Abstract: An adjustable shim for positioning a closure unit in a rough opening. The closure unit includes a frame with a perimeter edge and at least one longitudinal recess in the perimeter edge. The adjustable shim includes an attachment member adapted to releasably couple with the longitudinal recess and at least one deformable leg having a first end and a second end coupled to the attachment member. An elongated member is coupled to the deformable leg in a compact configuration. Displacement of the elongated member in a first direction deforms the leg into an expanded configuration. The adjustable shim in the expanded configuration creates opposing forces simultaneously acting on the perimeter edge and the rough opening sufficient to suspend the closure unit in the rough opening. A method and system for installing a closure unit in a rough opening using the adjustable shim is also disclosed.