Start by figuring out whether the furniture is actually rocking or just sitting on a slope. Put a spirit level on the seat or tabletop, then press each corner to find the high and low legs. If it is public furniture, check with whoever owns it before you alter anything—don’t drill or glue paving without permission.
For small gaps, adjustable glides or screw-in feet are the cleanest fix. If the piece doesn’t have them, use hard rubber, composite, or stainless-steel shims under the foot—not under a thin edge of the frame. Rubber is good for grip on smooth pavers; stainless or composite holds up better outdoors. Avoid stacking a tower of thin shims: use one solid wedge or a shim plate to spread the load. Timber wedges are okay for a quick temporary fix, but they rot and work loose.
Lift one leg at a time, slide the shim in, and keep checking the level. For a heavy bench or table, get a second person. Once it’s stable, trim or secure the shim so it can’t kick out, and make sure water can still drain away. If it’s permanent or in a windy public space, anchor the furniture using approved fixings or weighted bases rather than relying on shims alone. Finish by rocking it firmly and sitting or leaning on it—if there’s any movement, adjust again. The goal is stable, surface-friendly, and safe, not just level-looking.