FLEX PIPE! What is it good for?

By: Patrick Hooper

In addition to sewer lines and sanitary infrastructure, M.I.S. also performs inspections on footer drains  (a.k.a. foundation, perimeter or curtain drains) and downspout lines. The kind of pipe used for sewer lines is pretty standard, normally SDR 35 or SCH 40 PVC for newer construction and clay or concrete in older homes. On the other hand, the type of pipe used for the management of ground and storm water can vary—with one of the most common types used (in my experience) being flexible drain pipe/“flex pipe.”

Flexible drain pipe is a light-weight, corrugated, bendable drain line usually made from black polypropylene. This product is easy to work with and install, affordable and pretty common.

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Now, it’s not my intent to get into a technical evaluation of the maximum loads of various pipes made from/in any number of materials/sizes—if you want to do your own investigation, here are a few links you can check out:




What I do want to share here are my experiences with inspecting underground drains constructed of flex pipe.

By its nature, flex pipe isn’t very strong and can be compressed/misshapen by anyone of average strength with their bare hands. Depending on soil conditions, one square foot of dirt can weigh anywhere between 75 – 120 pounds, with a full square yard of soil weighing up to one ton or more. I’m guessing that you can see where I’m going with this…

I’ve been performing footer/downspout drain inspections for years now, and it is the rule rather than the exception that when flex pipe is used in an underground application, it eventually ends up collapsing—even when buried in less than a foot of soil. In fact, it’s not uncommon to see “ovaling” and compression with stronger, thin-walled PVC pipe, let alone the much-less rigid composition of flex pipe.


A perfect example of the problems I’ve seen with buried flex pipe is an inspection I completed for a construction company. They called me out after finding water in the basement of a new home, just days after they’d installed the exterior footer drain (using flex pipe). About 60 feet into the line, my camera hit a section of pipe that was completely crushed by the soil that’d been placed on it less than 72 hours prior.

So, what is flex pipe good for? Well, its affordability, light weight and flexibility makes it easy to work with and suitable for any above-ground irrigation/drainage application without straining your budget. I just wouldn’t recommend using it as permanent underground drain pipe.

So buyer beware—if you’re considering purchasing a home and you see the ridged ends of black, corrugated pipe in the sump-pump pit or day-lighted outside at the curb or in the yard, know that there’s a good chance that you may someday face a compromised or collapsed underground drain line, if it isn’t already. Furthermore,  flex pipe that is routed underneath a porch, sidewalk and/or a driveway can be significantly more expensive to re-mediate.

Additionally, if you you are considering a repair or replacement of a footer or downspout line–I would strongly recommend going with the much more durable SCH 40 PVC pipe.