Rotten wood is the main cause of the shingles to sink downward.

The chimney's failing mortar and caulk were in total damaged.

As the wood increases in moisture content, it can cause movement of the decking. As the new roof is installed, moisture is trapped in the system, and roofing felt used as underlayment can absorb moisture and wrinkle.

Excessive weight to the existing structure can cause roofs to sag. This includes snow, ice, wind as well as sheathing and roofing materials. Putting too many layers of shingles on your roof can also result in sagging and it can hold water on it.