The gray pipe worth identifying.
If your home went up between roughly 1978 and 1996, this is worth half an hour of certainty.
What it looks like
Flexible plastic supply pipe, usually gray but sometimes silver, blue or black, around half an inch or three quarters, marked "PB2110". Look at the water heater, under sinks, and where the supply enters — those are the visible ends of a system that is otherwise inside walls.
Why it matters
It fails at the fittings, generally without any warning drip beforehand, and the failure is a full-flow release rather than a seep. A number of insurers will not write a policy over it, or will exclude water damage arising from it, which turns a plumbing question into a coverage question.
What to do about it
Replacement is the only real answer, but it does not have to happen at once. Phasing — the accessible runs and the failing branch first, the remainder scheduled — keeps the cost manageable and gets you ahead of the failures.
What to do if you find it
Do not panic and do not let anyone panic you into a same-week whole-house repipe at an emergency price. Polybutylene fails unpredictably, which cuts both ways: some houses run for decades. What it does mean is that you should plan rather than wait, and know where your main shutoff is in the meantime.
Tell your insurer, because finding out at claim time that your policy excluded it is a lot worse than finding out now. And if you are buying, get it identified during the inspection period while it is still a price negotiation.
Common questions
Is polybutylene banned?
It is no longer manufactured for potable supply in the United States and is not permitted in new work, but existing installations are not required to be removed. The pressure to replace comes from insurers and from its failure record rather than from a code order.
Tell us what the house is doing