Repairing a sagging couch with non-removable cushions is quite a tricky job. You have to enter the couch via the underneath. You could find one of many possible repair jobs awaiting you. You may need to repair or replace the frame, the webbing, the springs or the foam. In a worst-case scenario, you will have to tackle all of these to get the job done.
Establishing the problem
Ask a friend to help you turn the couch upside down. Remove the bottom cover fabric. This is usually stapled in place. Use needle-nose pliers to remove the staples. Check the condition of whatever you discover inside. You may find wooden pieces supporting the springs. According to Afternoon Despatch and Courier, the frame may be compromised. You may find warped springs, perished foam or stretched webbing.
If some pieces of the frame are cracked or warped, you need to replace them. Measure them first and make a note of their various lengths. Then remove them using a hammer, screwdriver and brute strength, as required. Cut pieces of the same length from wood of the same dimensions. Nail or screw these in place onto sound parts of the frame.
Replace the webbing if it has stretched or snapped. Remove the old webbing by pulling out the webbing tacks with pliers. Cut new webbing to length. Nail it in place using new tacks. Master upholsterer Bill Eisenberg uses staples for a very similar job. Remove and replace the webbing and the foam support if both are sagging. Use a utility knife to cut a new piece of foam with the same dimensions as the old. Site it where the old foam was. You may need to cut it into more than one piece to get it in place. Replace the webbing as above.
Box in the springs if they have warped or bowed. This is usually easier than removing and replacing them. Measure the underside of the couch to the edges of the wooden supports. Buy a piece of chipboard to those dimensions. Lay it on the supports. Drill holes through the chipboard and into the supports, approximately every 30 cm (12 inches). Screw the board in place. The Nest suggests plywood is effective too. When all the work is completed, staple the bottom fabric cover back in place. Ask your friend to help you turn the couch over again. Sit and enjoy your repaired couch.