Billionaire Kerry Stokes's Seven Group lifted its stake in the ailing building products group Boral to 16.3 per cent in a renewed bout of share buying over the past few days.

Seven Group, which owns the WesTrac trucks and earthmoving equipment operations and Coates Hire, revealed late on Tuesday that its stake had risen from 12.2 per cent after buying 50.5 million shares in several tranches from July 3 to July 14.