Wall of fire: What's a 'solid surface' and what happens if it stops existing?

Question

Wall of fire has to be cast on a solid surface. What's a solid surface? Can it be the back of a dragon? Dragon scales are pretty solid..

If the solid surface disappears (ice melts away, roof collapses) does the spell then fail or stay where it was cast for the duration?

Answer 1

The rules are weirdly slippery on what a "solid surface" is.

Note that several "Wall of X" spells (ice, thorns, force) use similar language, as do monster attacks that create walls of stuff.

There are several cases of rules text using "solid surface" and "creature" as if they're distinct, which would imply that the surface can't be part of a creature. What makes it less clear is that the effect is usually the same either way, so "creature or solid surface" could just be inclusive language.

For example, the Otyugh can bash creatures "into each other or a solid surface", for the same amount of damage.

The catapult spell hurls an object in a straight line, "stopping early if it impacts against a solid surface." But if it would hit a creature, and the creature fails its Dex save, "the object strikes the target and stops moving."

You can break a Staff of the Magi "over your knee or against a solid surface"; the effect is the same, including the damage that you take for being in the center of the blast.

Melf's minute meteors is a tricky case. The meteor can be directed at a point you choose; it explodes when it "reaches its destination or impacts against a solid surface". This has a small functional effect: the meteors otherwise don't respect cover, so if a creature doesn't count as a "solid surface" then hiding under a big creature won't protect you from meteors coming down from above. Common sense says that it should protect you, so I lean toward "solid surface" being inclusive of creatures in this case.

So how do you rule on this?

One additional issue for the Wall spells is that "solid" implies that the surface isn't going to change shape during the duration of the spell. This matters mostly for Ice and Thorns, because they're rigid. If you cast a Wall of Ice on the back of a dragon, can the dragon shake it off? Does the wall flex as the dragon moves or does it splint the dragon in place? What happens if it tries to fly?

Another is that some of the walls do damage; if you allow a Wall of Fire to stick to a creature then that's 5d8 fire damage per turn, no save, for 10 turns.

Allowing walls to be used this way would introduce a lot of complexity that you might or might not want to deal with.

Answer 2

Vernacular Definition of Solid

Lacking a specific definition of a term in the rules, default to common language. The OED definition of solid:

Firm and stable in shape; not liquid or fluid. ‘the stream was frozen solid’

Consider how this affects the game play in two ways: by basic procedure, and for solid surface disappearing.

Default to Basic Procedure

The English definition of 'solid" implies a certain amount of rigidity as well. Further discussion of the terms will spiral into the semantics and debates of precise meaning, but we can short circuit that with the basic rules D&D procedure for adventuring:

1. The DM describes the environment.
2. The players describe what they want to do.
3. The DM narrates the results of their actions.

This is especially the case where the rule are defaulting the ambiguities of plain English. E.g. the effect of casting a wall of fire targeting the surface of a dragon's back is dictated by the interpretation of solid surface by the DM. However, the back of a dragon is not particularly stable in shape, and can probably be disqualified on that alone.

Xanathar's Guide to Everything Optional Rule for Invalid Targets

In the event the GM rules that the chosen surface does not count as a solid surface for the purposes of the spell, XGtE has a spellcasting section that offers optional guidance about ruling on casting spells at invalid targets.

If you cast a spell on someone or something that can't be affected by the spell, nothing happens to that target, but if you used a spell slot to cast the spell, the slot is still expended.

Solid Surface Disappears (Target Becomes Invalid)

For less ambiguous cases, e.g. ice melting, Jeremy Crawford offered a clarifying tweet. When the target of a spell (e.g. an ice bridge) no longer becomes a valid target (because it melted), the spell effecting it is suppressed. So the wall of flame is gone until the target becomes valid again or the duration runs out.