Shares of Gibraltar Industries tumbled by nearly 19 percent Thursday after the Hamburgbased building products manufacturer reported its biggest quarterly loss in 17 years.

The loss, even after excluding one-time costs associated with the sale of its steel-processing business earlier this month, surprised analysts who were expecting Gibraltar’s remaining operations to be profitable.

Company executives said they don’t expect Gibraltar to make money during the current quarter, either, as sales remain sluggish. That helped push the stock price down by $2.58, to $11.12.

“We’re not in a normal market yet,” Henning N. Kornbrekke, Gibraltar’s president and chief operating officer, said during a conference call with analysts.

“We think we’re starting to return,” he said. “We believe the economy and our end markets are on the front end of a recovery.”

But that didn’t help Gibraltar during the fourth quarter, when the company was stung by what executives described as “unexpected weakness” in its markets. As a result, Gibraltar lost $29.4 million, or 97 cents per share, which was worse than the $22 million, or 73 cents per share, the company lost a year ago, when the recession began hitting its construction and automotive markets hard.

While most of the loss was caused by $25.6 million in onetime charges related to the steel-processing unit’s sale and expenses stemming from the early repayment of some of Gibraltar’s debt, the company’s operating results in the quarter also were disappointing. Gibraltar’s plant on Walden Avenue in Cheektowaga is closing as a result of the sale of the steel-processing business.

Gibraltar lost $3.1 million, or 10 cents per share, from its continuing operations, which was an improvement from the loss of $9.5 million, or 32 cents per share, a year ago, but was far worse than the 10 cents per share that analysts expected the company to earn.

The company’s sales during the quarter fell by 25 percent, to $187 million, from $249 million a year ago, led by a 28 percent plunge in building products revenues and a 15 percent drop at its processed-metals business.

“We see the first quarter reflecting a similar environment and challenges as we faced during the fourth quarter of 2009, with improvement beginning thereafter and continuing throughout the year,” Kornbrekke said.

Kornbrekke predicted that Gibraltar would become profitable again in the second and third quarters as seasonal factors boost demand and increases in remodeling and repair activity bolster sales at some of its construction products businesses.

Gibraltar has been aggressively cutting costs as its business has sagged, with sales plunging by almost a third in 2009, to $834 million. Over the last two years, the company has closed more than a third of its facilities, slashed its work force by 38 percent, including a 50 percent reduction in staff at its Hamburg headquarters, and reduced its debt by 47 percent.

As a result, Gibraltar has lowered its break-even point to about $650 million in sales. Brian J. Lipke, the company’s chief executive officer, said he believes that the firm is in a position where its profitability can improve significantly with even a slight rebound in its construction markets.

Because of the weak demand, Gibraltar’s factories during the fourth quarter were running at only about 40 to 45 percent of their capacity, which Kornbrekke said could comfortably handle about $1.4 billion in volume.

At most of its factories, the company currently is running only a single shift, four or five days a week, rather than the double shifts six days a week that Kornbrekke would prefer.