Thank you for your question on the model 644 aluminum roof cap. The 644 is designed to be used with 3-1/4” x 10” rectangular or up to 8” round duct work. The duct work will come up through the roof and stick up approximately 1” above roofing material. The 644 will attach to the roof over the top of the duct work. The duct work does not actually attach to the roof cap. The 644 has a back draft damper and bird screen. This unit is made from aluminum and has a natural aluminum finish. The Broan model 634M is our roof cap for 6" round duct. This may be a better option for you. For further assistance please contact the Technical Service Department at 800-637-1453.