Abstract: A cover cap is provided for a screw fastener device of the type comprising a threaded shank and a head, with washer means below said head, said washer means including a rigid spreader washer located over a resilient washer part. The screw fastener device is especially intended to secure panelling, such as roof and wall cladding, in industrial and domestic buildings. The cover cap comprises a cap member for location over the fastener head, a peripheral wall of the cap member sealingly engaging said resilient washer part. Further the cap includes clip means for clipping to the fastener device below the rigid spreader washer to secure the cap on the screw fastener device. The clip means preferably comprises an annular series of indents, but alternatively the annular wall could include a peripheral skirt of indented U-form defining said clip means.