Derek Lightfoot, chief executive of The Rock Building Society, left the firm on Friday, the company announced yesterday. Lightfoot served six years as managing director.The appointment of a new chair to the board of directors (following the death of the long-serving prior chair) in April appears to have been the catalyst for the removal of the CEO six months later.While operational performance improved in the June 2010 half, the board was responding to long-standing unease over management of costs, including over-runs on the upgrade of the society's core banking system.There is also the mystery of the complaint to ASIC over one of The Rock's products that, according to the August financial statements, generated $190,000 in follow-up expenses in only a few weeks.Stuart McDonald took over as interim CEO of The Rock yesterday.McDonald worked at Suncorp's business bank before being made redundant, following the decision by the bank to wind down that part of its business.