I had a small water stain appear on my roof this winter and went up in the attic to find frozen condensation below the vent where both my water heater and my furnce exit the roof top. Do you have any suggestions on what could be causing this and how to rectify the situation.

Check the storm collar and/or stack flashing for leakage at the roofline.

Even if we had hardly zero rain in the last two months, can air leaking through cause the condensation.

If this is a double-wall b-vent, it could be warming the surrounding roof penetration to the point where any snow accumulation could melt and leak through. Another thought: Do you see any evidence of moisture on the outside of the vent where it passes through the attic? If the combustion draft velocity is insufficient, flue products may be condensing in the vent and leaking through. This could be due to insufficient combustion air or an oversized vent, or a combination of both.

If this is a double-wall b-vent, it could be warming the surrounding roof penetration to the point where any snow accumulation could melt and leak through. Another thought: Do you see any evidence of moisture on the outside of the vent where it passes through the attic? If the combustion draft velocity is insufficient, flue products may be condensing in the vent and leaking through. This could be due to insufficient combustion air or an oversized vent, or a combination of both.

Did not see any moisture on the outside, but went up on the roof and the entire vent stack is frozen solid. Looks like either the stack vent may not be functioning correctly or like you see the velocity is insufficient. Talked to a roofer and he recommended replacing the vent stack since he though it was defective and deflecting the condensation back down and causing it to freeze up. Which when it thaws is causing it to drain back down inside stack or through a cracks in collar. Is there anything that can be modified on this vent from the furnace to help this situation. This is a double wall vent that has two water heaters and a furnace tied to it.