Condensation will occur when warm, humid air extracted from the bathroom, shower, kitchen or utility room hits the cooler surface of your ventilation duct.
If the extracted air is significantly cooled and you have a vertical pipe the moisture will condense back to water and become trapped in your ducting, potentially causing damage to the fan and property.
Systems using Roof Vents or Ducting rising vertically carrying extract air to outside benefit with the use of a condensate trap, this is to prevent any moisture forming inside the ductwork, dropping back down and into the fan unit.
This should then be connected to a suitable soil vent pipe using a 21.5mm overflow pipe.
Constructed of PVC
More Info - https://www.bpcventilation.com..../ductwork-condensati