It’s surprising how often I come across this issue when doing home inspections: metal flue pipes serving furnaces and water heaters that come in contact with roof sheathing and underlayment in the attic. When this occurs, it’s a very obvious safety issue.
This issue is especially common when roofs are replaced. Why? Contractors may neglect to appropriately cut back roofing materials during installation.
If you want to make sure your home doesn’t inspire David Byrne to write another song about burning structures, you’d be well-served to provide 1-inch of clearance, free and clear, around the flue pipe at all times. And again, this is especially important if you have had your roof recently replaced.