What describes a roof with surfaces that are not horizontal?

A roof with surfaces that are not horizontal is best described as a pitched roof. Pitched roofs are characterized by their sloped surfaces, which facilitate water drainage and help mitigate the accumulation of snow and debris. The angle or pitch of the roof contributes to the overall aesthetic and functional design of a building. Pitched roofs can vary in steepness and are commonly used in residential structures for practical reasons, such as improved ventilation and insulation, as well as for aesthetic appeal.

In contrast, the other options refer to structural components or walls and do not pertain to the definition of a roof. A simple beam typically refers to a structural element that supports loads, while a partition wall divides spaces within a building. A party wall is a shared wall between two adjacent buildings, particularly in urban settings, and is not related to the sloping nature of a roof. Thus, the characteristics of pitched roofs accurately align with the description provided in the question.