Root's role will be a new position at the group, which also has operations in Europe and the Far East. The role was previously overseen by Bellwater chief executive Stephen Sproat.
Bellwater director Roger Francis said Root would concentrate on the London fit-out market as well as the firm's operations in Scotland. He said: "It's refreshing to have a new face at the firm."
Francis is confident about the 12-year-old firm's prospects. He said that he expected Bellwater to turn over at least £50m this year. He said: "We feel most markets are on the rise right now."
Root's departure from ISG comes just a month after founding director Ivan Millar announced his retirement from the group.
Millar will be replaced by John Knight in June.