In cold weather, water may freeze in underground pipes laid above the
frostline or in pipes in unheated buildings, in open crawl spaces under
buildings, or in outside walls.
When water freezes it expands. Unless a pipe can also expand, it may
rupture when the water freezes. Iron pipe and steel pipe do not expand
appreciably. Copper pipe will stretch some, but does not resume its
original dimensions when thawed out; repeated freezings will cause it
to fail eventually. Flexible plastic tubing can stand repeated freezes,
but it is good practice to prevent it from freezing.