A roof is the covering of the top part of a building, it protects the building against the effects of weather. The IfcRoof can either be represented:

• as a roof assembly that aggregates all parts (slabs, rafters or other included roofs, such as dormers) with own shape representation
• as a single roof without decomposition including all shape representations directly at the roof entity

If the IfcRoof has components (referenced by SELFIfcObject.IsDecomposedBy) then no independent geometric representation is defined for the IfcRoof. The IfcRoof is then geometrically represented by the geometric representation of its components. The components are accessed via SELFIfcObject.IsDecomposedBy[1].RelatedObjects. The geometric representations that are supported for the aggregated elements are defined with each element. See geometric use definition for IfcSlab, IfcBeam, IfcColumn, IfcBuildingElementPart and other subtypes of IfcBuildingElement. The image below illustrates roof placement, with an IfcRoof defining the local placement for all aggregated elements. 

This concept can be applied to the following resources:

• IfcSlab - A roof can be aggregated into slabs for each face

This concept can be applied to the following resources:

• IfcRoofType

The following restriction can be imposed by view definitions or implementer agreements:

• If the IfcRoof establishes an aggregate, then all contained elements are placed relative to the IfcRoof.ObjectPlacement

This concept can be applied to the following resources:

• Pset_ConcreteElementGeneral
• Pset_PrecastConcreteElementFabrication
• Pset_PrecastConcreteElementGeneral
• Pset_RoofCommon

This concept can be applied to the following resources:

• Qto_RoofBaseQuantities