Final answer:
The common minimum clearance to combustible materials from the top of a gas refrigerator is 18 inches, but one should refer to local codes and manufacturer guidelines as they can differ.
Step-by-step explanation:
The minimum clearance to combustible materials from the top of a gas refrigerator, except when specific allowances are made, is generally 18 inches or 457 mm. This clearance is a safety measure to prevent any heat produced by the refrigerator from causing ignition of combustible materials. It's important to always refer to local codes and manufacturer recommendations, as they can vary by location and appliance model.