I need to attach plasterboard to the bottom chord of trusses that are on 600mm centres. The plasterboard will then be wet plaster skimmed. The plasterer has recommended adding counter battens (e.g. 22x100) at 400mm centres. That works out as a lot of metres of batten to install. Is his concern reasonable or more based on 9.5mm plasterboard than the usual 12.5mm? I could go up to 15mm if that helps.