That depends on whether you have a standard efficiency or a high efficiency furnace.
Venting furnace through metal roof.
Metal roofing works well with this because the metal is unaffected by heat not being able to pass through it and enter the attic.
So which type of venting system should your furnace have.
Should you vent a metal roof.
In this video i cover installing a roof boot for a vent pipe in a metal roof.
The other option of course is a vented attic which requires an equal balance of intake and exhaust vents.
Match the vent pipe to the furnace maker s recommendations either sheet metal or vinyl.
Second during cold winters you ll be blowing warm moist air onto a cold surface the roof vent and roof plywood.
If there is no sub roof to support the metal roof consider installing slatted vents just under the eaves instead of on the roof.
Installing some type of ventilation mechanism on a metal roof is not a lot different from the addition of vents to any type of roof.
If your furnace has an afue rating below 90 percent it will most likely have a flue pipe that goes up through your roof.
That s because the combustion byproducts are in a gaseous form so the gases can float up through the flue pipe and out of your roof.
The water will condense and drip into the insulation below and perhaps into the house.
Run the vent pipe up through the roof and install a metal or pvc flashing collar over it.
In many cases for existing homes this is the easier way to go.