All screws should have rubber sealing washers.  Rubber washers should be EPDM quality or the equivalent and be free of carbon fillers.

Buy fasteners that are as durable as the roof sheeting you have purchased and that comply with class AS 3566 Class 3, (AS3566 is a performance based standard that specifies a minimum coating thickness for fasteners to be used in benign (Class 1) to severe marine (Class 4) environments.

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