Honestly, there is not one universal number because urban benches are built from very different materials. That said, the biggest warping risk comes from recycled plastic, often HDPE or a similar polymer. For those benches, the trouble zone usually starts around 150°F to 160°F. At that surface temperature, the material can begin to soften slightly, especially under sustained direct sun or heavy use. By 180°F, many plastic slats will show visible sagging, bowing, or permanent deformation. On a dark bench in full summer sun, surface temperatures can easily reach 150°F to 170°F, so color and location matter more than most people expect.
Metal benches are much more forgiving structurally. Steel and aluminum will not warp until several hundred degrees, which normal outdoor conditions will not reach. The real problems are surface burns and finish degradation long before the metal itself loses shape. Wood benches rarely soften from air temperature alone, but repeated cycles of direct sun, rain, and humidity can lead to cupping, twisting, and cracking. Long-term surface temperatures above 150°F can speed up that damage.
If you want a practical rule, keep recycled plastic urban benches below roughly 150°F to 160°F to avoid warping. For metal, material warping is not the main concern in normal urban heat. For wood, focus more on moisture and UV exposure than on a single maximum temperature. Placement, color, ventilation, and weight load all change the real-world failure point, so the safest approach is to choose lighter colors and avoid full afternoon sun where dimensional stability is critical.