The answer to 'why is my roof decking sagging' is that the roof decking is likely sagging due to structural issues or water damage. Roof decking is the plywood or OSB boards that form the base of the roof and support the shingles. If the decking is sagging, it can be a sign of underlying problems like water leaks, rotting wood, or inadequate structural support.
Some common causes of sagging roof decking include age and wear, poor installation, water damage from leaks, and excessive weight from snow or other heavy loads. Symptoms may include visible sagging or uneven areas on the roof, cracked or warped decking, and leaks or water stains on the ceiling below. Solutions can range from repairing or replacing the decking to addressing more serious structural issues.