The Millboard composite decking was still perfect in every way after 7-8 years. The joists Use Class III treated, fixed with stainless fixings and kept off the floor still suffered rotting.
The joists simply crumbled upon inspection. The likely cause was that that whilst sufficient gaps were left between boards they had clogged up and stopped the air from circulating.
The decking boards were lifted, joists and posts replaced and decking boards refitted.