IfcSlab
Definition from buildingSMART: A slab is a component of the construction that normally encloses a space vertically. The slab may provide the lower support (floor) or upper construction (roof slab) in any space in a building. It shall be noted, that only the core or constructional part of this construction is considered to be a slap. The upper finish (flooring, roofing) and the lower finish (ceiling, suspended ceiling) are considered to be coverings. A special type of slab is the landing, described as a floor section to which one or more stair flights or ramp flights connect. May or may not be adjacent to a building storey floor.
A slab may have openings, such as floor openings. They are defined by an IfcOpeningElement attached to the slab using the inverse relationship HasOpenings pointing to IfcRelVoidsElement.
A particular usage type for the IfcSlab can be given (if type information is available) by refering to the type object IfcSlabType, using the IfcRelDefinesByType relationship, or (if only occurrence information is given) by using the PredefinedType attribute. Values of the enumeration are 'Floor' (the default), 'Roof', 'Landing', 'Baseslab', 'Notdefined'. If the value 'Userdefined' is chosen, the user defined value needs to be given at the attribute ObjectType.
HISTORY: New entity in IFC Release 2.0, it is a merger of the two previous entities IfcFloor, IfcRoofSlab, introduced in IFC Release 1.0
Type Use Definition
IfcSlabdefines the occuurence of any slab, common information about slab types (or styles) is handled by IfcSlabType. The IfcSlabType (if present) may establish the common type name, usage (or predefined) type, common set of properties, common material layer set, and common shape representations (using IfcRepresentationMap). The IfcSlabType is attached using the IfcRelDefinedByType.RelatingType objectified relationship and is accessible by the inverse IsDefinedBy attribute.
Property Set Use Definition:
The property sets relating to the IfcSlab are defined by the IfcPropertySet and attached by the IfcRelDefinesByProperties relationship. It is accessible by the inverse IsDefinedBy relationship. The following property set definitions specific to the IfcSlab are part of this IFC release:
Material Use Definition
The material of the IfcSlab is defined by IfcMaterialLayerSet or IfcMaterial and attached by the IfcRelAssociatesMaterial.RelatingMaterial. It is accessible by the inverse HasAssociations relationship. Material information can also be given at the IfcSlabType, defining the common attribute data for all occurrences of the same type. It is then accessible by the inverse IsDefinedBy relationship pointing toIfcSlabType.HasAssociations and via IfcRelAssociatesMaterial.RelatingMaterial to IfcMaterial or IfcMaterialList. If both are given, then the material directly assigned to IfcSlab overrides the material assigned to IfcSlabType.
Quantity Use Definition:
The quantities relating to the IfcSlab are defined by the IfcElementQuantity and attached by the IfcRelDefinesByProperties. The following quantities are foreseen, but will be subjected to the local standard of measurement:
Name  Description  Value Type 
NominalWidth  Total nominal (or average) width (or thickness) of the slab. The exact definition and calculation rules depend on the method of measurement used.  IfcQuantityLength 
Perimeter  Perimeter measured along the outer boundaries of the slab. The exact definition and calculation rules depend on the method of measurement used.  IfcQuantityLength 
GrossFootprintArea  Total area of the extruded area (or foot print) of the slab. The exact definition and calculation rules depend on the method of measurement used.  IfcQuantityArea 
NetFootprintArea  Total area of the extruded area (or foot print) of the slab, taking into account possible slab openings. The exact definition and calculation rules depend on the method of measurement used.  IfcQuantityArea 
GrossVolume  Total gross volume of the slab, not taking into account possible openings and recesses. The exact definition and calculation rules depend on the method of measurement used.  IfcQuantityVolume 
NetVolume  Total net volume of the slab, taking into account possible openings and recesses. The exact definition and calculation rules depend on the method of measurement used.  IfcQuantityVolume 
GrossWeight  Total gross weight of the slab, not taking into account possible openings and recesses or projections. The exact definition and calculation rules depend on the method of measurement used.  IfcQuantityWeight 
NetWeight  Total net weight of the slab, taking into account possible openings and recesses or projections. The exact definition and calculation rules depend on the method of measurement used.  IfcQuantityWeight 
Containment Use Definition
The IfcSlab, as any subtype of IfcBuildingElement, may participate in two different containment relationships. The first (and in most implementation scenarios mandatory) relationship is the hierachical spatial containment, the second (optional) relationship is the aggregation within an element assembly.
The geometric representation of IfcSlab is given by the IfcProductDefinitionShape, allowing multiple geometric representation. Included are:
NOTE. If the IfcSlab is of type Landing and is used within an IfcStair or IfcRamp, the special agreements to handle stair and ramp geometry will also affect the geometric representation of the IfcSlab.
Local Placement
The local placement for IfcSlab is defined in its supertype IfcProduct. It is defined by the IfcLocalPlacement, which defines the local coordinate system that is referenced by all geometric representations.
Geometric Representations
Currently, the use of 'SweptSolid', 'Clipping', 'Brep' and 'MappedRepresentation' representations is supported. In addition the general representation type 'BoundingBox' is allowed. The geometry use definitions for 'BoundingBox', and 'Brep' are explained at IfcBuildingElement.
SweptSolid Representation
The standard geometric representation of IfcSlab is defined using the swept solid representation. The following attribute values for the IfcShapeRepresentation holding this geometric representation shall be used:
The following additional constraints apply to the swept solid representation:
EXAMPLE for standard geometric representation.
Clipping representation
The advanced geometric representation of IfcSlab is defined using the swept area geometry with additional clippings applied. The following attribute values for the IfcShapeRepresentation holding this geometric representation shall be used:
The following constraints apply to the advanced representation:
EXAMPLE for advanced geometric representation.
Definition of a roof slab using advanced geometric representation. The profile is extruded nonperpendicular and the slab body is clipped at the eave. 
EXPRESS specification:

PredefinedType  : 
Predefined generic types for a slab that are specified in an enumeration. There may be a property set given for the predefined types.
NOTE: The use of the predefined type directly at the occurrence object level of IfcSlab is only permitted, if no type object IfcSlabType is assigned. IFC2x PLATFORM CHANGE: The attribute has been changed into an OPTIONAL attribute. 
WR2  :  The attribute UserDefinedType shall be given, if the predefined type is set to USERDEFINED. 
