The answer to 'shingles sagging' is that sagging roof shingles are typically caused by improper installation, aging, or water damage. Symptoms include visible dips or depressions in the roof surface, loose or cracked shingles, and leaks. Solutions may involve repairing or replacing the damaged shingles, reinforcing the roof structure, or in severe cases, a full roof replacement.
Shingle sagging is often a sign of a more serious underlying issue, so it's generally best to have a professional roofing contractor inspect the roof and provide a recommendation. DIY repairs may be possible for minor sagging, but extensive damage or structural problems should be addressed by an experienced roofer to ensure the safety and integrity of your roof.