How To Install Interior Extension Jambs And Window Trim. Add finish trim to the inside of the window nailing into the window frame and the jamb extension to. Fastened to the window casing they rest on a sill which is pocket screwed into the framing.
Lay the jambs on the floor in the order they will go on the window making a box of sorts. Cut the protruding shims flush with the edge of the drywall using a sharp utility knife. First we added the board that would be placed at the top of the window jamb then moved to the sides.
The extension frame will be smaller than the rough opening so it will need to be shimmed to get it centred and keep it square.
This way I get a consistent reveal around the entire window. And again make sure you test your boards first to make sure they will fit. To create the PVC jamb extensions Roth followed several simple steps. Lay the cut pieces on a flat surface and attach them to the un-finished or the side towards the wall and the attachment flange facing the outer edge.