meccarroll wrote:........A lot of older roof failure occurs when a slate roof is re-covered with tiles (concrete tiles) which are much heavier than the original slate tiles........
True, and as an aside, most current rooves are designed with the assumption that the heaviest possible roofing materials are going to be used, whether or not they are actually used. Another major cause of roof failures from 60's and 70's rooves is the lack of roofspace ventilation: insulation stuffed hard up to the underside of the felt, no eaves or gable vents, and no ceiling-level vapour barrier combine to have some roof spaces dripping with moisture, leading to untreated timber rotting rapidly, and to the gang-nailed plates you describe failing through corrosion.