We encountered a Vampire Spawn in the 10-foot-high basement of an old church. The first strike was mine, after which the creature attacked my ally and retreated onto the ceiling as the thundering roar of my Booming Blade punished its retreat. Still inexperienced and unsure about my reach, after a short consultation with my mind, I decided to spend my turn switching to my trusty crossbow and make some distance.
However this turned out to be a mistake, since right after, the creature was in in my face trying to gouge out my eyes. Due to my nimbleness I managed to avoid its attacks. Regretting my earlier choice to change weapons I was left with the options to switch weapons and miss another turn, attack with disadvantage, or use my crossbow as an improvised melee weapon and trust in Booming Blade to hold the enemy in place after I disengaged.
Perhaps I should have switched weapons, but I decided that 1d4 (improvised weapon) + 1 (magic ring) + 1d8 (booming blade) was good enough. I missed.
On the plus side, it was my crossbow's sneak attack assisted bolt that finally slayed the Vampire by piercing its heart.
In this situation, would it have been possible to just attack upwards instead of changing to the crossbow?
In session, this was something to be looked up later and decided against, but I haven't found any rules on attacking in 3D.
With me being more than 5 feet in height and my melee attack range considered to be 5 feet, I should have been able to reach the ceiling itself, not to mention the Vampire Spawn hanging from it.
As for exact measurements (read this part only if you enjoy pointlessly over-analyzing things):
My character is 5 feet 6 inches/5.5 feet, which gives an estimated standing reach of 7 feet 4 inches/7.3 feet (for a human, my character is a Kenku), the blade of a short sword is 12 to 20 inches / 1 to 1.6 feet. This should give me a height of 8.3 to 8.9 feet from standing statically.
Testing it in real life, my estimated and standing reach match to less than 0.2 inches of the estimation on the site and I'm just shy of reaching my ceiling standing statically. As a test I tried moving as if I was attacking my ceiling and based on where I could feel my hand touch the ceiling, the length of my fingers and how much distance I lacked to reach my ceiling, I estimate about 0.4 feet in additional height. Considering that I'm taller than my character, let's say that gives 0.3 feet, increasing the height to 8.6 - 9.2 feet and that's still without changing how you hold the handle of the sword. In addition, just lying on the ground, my torso and legs measure about 0.6 feet.
Because my Kenku isn't a human, only a humanoid, it's impossible to estimate arm length which influences standing reach, however, considering that birds usually have a high wingspan and long legs, I think it's reasonable to assume that the arms should be as long, if not longer than, a human's.
So that would require the Vampire Spawn to be flat against the ceiling without any parts hanging down more than 0.2 to 0.8 feet for it to be outside of my reach. I consider that unlikely.
What are the actual rules for this type of scenario, or how do you handle it in your group? Should I be allowed to melee attack an enemy that's on a ceiling or low in the air?