A saggy sofa not only looks ratty, it does not provide the proper support for cushions and seating. But you don't have to toss that old couch out just yet. You can choose a temporary option for repair, or a more permanent one to fix your sagging sofa. Some couches have springs with webbing that keeps them in position, while others just use webbing for the seat support.
Measure the dimensions of the seating area after removing the cushions on the couch.
Remove 1/2 to 1 inch from the width of the plywood, so that when the plywood is set under the cushions, it doesn't rub against the backs of people's legs when they sit. Transfer these measurements to the plywood sheet.
Cut the plywood sheet using the circular saw. Before cutting, prop the plywood on the blocks of wood for easy cutting.
Sand the cut rough edges of the plywood to stop it from catching on the couch cushions.
Insert the cut end of the plywood onto the couch first. Replace the cushions on top of the plywood.
Remove the dust cover that sits atop the springs or the webbing. Pull the staples or upholstery tacks out along the edge with pliers. For a couch with springs, verify the springs are tied with cord. Retie, if necessary, or replace tied but broken strings in the same way as the originals were installed.
Turn the couch over if it has springs. If it does not have springs, skip to the next step.
Note the weave pattern of the webbing on the couch -- it follows an under-and-over pattern. Remove the broken webbing straps.
Secure the webbing on one end of the frame by folding it over 1/2 inch and securing it with a staple gun.
Set the rubber end of the webbing stretcher on the outside of the hardwood frame of the couch opposite from where you secured the webbing. Pull the webbing taut across the couch over the top of the webbing tool. Hold the webbing material and tool, and push down on it to get it as tight as possible. Staple the webbing in place; cut the strap and fold the end of the web over 1/2 inch, and secure again.
Repeat Step 5 for all the vertical straps.
Cut webbing straps long enough to attach to the hardwood frame, leaving enough to grab with the webbing stretcher. Secure the first strap as close to the front of the couch as you can, and weave it under and then over the vertical straps, alternating this with each vertical strap. Repeat the process for stretching the webbing over the web stretcher, and secure the webbing to the frame.
Measure and cut a piece of muslin large enough to include 1/2-inch seams folded under to act as a dust cover, if your couch had one. Staple the dust cover in place all along the edge of the hardwood frame.