The contractor, which has been campaigning to change sector for some time, was refused a re-listing by FTSE indices manager FTSE International in late May.
It is attracted to the sector because support services companies tend to have better price/earnings ratios than their construction counterparts. This tends to make them more attractive to investors.
Under the Comax deal, which will be partly financed by a £67m rights issue, Amey will repay £59m of Comax’s net debt.
Analyst Leslie Kent of stockbroker Seymour Pierce said: “Traditional contracting now represents only 9% of Amey’s turnover, and earnings should be more predictable. They are really dragging the building industry into the 21st century.”