Here's why:
* Indicative mood is used to state a fact or make a statement about something that is real or is perceived as real. In this case, the sentence is stating a fact about how the shack appears.
* Other moods like imperative (commands) or subjunctive (hypothetical situations) are not applicable to this sentence.
The verb "was" in the phrase "as if it was about to collapse" is also indicative as it describes a hypothetical situation that is presented as a fact based on the appearance of the shack.