Half Round Louver Vents are true half circles. Made from durable G90 galvanized steel with soldered seams for strength and sealant is applied when appropriate behind flanges. These decorative louvered gable vents are stocked in popular sizes. Sizes are listed in diameters (base measurement). Rough opening in wall should be 1/8" to 1/4" larger than listed louver size. Please contact us for oversize orders or specialty materials.