What Types of Loads Must a Roof Be Able to Support?

Live Loads

• Moving people, including the furniture and movable equipment they use, encompass live loads. According to the American Society of Civil Engineers, roofs must be able to support uniform and concentrated live loads. For instance, workers and maintenance equipment produce live loads that must be supported by rooftops. Minimum uniformly distributed loads can be calculated based on based on roof design, square footage and the type of live loads supported, according to the ASCE.

Dead Loads

• The ASCE designates dead loads as materials used in the construction of buildings. Dead loads include hardware and building materials, which are structural elements or components attached to the building. Roofs are constructed to resist forces created by beams, floors, walls, ceilings and ventilation systems. They must also support the weight of their own structure, covering and insulation. Movable items used by people, such as furniture, do not classify as dead loads.

Transient Loads

• Transient loads are lateral loads caused by weather conditions and natural phenomena. For example, roofs must be able to withstand the weight of snow, ice, earthquakes and wind force. Some of the variables builders must consider when calculating wind loads for roofs include the pressure, height and direction of wind. Timing, surrounding trees, nearby buildings and landscape are additional factors that can affect wind loading. Roofs must also be able to resist shaking and swaying caused by earthquakes and other seismic activity. This is especially true for roofs on tall buildings with complex shapes or special conditions.

Roof Types

• Not all roofs are made the same. Some roofs are flat or curved, while others feature pitched or sloped roofs. Roofs constructed for specific purposes such as promenades may have lower load requirements than other roof types. The ASCE publication “Minimum Design Loads for Buildings and Other Structures” specifies that the minimum live load for promenades is 60 pounds per square foot of roof. However, the minimum live load requirement for rooftop gardens must be 100 pounds per square foot of rooftop.