The answer to 'how do I know if my roof needs to be replaced' is that there are several signs that indicate your roof may need to be replaced. These include missing, cracked or curling shingles, significant roof sagging, and leaks or water damage in your home's interior. Older roofs, typically over 20 years old, often require replacement due to normal wear and tear.
If you notice any of these issues, it's best to have a professional roofing contractor inspect your roof to determine if replacement is necessary. They can assess the condition of your roof and provide recommendations on the best course of action.