Outdoor plumbing fails more reliably than indoor plumbing because it is fully exposed and because protecting it requires an active step people forget until it is too late.
Hose bibs
The most common single failure. A hose left connected holds water in the bib and in the pipe immediately behind it, and that water freezes first.
What to do: disconnect every hose, shut off the interior isolation valve if one exists, open the outside tap to drain the line, and leave it open through winter so any residual water has somewhere to expand.
Frost-free bibs are designed to drain automatically to a point inside the heated envelope — but only if the hose is disconnected. With a hose attached, a frost-free bib cannot drain and offers no protection at all. This surprises people who believe they are covered.
Irrigation systems
The failure people most often get wrong, because draining feels sufficient and is not.
Irrigation lines follow the contours of the ground, which means low points where water collects regardless of how the system drains. That residual water is enough to split a pipe or crack a valve body.
Blowing out the system with compressed air clears those low points. It is worth having done properly, both because it works and because too much pressure damages the system — which is a genuine risk with improvised equipment.
Do it before the first hard freeze, not after the first frost warning when every irrigation contractor in the county is booked.
Backflow preventers
Above-ground backflow assemblies are exposed and vulnerable, and they are expensive to replace. They need draining and insulating, or removal where the design allows.
They also typically require periodic testing. Worth handling at the same time rather than as a separate task.
Other outdoor water
Pool and spa equipment, outdoor showers, garden taps at outbuildings, and any supply to a detached garage or shop. Anything carrying water outside the heated envelope needs the same treatment.
Detached structures are the ones most often forgotten, because they are not part of daily routine. A shop tap that nobody has used since August is exactly the sort of thing that bursts unnoticed.
Western versus Eastern Washington
East of the Cascades this is mandatory annual maintenance. Sustained cold means anything holding water outside will freeze, and thoroughly.
West of the Cascades most winters would forgive skipping it, which is precisely the problem. The occasional hard snap catches properties that have got away with it for years, and the damage is no less expensive for being infrequent.
Timing
Do it while it is still comfortable to be outside, in autumn. The alternative is doing it in a hurry on the evening of the first freeze warning, in the dark, badly.
Spring reversal
When turning systems back on, do it slowly. Opening a valve fully on a system that has been dry all winter sends a pressure surge through it, and that is a common cause of spring failures in irrigation systems that survived the winter intact.