Overview
A projecting sill becomes the lower measuring surface. Check the drop where the bottom frame will actually sit, particularly if the ledge slopes. Confirm clear recess depth and specify the sill detail with the order.
Measuring Inside Mount for a Square-Set Window with a Sill
Take three drops from the sill's upper surface to the inside top of the opening. Take three widths within the recess. Use the smallest width and smallest drop, with no manufacturing deduction of your own.
Measuring Inside Mount for a Window with a Sill and Architrave
Keep width measurements inside the recess even though trim surrounds it. Measure three drops from the top of the sill to the inside top and three widths across the opening. Record the smallest of each and specify the projecting sill in the order.
