Final answer:
To fix water backing up in a skylight, the initial step is to clear the skylight drain. If problems persist, check for damage to seals or flashing, and as a last resort, consider replacing the skylight.
Step-by-step explanation:
If water is backing up in a skylight, the most appropriate and immediate action to address the issue would be to clear the skylight drain. Skylights are designed with drainage paths that can become blocked with debris, causing water to back up. It's essential to safely access the skylight and remove any leaves, dirt, or obstructions from the drain area. If this doesn't resolve the issue, inspect the seals and flashing for damage which might require sealing or repair. However, only after addressing these common issues, and if the skylight is old or damaged beyond repair, should you consider the more costly action of replacing it. Installing a drainage system is typically not necessary if the skylight is properly designed and maintained.