The answer to 'why is my flat roof sagging' is that a sagging flat roof is often caused by structural issues or water damage. Common causes include inadequate roof support, excessive weight from snow/debris, rotting roof joists, and water pooling on the roof. Symptoms include visible sagging, cracks in the roof surface, and water leaks.
If the sagging is minor, it may be possible to DIY with repairs like reinforcing the roof structure or removing excess weight. However, significant sagging often requires professional roof replacement or major repairs to address the underlying structural problems. It's best to have a licensed roofer inspect the roof to diagnose the issue and recommend the appropriate solution.