A saggy seat on your chair may indicate that the padding and/or biconical springs may need to be replaced. Here are some hints and tips to get your started.
Check the condition of your biconical seat springs by removing any loose seat cushions and checking underneath the chair. If possible, remove the dustcover. If you can see coil springs pushing through the jute webbing base, it might only require a new base of webbing, which isn’t hard to do.
You might need to also replace tired springs. To check a tired spring, remove the spring from the frame and place it on a table. Use the flat of your hand to press it down. If it starts to lean over to the side, replace it.
Before you install new springs, replace the jute webbing that the springs are sitting on. Use 10 gauge jute webbing for seats, and 9 gauge jute webbing for backs or arms.
Coil springs are positioned so that the spring joints (also called the open end, or knot) face towards the inside of the chair. Sew the springs onto the jute webbing using spring tying twine and a strong needle.
Compress the spring with your hand until you feel it just starting to spring back. That’s the height to tie it off at. The spring needs to be compressed enough to give sufficient resistance to the top padding, without forcing out the bottom. Use spring tying twine to tie them down, then cover with a layer of hessian.