I agree that the climax to "The Tell-Tale Heart" comes at the very end when the planks are torn up to expose the still "beating heart." It could be argued that the climax occurs earlier, when the narrator finally kills the old man after so many days and nights of patient waiting. Certainly, there is a suspenseful build-up to this action. But because the story continues to build even after the murder, and an even more surprising and terrifying event occurs, I find the true climax at the end when the body is exposed to the police.
"The Tell-Tale Heart" is an interesting story to map because the action continually rises until the very end of the story. The narrator experiences an increasing sense of "madness" until the end when he cannot take the beating of the heart any longer. The moment when he decides to tear open the floor is the climax or turning point of the story. At this time, the narrator is overwhelmed by his guilt from having killed the old man, and he cannot suppress this guilt in the face of the policemen. So he decides to out himself as a means of dealing with his guilt making this the climax of the story.