My gas range vents throgh the roof it is getting water in the pipe, i tried changing the roof cap put on rainproof cap, still getting water in the vent pipe i noticed the pipe going through the attic and out the roof is thin alluminum but it is sealed and in good shape on roof, could this problem be from condensation or something else.
Probably condensation. Do you use it a lot, and is it cold outside where you live? The warm air in the vent hitting the cold pipe in the attic could lead to condensation. If it is a small residential range hood you can probably insulate the pipe to reduce this. If it is a larger hood, there may be clearance requirements to its vent. Check with the manufacturer of the hood to see if you can insulate the vent pipe.