From time to time I see accordion-style flexible drain pipe (also sometimes called flexible waste pipe) in use, much like the one below. St. Louis County inspectors take an exceptionally dim view of these, and I always wondered what the big deal was, since literally every hardware and home-improvement store in St. Louis County sells them. Why would they sell something if it isn’t okay to use it?
I still don’t have a good answer to the first half of the question except that people buy them. S traps are another example of plumbing parts that are legal to buy but illegal to use. Now that I’ve taken apart several bathroom lavatories that had flexible waste pipes in use, I understand why the building inspectors hate them. If you want a plugged-up drain, installing these is a good way to get one.
Problems with flexible drain pipe
The major problem with them is that they collect the dirt and grime that’s supposed to flow down the drain. The zigzag sides are great for slowing down the flow of water and letting the gunk sit and collect. Making matters worse, since they slow down the flow of water, gunk can easily collect further down the pipe, especially if they pipe already isn’t 100% clear.
I recently replaced one of these flexible accordion waste pipes with a regular, approved PVC pipe. The accordion drain had a lot of debris in it. The p-trap underneath it was full of a really scary, thick soup of gunk. I immediately took that outside and dumped it out. But when I put it all back together, the sink still had a very slow drain. It would drain eventually, but it took several hours to do it. The clog was far enough into the wall that I couldn’t reach it, so I had to use a heavy-duty lye-based drain opener–something called Instant Power if you’re interested–overnight to clear it.
Why buy flexible drain pipe?
I’m sure one of the reasons people buy these flexible waste pipes is because they don’t want to cut PVC pipe. But that’s not extremely hard. PVC cuts in seconds with a miter saw. You can also cut it with any other power saw you may have available. Just secure the pipe in place while you’re cutting it and keep your hands far out of harm’s way. Measure it against the old pipe and add an additional 1/8 of an inch to be safe. PVC fittings have a good quarter inch of flexibility in them. Double check everything, then cut.
Alternatives to flexible drain pipe
The other reason people buy them is because sometimes plumbing just doesn’t line up easily. But there are safe, approved ways to deal with that. Often you can deal with large gaps by exercising some creativity when putting the fittings together. There’s no rule that the 90-degree elbows all have to run completely parallel to each other. You can rotate them to fit the space you have to work with when the pipe coming from the sink and coming out the wall aren’t in a straight line perpendicular to the wall.
To deal with smaller gaps in alignment, use another type of fitting. Chances are there’s an elbow that goes from the trap up to the drain. Rather than using regular white PVC for that part of the connection, buy a 1 1/2″ flexible PVC elbow or 2″ flexible PVC elbow (whichever matches what you have) and sub that in. The flexibility will allow you to force a connection that wouldn’t quite line up otherwise. You can also splice one or more straight couplings into regular straight lengths of PVC to get a bit more flexibility if you need it. Or to get out of a real bind, Fernco makes an entire trap out of flexible rubber.
It’s not quite as easy as pulling and twisting an accordion-style fitting. But the 30 extra minutes now can save hours of frustration in the future.
Predictably, someone started making flexible p-traps too. In case you’re wondering, I don’t recommend flexible traps either.
What about metal pipe? Is metal pipe against code?
Someone asked me if metal pipe is OK, or if it has to be PVC. Metal pipe is fine, code-wise, as long as it’s not leaking. If it’s corroded and leaking, an inspector will make you replace it. I don’t like a metal trap because it will eventually rust through and leak, so I always replace metal traps with PVC. Frequently I replace any other metal fittings that I can with PVC while I’m in there. That’s about a $20 project and takes less than 30 minutes, so I think it’s worth it. It saves me from having to come back and replace another piece in a year or two.
What about inside the wall? Some people replace all the metal drainage pipes in a house when they see them. I’m inclined to leave it alone until it starts developing problems, as replacing the plumbing stack and everything running to it can quickly become an expensive project.