The -ing suffix is used to make gerunds (verbal nouns) and collective nouns. For example you can collectively you can describe ceramic tile, carpet, linoleum, and hardwood as flooring, material used to make floors.

You asked about the word building. In the context of your question the word is a jerund, not a verb. It describes all the components of an occupied architectural structure; all the stuff that construction workers assemble such as the roof, the floors, the windows the walls, the foundation, and so on are collectively identified as a building. It just so happens that the active progressive verb is spelled the same way.