Federal Reserve Chair Janet Yellen said banks’ reluctance to lend to any but the best credit risks is holding back the housing recovery.

“It is difficult for any homeowner who doesn’t have pristine credit these days to get a mortgage,” Yellen said at a press conference in Washington following the central bank’s policy meeting. That, she said, is ’’one of the factors that is causing the housing recovery to be slow.’’

“It is important for us to work to clarify the rules around mortgage lending to create an environment of greater certainty” so lenders can extend more credit, she said.

Builders broke ground on 1 million U.S. homes at an annual pace in May, indicating the industry is picking up this quarter after a weather-induced slump to start the year, a Commerce Department report showed yesterday.

A strengthening job market and a retreat in mortgage costs in recent weeks are helping support residential real estate following a lull in building in early 2014. Yellen flagged housing as a risk to the outlook in testimony to Congress in May.

Residential investment subtracted from growth over the past two quarters after boosting the economy since the end of 2010, according to Commerce Department figures. Gross domestic product shrank at a 1 percent annualized rate in the first three months of 2014 as homebuilding slowed.