Damp patches occur on fibre cement sheets when the roof falls below the dew point of the air.
The reason why damp patches occur on fibre cement sheets is because water vapor in the air can condense on the surface of the roof when the temperature of the roof falls below the dew point of the air. This can happen when the roof is exposed to cool night-time air or when it is shaded from the sun during the day. When the water droplets form on the surface of the roof, they can pool together and create damp patches.
The dew point is the temperature at which moisture in the air condenses into liquid water. On a fibre cement roof, this can happen when the temperature of the roof falls below the dew point of the air surrounding it. This can lead to the formation of damp patches on the roof.
In general, it is important to make sure that a fibre cement roof is properly ventilated to prevent the formation of damp patches. This can help to keep the temperature of the roof above the dew point of the surrounding air, which can help to prevent condensation and the formation of damp patches.