The Union of Construction, Allied Trades and Technicians (UCATT) says many staff are paid via accountancy firms which take admin fees and costs.
It says affected workers include those working on the Heads of the Valleys road expansion in Blaenau Gwent.
A Welsh government spokesman said the deal signed with contractors Carillion includes fair payments for employees.
Workers said they were told they had to stop being classified as self-employed to comply with HM Revenue and Customs (HMRC) rules and be paid as an employee through an agency or accountancy company.
A spokesman for Carillion said legislation instigated by HMRC in April 2014 made a self-employment scheme unfeasible for workers who are "under direct supervision, direction and control".
"It is for the individual to decide which payment route they opt for," he added.
This affects 48 people working on the A465 contract.
Nick Blundell, regional secretary of UCATT Wales and South West, said: "Workers are being ripped off by the umbrella company con-trick and they are now struggling to make ends meet."
HMRC said it would never require the use of an umbrella company or any other intermediary structure.
Huw Irranca Davies, Labour MP for Ogmore, added: "It's not illegal, but it's completely immoral and I'm even now hearing people from within these umbrella companies that they don't want to be doing it but they have to, because everyone else is."
Companies in Wales are being asked to change how agency staff are paid amid claims workers are being "ripped off".