Jim Baker, chief executive of the Forge Company, said the buyout had occurred because Rok wanted to stick to its core business.
He added that the split, which took place last week, was amicable and that the firm was still working with Rok Property Solutions on six projects.
Forge Lewellyn was the product of a joint venture agreement between Lewellyn Construction and American company Forge.
Llewellyn, which has a turnover of £179m, was acquired by Rok in September last year. Under last week's deal Forge Inc has acquired the assets of Rok's half of the joint venture agreement.
Mike Bailey, Rok group finance director, said: "The business was transferred for a nominal amount."