Construction firms added jobs in 40 states and the District of Columbia over the past 12 months and in 30 states and D.C. between April and May, according to an analysis of Labor Department data by the Associated General Contractors of America.

Association officials say the employment gains help, but caution that construction employment remains below peak levels in every state and the District of Columbia, except North Dakota.

“With demand for construction growing in most states, many firms are slowly rebuilding their depleted payrolls,” says Stephen E. Sandherr, the association’s chief executive officer. “But if overall economic growth slows, construction employment could backslide in many states.”

Nevada led all states in percentage gains in construction employment (12.5 percent, 7,000 jobs) between May 2013 and May 2014, while California added the most new construction jobs for the year (37,700 jobs, 5.9 percent).

Ten states shed construction jobs during the past 12 months, with West Virginia losing the highest percentage (-6.8 percent, -2,200 jobs) and New Jersey losing the most construction (8,500, 6.2 percent).

Minnesota (3,800 jobs, 3.6 percent) added the most jobs between April and May, while Wyoming (4.1 percent, 900 jobs) had the highest percentage increase for the month.

Nineteen states lost construction jobs for the month, with Florida (-6,100 jobs, -1.5 percent) losing the most. Arizona (-4,100 jobs, -3.3 percent) experienced the highest monthly percentage decline.