Watch a Youtube video or three, both before and during if that’s what it takes. It might take you 45 minutes to get it done, but you’ll get it. You don’t have to resort to using a flexible p-trap.

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