The answer to 'shingle replacement sagging' is that the most common causes of shingle sagging are improper installation, old or damaged shingles, and poor roof structure. Symptoms include visible dips or depressions in the roof surface, and the shingles may appear loose or detached. Solutions can involve replacing the damaged shingles, repairing the roof structure, or in some cases, a full roof replacement.
If the sagging is limited to a small area, it may be possible to DIY the shingle replacement. However, if the sagging is widespread or the roof structure is compromised, it's best to call a professional roofing contractor to assess the issue and provide the appropriate repair solution.