Venting The Range 90 Degree Turn . The sealant won't deteriorate because it is under the pipe flashing, protected from the sun's uv. From there the duct can utilize a framing joist bay and extend to the envelope or, more commonly, the duct can continue to the roof. NEW 4" 90 Degree Turn Close Elbow Aluminum Dryer Vent Exhaust Duct from www.pumpenscout.de There is also a skylight directly above the area where the range is leaving very little, if any room for an exhaust vent. You may also need adaptor pieces to fit the round microwave exhaust into the square ductwork. For most homeowners, 900 cfm will be enough to vent a 30” or 36” gas range.