Honestly? There’s no single number, because it depends almost entirely on what the bench is made of and how it’s put together. Rain itself isn’t the only killer—it’s the cycle of wetting and drying, plus UV exposure, mildew, and freezing water that causes most of the damage.
If you leave a cheap big-box pine or fir bench out in the rain year-round with no protection, expect visible problems within 6 to 14 months. The wood will start to cup, crack, or rot around the joints, and any mild steel hardware will rust. By year two or three, it may still look like a bench from a distance, but it will feel spongy or wobbly and probably won’t be safe to sit on.
Pressure-treated pine does better, but only if it’s sealed or painted and kept off standing water. Even then, in a consistently rainy climate, you might get 5 to 10 years before the bottom of the legs or the joint areas start to soften. If it sits directly on wet soil or grass, cut that estimate in half.
Cedar and redwood are a step up because they have natural rot resistance. A well-built cedar bench left out all year can last roughly 10 to 20 years, depending on how much rain and shade it gets. It will turn gray and may develop surface cracks, but it usually stays structurally sound much longer than pine. If you want to keep the color, you’ll need to clean and oil or seal it every year or two.
Teak is the classic “leave it out in the rain” wood. High-quality teak contains natural oils that make it extremely resistant to rot and insects. A solid teak bench can easily last 20 to 30 years outdoors in rain, and often much longer—some teak garden benches from the 1970s are still going strong. The catch is that it’s expensive, and over time it develops a silvery-gray patina. That gray look is not rot; it’s just surface weathering. You can leave it alone, or scrub and oil it to bring back the golden color.
Metal benches are all over the place. Cheap tubular steel or wrought-iron benches will start rusting within months if the paint gets scratched or if water pools on the seat. They may become unsafe in 2 to 5 years. But a good powder-coated aluminum bench won’t rust at all. Aluminum can easily last 15 to 25 years outdoors, even in rain, as long as the hardware is stainless steel or coated. Cast aluminum with a thick powder coat is one of the best low-maintenance choices.
Recycled plastic or poly lumber benches are probably the most practical for full rain exposure. They won’t rot, splinter, or rust, and they don’t need sealing. A quality poly or HDPE bench can last 20 to 30 years or more, though cheaper plastic may fade, get chalky, or warp in hot sun. Marine-grade polymer is heavy and stable, but it can be pricey.
Concrete or stone benches are nearly indestructible in rain, but they’re not comfortable for long sitting and they can crack in freeze-thaw cycles if water gets into small pores. With good drainage, they can last decades.
So, if you’re asking for a realistic rule of thumb: a cheap softwood bench left out in the rain all year may be trash in 1 to 3 years. A quality cedar, teak, powder-coated aluminum, or poly lumber bench can realistically last 15 to 30 years, and often longer if you keep it slightly off the ground, let it drain, and do basic maintenance like cleaning or tightening fasteners. The rain will age it—but it doesn’t have to destroy it.