Equally suitable for rear-vented freestanding rangehoods and top-vent wall mounted canopies, roof ducting is the most efficient way to vent your rangehood. With no major bends and nothing to obstruct the passage of air your rangehood can work to its maximum capacity. Roof ducting can of course be achieved with a straight run of duct in many instances but the conviguration shown in the illustration allows the duct to go around rafters and purlins where this isn't possible.
One word of warning though, many kits are sold with a downdraft cowl, which is a roof cowl designed for reducing backdraft on wood or coal fires. Due to it's design and purpose it's not the best solution for rangehoods. Whereas a fireplace produces slow moving smoke, a rangehood emits fast moving air under pressure. The design of the downdraft cowl has a cone that's great for reducing the downdraft of smoke However, this cone creates a high pressure area that results in back pressure that makes the rangehood less efficient and potentially more noisy. The illustrations below indicate the cowl to avoid.
Bird proof rangehood roof cowl below. Avoid the downdraft cowl on the right.
Incorporating a muffler on top of the rangehood over the outlet (the noisiest part), thiese kits are the quietest ducting solutions you can buy. Rangehood Tamer is available for roof or soffit ducting
With no acute bends to cause back pressure, this is the most efficient way to vent a rangehood. Roof venting allows the rangehood to operate to its full capacity by allowing the air free unrestriced passage.