I prefer to run a bath vent to outdoors via a horizontal line that goes across an attic and out through a gable end wall or one that vents down and outside through a roof overhang or soffit.
Bathroom exhaust fan through flat roof.
If you have access to the attic the fan can vent either through a gable wall or roof.
This article describes routing bath exhaust fan duct upwards through an attic or roof space or downwards through a floor or crawl space.
It also indicates that on sloped roof the termination should be.
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The warm air will exhaust out the duct and enter back into the attic through the soffit vent or ridge vent.
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Avoid venting through a soffit vent or ridge vent.
Vent your bath and kitchen exhaust fans through the roof through a special roof hood.
Bath exhaust fans routed up through the roof.
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Bathroom ventilation fan duct routing routing a bath vent duct down out or up through an attic or roof out.
Venting through a roof vent or exhausting them in the attic could cause moisture problems and rot.
My interpretation is that you are not allowed to place a fan vent termination on a flat roof.
Letting the fan exhaust into an open attic will cause moisture buildup on the underside of the roof.
This discussion is now found at bathroom vent up through roof.
The best exhaust fan venting is through smooth rigid ducts with taped joints and screwed to a special vent hood.