A crucial indicator of homebuilder sentiment just suffered its biggest monthly drop in the index's 35-year history as the coronavirus pandemic hammered the American economy.

Builder confidence in the market for single-family homes plunged 42 points to a reading of 30 in April, the lowest point since June 2012, according to the latest National Association of Homebuilders/Wells Fargo Housing Market Index. The survey dates to January 1985.

The HMI reading was expected to drop to 55. Anything above 50 is considered positive. The last negative reading was in June 2014.

This month's survey was conducted from April 1 to April 13, after millions of Americans had been issued orders to stay home to stem the spread of the virus, prompting historic waves of business closures, layoffs and furloughs.

"This unprecedented drop in builder confidence is due exclusively to the coronavirus outbreak across the nation, as unemployment has skyrocketed and gaps in the supply chain have hampered construction activities," said NAHB Chairman Dean Mon, a homebuilder and developer from Shrewsbury, New Jersey.

Of the index's three components, current sales conditions dropped 43 points to 36, sales expectations in the next six months fell 39 points to 36, and buyer traffic decreased 43 points to 13.