A Missouri American Water main broke around noon Wednesday on south Ladue Road in the Schellridge neighborhood.
The main - an 8-inch cast iron pipe - was likely first installed in the 1960s, said Kevin Everler, Missouri American operations supervisor.
Everler said workers were able to turn off the water, dig the pipe up and repair it by 4 p.m. the same day. The pipe broke in a resident's yard; the job did not require workers to dig up the road.
"It probably was caused by metal fatigue and the ground shifting," he said. "It's been pretty dry and hot out."
No boil-water order will be needed, he said.