This board sits below the main board, which includes chief executive Andrew Wyllie, and oversees the day-to-day running of the £900 million turnover firm.
Alan Kay, who was the managing director of its major civils arm, is promoted to a new role of chief operating officer.
The firm has created four new contracting operations – environment, infrastructure, community and oil, gas and process.
Darren James, who was previously the firm’s highways director, is promoted to the executive board as managing director for infrastructure with responsibility for highways, rail, airports and nuclear.
Mr James, along with Alex Vaughan, who is now the managing director of community, which involves the firm’s health, education and retails sectors, will report to Mr Kay.
Elsewhere, David Jenkins becomes managing director of environment which houses Costain’s water, waste and marine businesses. And Charles Sweeney becomes managing director of oil, gas and process. The pair will report directly to Mr Wyllie.
The second addition to the executive board is Tracey Wood who is promoted to the position of human resources and legal director.
Current board member Stephen Wells assumes responsibility for strategy as well as his ongoing business development role.
Mr Wyllie said the moves had been made as part of a greater focus on the firm’s target customers. These include blue-chip clients which Mr Wyllie has previously said he wants to see more off after adding retailers Marks & Spencer and Sainsbury’s to its list last year.