How does the phoenix’s Fire Form work?

Question

The phoenix from MToF has an ability called “Fire Form”, which states:

Fire Form. The phoenix can move through a space as narrow as 1 inch wide without squeezing. Any creature that touches the phoenix or hits it with a melee attack while within 5 feet of it takes 5 (1d10) fire damage. In addition, the phoenix can enter a hostile creature's space and stop there. The first time it enters a creature's space on a turn, that creature takes 5 (1d1O) fire damage. With a touch, the phoenix can also ignite flammable objects that aren't worn or carried (no action required).

I’m specifically interested in the latter part of the description: the ability to ignite with a touch.

What counts as a touch?

Let’s say that the phoenix is flying over a city, and decides to glide right over the city rooftops, could it tilt its wings slightly to graze the rooftops, and light the entire length it flies over? Mechanically, you could say that it stops every 5 feet, to use that non-action to ignite a roof, but why bother if its body isn’t entirely corporeal? Couldn’t it just do it whilst flying?

Answer 1

You'll have to ask your DM

To start with, a phoenix could definitely light something on fire while flying. As you state, all that is required is a "touch" and there is no reason the phoenix would need to stop to touch something. Touching is not in itself defined in the rules (since we are not casting a spell), so we are forced to use the plain definition, which your example satisfies.

However, the issue with your specific example is whether a rooftop is a "flammable object".

Flammable?

Most rooftops, even historic ones from ages past, were designed to be as fire resistant as possible/affordable. Now, if the phoenix is flying past an un-oiled thatch roof, that is one thing, but a DM may say that other types of roofs are not readily flammable.

Object?

There is considerable debate on what the rules consider an "object" (see this Q&A for more details). I'm forced to conclude that objects aren't clearly defined in the rules, but they are at least considered distinct from a "structure" (as many features call out both objects and structures as separate things). That being said, the Dungeon Master's Guide and this Q&A discuss that structure are composed of objects. Whether or not a rooftop (likely a portion of a structure: a building) is considered an object, though, is ultimately up to the DM.

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As a side note: Keep in mind that moving 5 feet and using a non-action would only be a necessary explanation if grid rules are being used which are considered a variant rule.