If the roof pitch is not a problem, it can be a DIY repair. Rain caps are sold at most hardware stores. A roofing contractor is best equipped to handle steep pitches and can check the condition of flashing while he makes any repairs. Probably either trade could do that repair. If you are planning to service the furnace anyway,ask the HVAC company if they can make the stack repair. If you want to check any other issues on the roof like flashing, gutters, vents etc, then call a roofing specialist.
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