Abstract: A blocking system or backing system for stud buildings provides expandable and retractable backing spans for placement and fitting between the studs. Each backing span is composed of multiple elements including a first backing element having a stud face for abutting against the web or wider dimension of a stud, and a second backing element for bearing against the first backing element. The first and second backing elements are formed with complementary bearing surfaces and are movable relative to each other with a component of motion in the lateral direction. The relative motion permits expansion and retraction of the backing span for spanning variable spacing between the studs. In the preferred example the backing span further includes a third backing element having a stud face for abutting against the web or wider dimension of an adjacent stud.