D&D 5E Radius AoE also on walls?

[Oofta]

As others have said, there is no definitive answer. I'll add that as a DM, your ruling is correct when it comes to things like this. Just try to be consistent and logical, and remember that when the player's use your rulings against you (which they will inevitably do) that it's your own fault.

Having said all of that, there is a difference in wording for some spells. Fireball for example states that it's a "20-foot-radius sphere centered on that point". So pretty obvious it's a ball. Of fire.

Spike growth states it's area is "The ground in a 20-foot radius centered on a point".

I would personally rule it like you did - that the wall of a cliff is effectively ground. Why? Because it's the simplest ruling. Imagine you cast the spell at the bottom of a gentle slope. Do the spikes go up the slope? I can't imagine why they would not. But increase the slope gradually until it becomes a "cliff". At what angle do the spikes cease to grow?

Of course the person using the spikes to climb would still take damage, and move at half speed (or half of half if they don't have a climb speed).

 

[Barolo]

I, as a DM, would definitely allow, as long as the surface is suitable. First because I won't need to change anything on my interpretation of the rules to accommodate uneven terrain, if something would change the orientation of gravity, or something weirder were to happen. Second because the cases where this could be used for good effect would be so few and far between, it gets kinda irrelevant to say no.

 

[AaronOfBarbaria]

I think I wouldn't have a problem letting the spikes grow out of the cliff too... but I also think I'd ask the player "If the cliff face is part of the ground, are wall part of the floor?" to see if they were reasoning things out and thinking spikes should come out of the cliff, or were just having a poor reaction to the baddy having a way to reduce the effect of the resource they just spent.

 

[Valetudo]

Just remember one of the golden rules for dnd"Dont be a dick dm". Let your player have fun with his spell.

 

[smbakeresq]

I would have no problem with it. For all area of effect spells, remember to think in 3 dimensions. A fireball is a sphere, so it is a sphere 40' across at it widest then if it is cast 10' in the air the area on the ground would be about a 20' diameter sphere. This can be used to avoid hitting your teammates.

This also applies to everything if you PC pick up on it. A huge creature that is huge because it is tall wouldn't get any cover from a medium creature in front of it if the player said he as targeting only the top half.