The head flashing comes in 6' pieces, and one piece can easily do a 3' half round. using two pieces you can do up to a 7' diameter. If you have a larger unit to protect simply add more pieces. You seal the joint by applying a foil backed butyl tape. For head flashing you can apply the tape to the underside of the joint and up the back and then forward over the front of the fins. You can stop before it shows. Works really well to seal the joint. DO NOT CAULK