How far down can I attack when standing on the ceiling?

Question

I have a character using the Spider Climb spell and a whip in a dungeon with 20 foot ceilings. Character is 6 foot tall. Average Medium sized creature is 4+ feet tall. Average Human 5'9". If PC is on the ceiling what is the range they can attack into?

If PC threatens 5 foot base square on ceiling + 5 foot standard reach + 5 foot extra reach from whip, that is makes 15 feet total. From a 20 foot ceiling, would that mean any Medium creature over 5 feet is within the 15 foot reach? Any creature over 5 feet would extend into the square threatened by the 15 foot reach. Or is any Medium sized creature only considered to occupy 5 feet square no matter how much taller they are.

I think it is slightly different than asking how much reach a creature has. It is more of a question about how many squares a creature is vulnerable and can be attacked into. The question is there are two different numbers given for "Medium" creatures. I realize 5 foot square is mainly to determine horizontal ranges, but it also clearly states medium creatures can be over 5 feet in height. Probably the majority are.

I realize one can say an enemy can just duck down. But maneuvering to avoid an attack is usually thought to be already included in fighting mechanics. As normal characters don't occupy the entire 5 foot width or depth of a horizontal square and can move around to avoid attacks.

Also, I realize this might be over scrutinizing things, but there really are two separate numbers given for medium creatures: 5 foot square (horizontally), and 4-9 foot height vertically. Is there any guide given to 3D or vertical workings?

Answer 1

I think it's useful to break this into a two different questions. First:

Does it work mechanically? Yes, if the DM says so.

Your strict reading of the rules (aka: Rules As Written or RAW) is mostly correct, except for the part about grid cubes. Something that can be easy to forget is that the 2D grid many groups use is not actually a direct part of the rules. Everything in DnD 5e is based off of distances. It just so happens that those distances tend to come in multiples of 5, and the books strongly suggest using a grid for combat, with rules to facilitate it. However the grid doesn't actually exist in-game, and is only there as a convenience.

When it comes to attacking things above you, it's up to you as the DM to decide what your players can do. If you think being able to attack the orc with a whip from the top of a 15-foot high chamber makes sense, then that's all that's needed.

The next question to ask is:

Will it break the game? No, not at all.

Using Spider Climb to stand on the ceiling hitting enemies with a whip is not appreciably different from using Spider Climb to stand on the ceiling hitting enemies with a crossbow, or other spells, or dropping rocks. The caster certainly has an advantage, but this is at the not insignificant cost of a second level spell and concentration. Also remember that while the wizard might be out of melee reach, they will have no such protection from ranged attacks, and every time they take damage, they must make a concentration save or fall to the ground.

Most likely your player will be able to dominate a handful of encounters using this strategy. However, it shouldn't take long before they run into a fight somewhere with a ceiling only 10 feet tall, or 20+ feet tall, or a fight outside, or just a fight against enemies with ranged attacks. If anything, I would suggest specifically creating one or two fights where this strategy is strong, just to be sure that your player is able to use the cool trick they figured out at least some of the time.

Answer 2

A medium creature fills a 5x5x5 foot cube

It’s an abstraction to facilitate gameplay - don’t overthink it.

Similarly, diagonals don’t exist - count each cube up, down or sideways or diagonally as 5 feet.

Answer 3

The spiderwalking character would not be able to reach a 6 foot character who was aware of them.

Just because the target is tall enough to project their height into range does not mean they would.

If I'm 6 feet tall and there is a spiderwalking freak on the ceiling wielding a whip, I would take a defensive position, ducking. This could make my height effectively 3 feet, putting me well out of range.

If you allow the Wizard on the ceiling to attack the character on the ground from 20 feet up, you are biasing to the benefit of the Wizard I think to an unreasonable degree.

The Wizard has a reach of ten feet with the whip and is attempting to attack vertically. Thinking logically about this, the attacker would have more trouble applying force with the whip in this manner and also the target is much smaller than when standing face to face. The target when standing face to face is 6 feet tall and 2 feet wide, when attacking from above, they are maybe 1 foot tall and 2 feet wide. In the most ideal of circumstances, the target is standing as tall as they can extending only 1 foot into your range, and waiting to be hit from above.

As a DM, I would allow the Wizard to attack if it was a surprise, however they would get disadvantage on the attack roll considering the target is effectively tiny for the purpose of this type of attack.