Thread: Flashing for an old sash window in a weatherboard

Lets start from the bottom of your question and work up. The window must be fixed through the sides, weights or no weights. The reason for this is any nails through top and sill would puncture the flashing.
There is the method called "beaded in" where the frame has an outer and inner architrave or moulding that holds the window in position. As for clearance around the window, the top is the most important 10-12mm to allow for frame settlement. The sides only require enough clearance to allow for flashing up turns at each end and are packed back to the frame where the holding nails are located. The sill is raised only enough to allow the flasing to drain to the outside. The sill flashing extends the full width of the window and turns up against the studs which are notched with a saw cut, angled down towards the ouside, into which the ends of the flashing are tucked. The internal edge of the flashing is turned up to finish just below the top edge of the architrave. The outer edge turns down over the external cladding.
Top flashing runs behind first line of cladding and sarking if present, on face of studs/head then lips over the front edge of the top of the window frame. The ends run past each side of the window at least 100mm with the bottom edge of these extensions trimmed up to the bottom of the weather board.
Hope this helps.