Can an extractor fan be vented through a soffit vent?

Why the soffit is the wrong place

Soffit vents are intake vents. On a working attic, air flows in at the soffits and up to the ridge. A fan duct that ends at or behind a perforated soffit panel releases warm, wet air exactly where the attic is breathing in. In a Georgia winter that moisture condenses on the cold underside of the roof deck; in summer it adds to an attic that is already humid. Signs include mildew on the soffit panels, rusty nails at the eave, peeling paint, and dark stains on the decking near the outlet.

What the code says

IRC Section M1501.1 requires exhaust air to go outdoors and specifically excludes attics, soffits, ridge vents and crawl spaces. Some building officials accept a duct sealed tight to a dedicated exhaust fitting mounted in a solid section of soffit, since the air then goes outside rather than into the soffit cavity. Others do not accept any soffit termination. Ask your local building department before going that route.

Better terminations

• Roof cap: a hooded exhaust cap with a damper, flashed into the shingles. Short, direct runs work well, and it is easy to add during a reroof.
• Wall cap: through a gable wall when the fan is near an outside wall.
• Insulated duct: whichever way you go, insulate the duct in the attic and keep it short and straight so the air does not condense inside and run back down to the fan.

For how intake is supposed to work, see roof ventilation basics; for how new caps get flashed, see roof flashing explained.