Quick answer: no! Bouldering routes (a.k.a. "problems") can be vertical, overhanging, or on a slab (less-than-vertical, the opposite of the overhang).
However, gyms that only have a small bouldering section may tend to make things vertical to overhanging because it's a quick and lazy way to challenge strong people -- and also because it's safer: if you fall off an overhang, you fall cleanly away from the wall and straight down onto the mats. Whereas on a slab, you can slide down the wall and bang yourself on the holds on the way, which tends to be painful.
If it is mostly overhanging, don't be daunted: overhangs look impossible at first, and they do demand a bit more strength just to stay on the wall, but they're not all about muscle: a huge part of it is body position and footwork.
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Quick answer: no! Bouldering routes (a.k.a. "problems") can be vertical, overhanging, or on a slab (less-than-vertical, the opposite of the overhang).
However, gyms that only have a small bouldering section may tend to make things vertical to overhanging because it's a quick and lazy way to challenge strong people -- and also because it's safer: if you fall off an overhang, you fall cleanly away from the wall and straight down onto the mats. Whereas on a slab, you can slide down the wall and bang yourself on the holds on the way, which tends to be painful.
If it is mostly overhanging, don't be daunted: overhangs look impossible at first, and they do demand a bit more strength just to stay on the wall, but they're not all about muscle: a huge part of it is body position and footwork.