Fenton sold the shares raising almost £4.8m. He has stepped down for family reasons after over eight years, which saw him lead a successful IPO in 2016.
Mark Cutler, former regional managing director of Balfour Beatty, will succeed Fenton as chief executive.
Cutler, 49, is expected to take the helm no later than October or earlier if prior agreement can be reached.
Most recently Cutler was managing director of the Balfour Beatty/ Vinci joint venture, with responsibility for the High Speed 2 programme.
From 2010 to 2014 he was chief executive at privately-owned civils contractor Barhale Construction.