The answer to 'why is my roof decking sagging' is that the roof decking is likely damaged or deteriorated, causing it to sag or sink. This can be caused by several factors, including water damage, structural issues, or simply age and wear over time.
Some common signs of sagging roof decking include visible dips or uneven areas in the roof, cracks or splits in the decking, and potential leaks or water damage. In severe cases, the sagging may be severe enough to affect the structural integrity of the roof.