First, I’d check what “sloped” actually means in practice. Put a spirit level on the tabletop or seat, then press each corner to find the wobble. On most urban terraces, a slight slope is there for drainage, so you don’t want to level the whole terrace or block water from running away. You only need to level the furniture itself.
The cleanest fix is adjustable furniture feet or threaded glides. Many outdoor tables and chairs have them, and you can simply turn the low-side foot out until the piece sits firm. Add rubber foot caps for grip and to protect tiles. If your furniture does not have adjustable feet, use UV-resistant rubber, composite, or hard plastic shims—not cardboard or random wood offcuts that will rot. Stack thin shims under the low leg, keep full contact with the floor, and put an anti-slip pad between the shim and the tile so nothing slides.
For heavier pieces, such as a dining table or sectional, use wider leveling pads under the frame rather than tiny shims under one corner. Keep a small gap underneath for airflow and drainage, and never block drain slots or the edge where water escapes. If you rent, avoid drilling into the waterproof membrane; use non-penetrating anchors or weights where wind uplift is a concern.
Once it’s level, give it a real stability test: push gently from several sides, sit on the chairs, and recheck after rain. If the terrace slope is steep or the furniture is tall and heavy, don’t fight it with a stack of shims—use proper adjustable feet, a low-profile leveling platform, or get a professional to secure it safely.