On stripping a small flat roof recently over a bathroom we soon could see that the decking was rotten. Surprisingly this was a close boarded roof, of 6x1 planks, and should have been in good shape - but all were totally perished as were a few joist ends. The Culprit - Badly Installed Extractor The actual flat roof itself only had a few cracks so we were surprised as to why the timbers were so bad. On further investigation we found an extractor fan was fitted in the bathroom ceiling but had no way of venting outside, the hot wet air was simply discharging straight into the ceiling void beneath the decking. And slowly rotting the timbers supporting the flat roof. Apparently the fan was fitted 20 years ago, no wonder the timbers were gone.