19 in fig1 c a third fold line 22 is added that is parallel to the first fold line 16 at a third distance d 3 as measured from the outer edge and bisects the sheet 10 fold line 22 defines the width of the seat 30 of sill pan 10 for seating the window frame and the downward flap 24 that extends down the wall 34 see fig2 below the window sill the fold along third fold line 22 also creates a drip edge additionally in fig1 c the back dam 20 is folded upward along fold line 16 relative to the seat 30 in fig1 d a cut is made along second fold lines 18 that extend from the outer edge 13 until the third fold line 22 and the segments 26 and 28 that form the side flaps and the end dams respectively are bent upward and perpendicular to the seat 30 the side flaps 26 are subsequently bent away from the seat 30 and made perpendicular to the end dams 28 the opposing ends 15 of back dam 20 that are defined by the area between the inner edge 11 first fold line 16 and second fold lines 18 are folded upward after a small cut is made to first fold line 16 that extends from the side edges 19 to the second fold line 18 the upward opposing ends 15 seal against the end dams.