Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Wide-Ranging; Somber
Although Roots deals with some heavy topics, things would get boring if its tone were steadily depressing the whole way through. And it wouldn't be accurate, either—the characters we meet experience joy, sadness, fear, love, anger, pride, and every other emotion under the sun over the course of their lives. This rainbow-wide palette of tones is needed if you want to portray real life in all of its complexity.
Despite this, there's definitely a heavy-hearted air hanging over the bulk of the novel. Maybe that's because, as dwellers of the future, we already know the historical background of the events we're witnessing, which makes happy scenes feel like calm moments before a storm. This lends the novel a heaviness that befits its heavy subject matter, not to mention the true horror that was American slavery.