What Causes a Bathroom Vent to Stain a Painted Metal Roof?

Uninsulated Ventilation Pipe

• The fan vent pipe carries the moist, warm air out of the bathroom at a rate between 60 and 130 CFM (cubic feet per minute). This warm air will heat up the ventilation pipe and, if the pipe is uninsulated, will convect from the pipe to the metal roof where it will form condensation. This process can cause the area around the roof exhaust to rust through creating other problems. The best solution to this problem is to always use insulated ventilation pipe for bathroom ventilation. This prevents the convection process from the pipe to the roof panels.

Improper Roof Vent

• Not all roof vents are designed to have bathroom vent pipes attached to them. Attachment of ventilation pipes to standard roof vents simply blows the exhaust air from the bathroom at the vent and not necessarily out of the attic completely. This leads to moist air blowing along the bottom of the roof surface around the roof vent causing water damage. This damage will stain the roof surface as it will change the composition of the metal as it rusts. Installing a proper bathroom roof exhaust vent where all of the exhaust air is expelled from the bathroom will prevent this.

Low Pitched Roof

• Some times even when everything is installed correctly, staining and rusting can occur around the roof vent. The build up of moisture on low sloped roof that have low profile exhaust vents allow the ventilated moisture to sit on the roof surface directly in front of the vent. Streaks of rust are common as the galvanized coating on the screws holding the flashings around the vent become corroded as well as the flashing. If there are large stains from the bottom of the flashing, then the galvanized flashing is rusting and should be replaced to avoid further staining.