Can I install I install Onduline sheets upside down?

No, definitely not. The underside does not have any UV protection, has less bitumen saturation and also the side laps will not seal properly, as they finish incorrectly.

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The topside of the roof sheet is sprayed with a resin which helps to combat UV degradation. The colouring is a natural pigment, which is impregnated into the sheet, prior to the bitumen dip. This process ensures an equal colouring and means the colour cannot peel from the sheet.

The underside of the roof sheet has a rippled finish. This increases the surface area, enabling it to combat condensation. It will hold the moisture for as long as it can, until the airflow dissipates the liquid, by turning it back into vapour.