Abstract: A deformable shim assembly for positioning a closure unit in building framework defining a rough opening. The system includes a plurality of flexible, generally identical shims each having a cylindrical surface curvature to provide a convex outer surface and a concave inner surface. The forward end of each shim is tapered and is defined by side edges with an angle of intersection bisected by the axis of symmetry. The side edges also define a plane that intersects the cylindrical outer and inner surfaces at an acute angle to define a wedge-shaped space. The space has a transverse cross section in the shape of a circular segment that diminishes progressively toward the forward end of the element.