The answer to 'my roof decking is failing' is that the underlying plywood or OSB (oriented strand board) that forms the roof's structural base may be deteriorating or failing. This can be caused by several factors, including water damage, rot, or structural issues.
Some common symptoms of failing roof decking include sagging, soft or spongy spots, and visible cracks or holes. If left unaddressed, this can lead to more serious problems like leaks, structural collapse, and the need for a full roof replacement.