Over Sill (2.5m) – 110mm depth The Over-Sill is designed to extend the length of an existing window sill, ensuring that it overhangs the insulation. The purpose of this particular type of Verge Trim is to prevent water ingress behind the insulation, which can cause serious damage. This Over Sill is 110mm in length, therefore is suitable for any insulation thickness between 50 – 80mm.