Sexuality and gender are the two main overarching themes in "A Woman on a Roof." When looking at it in a more specific manner, however, the emerging theme becomes the objectification and patriarchal view of women and their behaviors. Throughout the story, Doris Lessing's three male characters wolf-whistle at a bikini-clad woman, form expectations of her and her behavior, and in the case of Tom, fantasize about her to the point where he feels entitled to her affection.

When she is initially introduced, the woman immediately becomes objectified—by the three men as well as the narrator. Though she is covered by clothing one would typically find at the beach, the three men marvel at her audacity to sunbathe on a hot day. Later in the story, they, themselves, decide to briefly take off their shirts and vests, yet the narrator does not go into the same sexualized detail as with the sunbather.

Stanley, the middle-aged man of the trio, is shocked at her "naked" activity, going as far as to say that he would never allow his newly-wed wife to be seen in public like that. He later gets angry when she refuses to respond to his wolf whistles; in his eyes, even though he's being disrespectful to her, she is expected to acknowledge him.

Tom, the youngest of the trio, dislikes the attitude and demeanor Stanley directs toward the woman on the roof. He wishes to distance himself from Stanley in the woman's eyes to gain her affection, yet he does nothing to indicate his disapproval:

He stood beside Stanley, excited, grinning; but he felt as if he were saying to the woman: Don't associate me with him, for his grin was apologetic.

As the story progresses, Tom becomes more and more obsessed and possessive. He fantasizes about caressing her and assaulting her with a crane, and at one point he feels she "betrayed" him by being absent from the roof. Despite his previous scorn for Stanley's attitude toward her, Tom eventually falls into the same pattern of behavior. When he finally interacts with her at the end of the story, she ignores him. He responds by becoming angry that his expectation—that she would swoon over him—was not met. In the end, it is shown that Tom's dismissal of Stanley's behavior has less to do with the woman's welfare and more to do with his own desires for her. Tom, more than the other two men, objectified, sexualized, and developed expectations for the woman on the roof. This is the crux of the story's theme; Tom is the epitome of the patriarchal treatment and objectification of women.

There are two themes in Lessing's story that are sometimes separately addressed and at other times seamlessly interact: those themes are sexuality and class and gender barriers.

Tom, the protagonist, wrestles with his ever-increasing sexual desire and his intense insecurity about himself as an attractive or sexually desirable person. He observes a young woman sunbathing and concocts elaborate fantasies about her affection and longing for him. Unfortunately for Tom, the sunbather has no idea what is going on in his head, and he is too insecure to approach her in real life.

Class and gender barriers are evident in the way the woman and the workers interact (or fail to interact). The woman ignores their comments and cat-calls. She feels objectified; the men feel rejected. Class-wise, the woman has the money and leisure to relax on the rooftop. The men, however, must engage in manual labor.