Listing Requires:
No combustibles within 20 inches of the back of the fireplace.
No combustibles within 14 inches of the sides of the fireplace.
No combustibles within 10 inches of the top of the fireplace.
UL approved heat shields reduces clearance to 3 inches from combustibles

The heat shield is constructed on site. UL Listing requires the Combustible Wall Heat Shield the top of the heat shield to remain open for proper air flow. A MINIMUM 2 inch x 3 inch wide Air Flow opening at the bottom of the heat shield between the floor and metal shield to remain open.