Choose the optional flashing if the opening of your fireplace is too large and the trim is not able to seal the gap between the fireplace opening and the insert. The flashing is available in 3 sided and 4 sided flashing. Choose the 3 sided flashing if you have chosen the 3 sided trim. The 3 sided trim can cover a fireplace opening smaller than 26.75" H x 44.75" W Choose the 4 sided flashing if you have chosen the 4 sided trim. The 4 sided flashing can cover a fireplace opening that is smaller than 36.5" H x 44.75" W. Available in black color to match the fireplace.