The answer to 'why is my roof decking curling shingles' is that the roof decking may be warping or cupping, causing the shingles to curl upwards. This is often due to improper installation, water damage, or the use of substandard materials. Curling shingles can lead to leaks, reduced lifespan, and an unsightly appearance.
To address this issue, you'll need to inspect the roof decking for signs of damage or warping. If the problem is limited to a small area, you may be able to replace the affected decking yourself. However, if the issue is widespread or you're unsure of the cause, it's best to call in a professional roofing contractor to assess the situation and provide a solution.