Pipes rarely fail all at once. They give hints for years: a little rust in the first glass of water, a shower that loses pressure whenever the washing machine starts, a pinhole leak patched with a clamp and forgotten. At some point the patches add up, and the question shifts from fixing the next leak to replacing the supply system outright.
What usually wears out
Galvanized steel is the usual culprit in older houses. It corrodes from the inside, and the buildup slowly narrows the pipe until fixtures at the far end of the house barely trickle. Because the damage is internal, the pipe can look fine from the outside right up until a joint lets go. Early copper holds up better, but it can pit, especially at fittings and in spots where water flows fast and turns sharply. Some mid-century and later houses also have plastic lines from product runs that turned out to be failure-prone.
Mixed systems are common too. Many houses have been repaired piecemeal, with copper spliced into galvanized and a run of PEX added during a kitchen remodel. Each transition is a potential weak point, and dissimilar metals joined without the right fitting can corrode where they meet.
Signs it is time to stop patching
Discoloured water that clears after running, recurring leaks in different parts of the house, pressure that drops sharply when two fixtures run together, and visible corrosion at exposed joints all point the same way. A single leak is a repair. Several in a short span, especially on the same type of pipe, usually means the rest of the line is close behind.
Cold weather adds its own stress. Pipes running through crawlspaces, exterior walls or unheated garages are the ones that freeze and split, and older lines that are already thin give way first. It is worth reading Keeping Pipes From Freezing in West Seattle's Older, Wind-Exposed Houses if your lines run anywhere exposed; the same vulnerable runs tend to be the ones that need replacing first.
How a repipe actually goes
The work starts with mapping where the existing lines run and deciding on a route for the new ones. Crawlspaces and unfinished basements make this much easier, since new lines can often run underneath and come up through floors with minimal wall opening. In finished spaces, the installer cuts small access holes at strategic points and fishes pipe through, then patches afterward.
Most households can stay put through the job. Water is shut off during the day and restored in the evening, with the final switchover done once the new system is tested. Material choice comes down to the house and the budget. PEX is flexible and quick to route through tight framing; copper is durable and familiar. Either way, the new system should include proper shutoffs at each fixture and a main shutoff that actually works.
For more background on what a full replacement covers, this overview of replacing aging supply lines in older homes walks through the stages.
Things to sort out before work begins
Ask how drywall and plaster openings will be handled and who patches them. Confirm whether permits are being pulled, since supply-line replacement generally needs one. Find out whether the water heater connections, hose bibs and the main line from the meter are included or treated separately. If the house has a cast iron drain stack, ask for a look at that too; it is often the same age as the supply lines and may be due for attention.
Finally, think about future plans. If a bathroom remodel is coming in a year or two, it makes sense to rough in for it now rather than open the walls twice. A good conversation with the installer about what a full system update involves can save a lot of rework later.