Flexible DUCT Connection AutoCAD File; download DWG File of Duct Branch connection CAD drawing shows opening in floor or wall shall be 3 mm per foot greater than damper dimensions (4.5 mm. greater per foot for stainless) minimum clearance of 6 mm and required for any installation. curtain type vertical fire damper, complete with fasteners, sleeve and perimeter angles, refer to manufacturer's installation instruction for details