The provided context is insufficient for a comprehensive guide on repairing sofa springs. Therefore, this answer is based on general principles of furniture construction and upholstery repair.
The process of repairing sagging sofa springs fundamentally begins with a diagnostic phase to identify the type of spring system and the precise point of failure. The most common types of spring systems are serpentine, also known as sinuous or zigzag springs, and coil springs. To inspect the system, the sofa must be overturned and the dust cover, a fabric layer typically called cambric, must be carefully removed from the underside of the frame.
For sofas with serpentine springs, sagging is often caused by one of three issues: the metal clips that secure the spring to the wooden frame have broken or pulled loose, the spring itself has lost tension and become stretched, or the tie wires connecting the springs to one another have failed. If a clip is the culprit, it can be replaced by unhooking the spring, prying out the old clip, and screwing a new one into the frame. The spring is then re-tensioned and hooked into the new clip. If the spring itself is stretched or broken, a replacement of the same length and gauge is required. For general weakness, adding new perpendicular support wires or strong upholstery twine across the springs can redistribute the load and restore firmness.
Repairing a traditional coil spring system is a more intricate task. These systems often feature springs that are hand-tied to each other and to the frame with specialized upholstery twine. Sagging in this case usually results from broken or stretched twine, which compromises the integrity of the entire interconnected unit. The repair involves re-tying the springs, a skilled process requiring specific knotting techniques to achieve the correct tension and crown shape. If a coil spring has broken or the webbing foundation it rests on has torn, both the webbing and the spring may need replacement, a process which necessitates tools like a webbing stretcher to ensure a taut base.
While not a structural repair, a common temporary solution for sagging is to place a rigid support, such as a thin piece of plywood or a commercially available sofa support board, underneath the cushions. This measure addresses the symptom of sagging by providing a level surface, but it does not correct the underlying mechanical failure of the spring system. For permanent and structurally sound repairs, especially on valuable or antique pieces with complex spring systems, consulting a professional upholsterer is advisable.
