My bathroom exhaust fan and kitchen exhaust fan ductwork are joined in the attic and only vented to within an inch of the main roof vent rather than their own vent through the roof. We are preparing to re-roof our house, should these ducts be split and run directly through the roof, or is it OK to have them joined and end just shy of the main roof vent?

Venting into attic not a good idea. Adds moisture to attic and can cause condensation.

Chuck

OK, so I will need to put it through the room, Is it OK to have the two fans exhaust joined into the same ductwork?

Oops, I meant through the roof, not room.

These fans should not be joined. Each exhaust fan should have its on exit. Usually kitchen and bath fans exit through the wall or the soffit, not through the roof.

Harold's dead on! Venting into the attic would trap all the moisture in your home instead of venting it outside. The two exhausts should have separate terminal exits. To connect the two together would run the risk of exhausting one fan into another. Good luck, Tom

Even UPC is with agreement Harold... Go to HD, get vent roof jacks and install them before roofing goes on. Each vent should have its own, separate, outlet.