How to Fix Sagging Sofa Cushions

Sofa cushions sag for one reason: the foam has compressed or the support structure underneath has failed. This happens to every couch eventually—it's not a manufacturing defect, it's physics. The good news is you don't need to replace the whole sofa. You can rebuild the cushion from inside, add bracing to the frame, or swap in new foam. The work is straightforward, and the results are immediate. A sagging sofa isn't just uncomfortable; it looks tired. Fixing it brings your living room back to life and buys you years more use from furniture that's otherwise perfectly fine.

1. Remove the cushion and inspect the damage. Pull the cushion out completely and feel the foam core with your hands. Press down—if it compresses and doesn't spring back, the foam is shot. Look at the underside of the cover: if the seams are split or the fabric is thin, you may need to recover the cushion too. Check the frame and support strips beneath where the cushion sits; wood rot or broken slats are a separate fix.
2. Decide between foam replacement and support reinforcement. If only the foam is compressed, replacing it is the fastest fix. If the frame or support slats are sagging, you need to add plywood bracing underneath first, or both the cushion will sag again. Most jobs need both: new foam on top, solid support below. This is not a situation where you pick one or the other—you pick both.
3. Add plywood support underneath the cushion frame. Cut a sheet of 3/4-inch plywood to fit snugly inside the frame where the cushion sits. If there are existing slats, remove them or lay the plywood on top of them for extra height. Secure the plywood with L-brackets or pocket hole screws so it doesn't shift. This creates a rock-solid platform that won't flex under weight. If the frame itself is broken, repair or reinforce it first before adding plywood.
4. Remove the old foam from the cushion cover. Carefully cut the seams along one side of the cushion cover with a seam ripper or small scissors. Work slowly—you want to save the cover if it's still in good shape. Peel out the old foam in pieces. If the foam disintegrates, wear a dust mask; old foam breaks apart into nasty particles. Rinse the inside of the cover with a damp cloth to remove any foam debris.
5. Cut and install new high-density foam. Measure the cushion cover's interior length, width, and depth. Order high-density foam (3.5 pounds per cubic foot minimum) cut to those exact dimensions. Most foam suppliers will cut it for you. Slide the new foam into the cover. It should fit snug but not so tight that you tear the seams. Work it in gradually, corner by corner.
6. Stitch the cover closed with a curved needle. Use a curved upholstery needle and heavy thread (buttonhole thread or waxed thread) to hand-stitch the opening closed. Make small, tight stitches about 1/4 inch apart. This is not furniture-grade seaming—it doesn't have to be perfect, just strong enough to hold. If you're uncomfortable sewing, ladder-stitch from the outside (the cover seam stays in place, you're just closing the gap).
7. Test the cushion and reset the sofa. Press down on the cushion with your full weight. It should feel firm and spring back immediately. No sag, no soft spots. Now slide the cushion back onto the frame. If you added plywood support, the whole assembly will feel noticeably more solid. Sit normally and feel the difference. The couch should now have the support it had when it was new.