A sagging sofa can be a pain in the backside, literally. A sofa that is no longer firm and supportive can be uncomfortable to sit on. It can also be hard on your body, and cause muscle aches and pains. There is a good way to fix a sagging sofa and make it feel like new again. It's not expensive or hard to do. You may have the common materials you need on hand.
Remove the cushions from the sagging sofa. Measure the width and length of the base of the sofa.
Measure and cut a piece of plywood you have on hand to match the measurements. Or, take the measurements to a home improvement store and have them cut a piece.
Wrap the plywood piece in cotton batting or in sheets of foam to protect the sofa from the rough edges. Staple the edge of the material in the middle of the bottom side (rough side) of the plywood.
Pull the material across the rest of the middle and then wrap it around the edge. Staple the cotton batting or foam near the edge to hold it in place to fix the sagging sofa.
Pull the material across the top side (smooth side) of the plywood piece and around the other edge. Overlap the beginning edge with the cotton batting or foam an inch, then staple the edge in place. Cut off the excess material.
Position the wrapped plywood piece on the base of the sagging sofa. Replace the cushions.