I'd start by saying that someone could not climb 15 feet across a ceiling without a climber's kit or a clever solution to the problem (throwing a small character with a rope to the other side, building some sort of bridge, etc.). Just looking at it from a real-world perspective, that's just not something that could be done without the assistance of tools. This is assuming that it's a typical stone ceiling, and not, say the roof of a cave (which might have hand-holds), but even for a cave roof, I'd give that an Athletics DC 25 without tools.
Knowing my players, they'd solve that by tying a rope to the halfling rogue and having the dragonborn fighter throw him. I'd give that an Athletics DC 15 for the throw and a Acrobatics DC 15 for the landing without damage, which they should have no trouble with. At that point, they'd tie the rope to something on both ends (or go across by low-to-high weight/strength), which is another Athletics DC 15 for either the person climbing across or the person pulling the rope. If they thought ahead to having another rope tied to each person going across, they could easily get across without any real danger.
Using Ghost Hand would also be a good use for moving the rope across and tying it to something.
Of course, it's also only 15 feet, so most would be able to jump it with at most a few tries (Athletics DC 10 with a running start?), and if they had a rope tied to them, there would be no real danger again.
Given all that, I'd make it 30 feet.
But you also mention that they have to climb up first, so if that's a significant distance up, that would put a kink in solutions such as that.
Knowing my players, they'd solve that by tying a rope to the halfling rogue and having the dragonborn fighter throw him. I'd give that an Athletics DC 15 for the throw and a Acrobatics DC 15 for the landing without damage, which they should have no trouble with. At that point, they'd tie the rope to something on both ends (or go across by low-to-high weight/strength), which is another Athletics DC 15 for either the person climbing across or the person pulling the rope. If they thought ahead to having another rope tied to each person going across, they could easily get across without any real danger.
Using Ghost Hand would also be a good use for moving the rope across and tying it to something.
Of course, it's also only 15 feet, so most would be able to jump it with at most a few tries (Athletics DC 10 with a running start?), and if they had a rope tied to them, there would be no real danger again.
Given all that, I'd make it 30 feet.
But you also mention that they have to climb up first, so if that's a significant distance up, that would put a kink in solutions such as that.
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