Soffit is 10mm so I need a fixing screw which will drill through both and secure itself to a 4 - 5mm hard old soffit which will no accept a screw w/o breaking up. Note the facia is already UPVC and there is 18mm gap to insert the new soffit in but I need to fix the soffit at the wall side which has no such ledge to support it. There is no opportunity to remove the old soffit which extends over the outer leaf. MAYBE though a grip adhesive will do the job but I would not like to trust the old painted surface. Advice welcome. Many thanks.
