I left a typical urban bench on my lawn just to see what would happen, and yes—the legs absolutely sank into the ground over time. It wasn’t dramatic overnight, but within a couple of weeks I noticed the bench tilting slightly, and by the end of a rainy month each leg was buried about an inch and a half into the soil. If your grass sits on softer, loamy ground or clay that holds water, gravity does its work slowly and steadily.
The biggest factor turned out to be moisture. When the grass was dry and the soil was firm, the bench barely left an impression. After a heavy rain, though, the ground turned spongy and the legs pressed in like a cookie cutter. The bench’s weight matters too—a light aluminum slatted bench might only sink superficially, but a heavy cast-iron or hardwood slatted bench with narrow feet concentrates the load on tiny points, making it dig in surprisingly fast. Even the shape of the legs plays a part: flat metal straps or thin cylindrical legs disappear into wet turf faster than wide, rounded wooden legs.
I found three things that actually stop the sinking. The simplest fix is to place pavers, flagstones, or even pressure-treated wood blocks under each leg to spread the load. Another trick that works well is sinking gravel-filled bases just below grass level; water drains away faster and the bench stays stable. If you don’t want to alter the lawn, reposition the bench every few weeks and avoid leaving it in low spots where water pools naturally. Since I started using square concrete stepping stones under the legs, I haven’t had any sinking at all—even through a week of April showers. The bench still looks great on the grass, but now it stays perfectly level no matter what the weather does.