The answer to 'why is my roof decking missing shingles' is that there are a few common causes, including age, weather damage, improper installation, or lack of maintenance. Shingles can become loose, crack, or fall off over time, exposing the roof decking underneath. This can allow water to penetrate the roof and cause further damage.
If you notice missing shingles or exposed roof decking, it's important to have a professional roofing technician inspect the roof as soon as possible. They can identify the root cause and recommend the best course of action, whether that's repairing or replacing the damaged shingles, or potentially needing a full roof replacement.