You could calculate the unexpected expenditure on repairs (not defects) over the past five years, average it out and add 5 per cent, then factor that into your budgets.
The chairman of a building I know well used to announce at every AGM that “there is no slush fund” until I pointed out he was defaming his predecessors by implying they had been finagling the finances. Also they eventually, inevitably needed an emergency payment. They now have a “contingency” fund which, of course, is a slush fund by any other name.
If you announce your plan at your next AGM, the owners can decide whether or not they want a contingency fund or would rather pay special levies as they arose.