NEVER use an extension cord. If you do not understand the electrical requirements needed regarding the electrical receptacle plug, contact your local electrical inspector or a licensed electrician.
The reason for this is due to safety and performance of the system. Extension cords are often not properly selected and will degrade, potentially leading to electrical shorts or other hazards such as fire. Roof de-icing cables are designed to be connected directly to a properly rated outlet, and the length and type of extension cords may not be rated for the same power load, leading to insufficient power supply, potential damage to the cables, and further safety risks including their ability to prevent ice dams on your roof.