What is the opposite of word roof?

There isn't a single perfect opposite for "roof" because it depends on what aspect of the word you're trying to contrast. Here are some possibilities:

Based on location:

* Floor: The opposite of the top of a structure.

* Ground: The surface beneath the roof.

Based on function:

* Foundation: The base of a structure, supporting the roof.

* Basement: The lowest level of a building, opposite the roof.

Based on imagery:

* Sky: The vast space above the roof.

Ultimately, the best opposite for "roof" depends on the context of your sentence.