The move comes despite claims made by the Construction Confederation earlier this month that the pilots do not provide an adequate yardstick for measuring safety performance.
Representatives from construction unions UCATT, GMB and T&G have since been meeting with firms in a bid to sign them up to the pilots.
George Brumwell, general secretary of UCATT, has appointed former Mowlem safety director Tom Gallagher as an independent consultant.
A UCATT insider said Gallagher had met Skanska and Bovis as well as O'Rourke and Mace, and is due to meet HGB, Ballast and Sir Robert McAlpine next week.
Brumwell said there was much support among contractors for the roving rep pilots. He said Mowlem would be signing up to the scheme and that he would be meeting John Gains, the firm's chief executive, next week. Gains is president of the Construction Confederation.
Gallagher said in the new year there would be at least six contractors taking part in the pilots.
He told Building: "The idea is to ensure larger contractors support the pilots so that the smaller and regional firms are influenced into taking part."
We are a little surprised the unions are targeting major contractors
Construction Confederation spokesperson
Contractor O'Rourke confirmed that it would be taking part and project manager Mace has agreed in principle to support the pilots.
A Bovis spokesperson confirmed the firm had been in discussion with Gallagher but refused to be drawn on whether is supported roving reps.
The spokesperson said: "Bovis always takes seriously any initiatives that can help to improve health and safety on site."
HBG told Building that it would not be joining the pilot schemes. Skanska, Ballast and McAlpine were unavailable to comment.
GMB health and safety director Nigel Bryson said the union was also in talks with firms to help win support for the pilots.
Bob Blackman, national officer for T&G, said contractors and clients had expressed interest in the scheme at meetings.
A spokesperson for the Construction Confederation said it could not tell members what to do, but only advise them.