BIM for the Baffled: Part 5 - What are objects, layers and assemblies in BIM?

4 December 2015 Kingspan Insulation UK
BIM for the Baffled Part 5 - What are objects, layers and assemblies in BIM?

An object is an amalgamation of different types of information that provides a physical definition of the product in order for it to be used in a virtual model of the building, as well as a functional definition, showing how it works.

There are two types of object, the first are those like doors and windows which have a fixed space. These are called components. The other type of object, called families or system families, are those that can be any size, like floors or walls. They can often be a layer within an assembly/system (such as the insulation within a wall) or they are the system (the entire wall).

The information that makes up an object (or layer) can include:
•    Information content which defines the product (including its name)
•    Geometry data; the physical dimensions such as height, width, thickness
•    Functional data, including performance data which enables the object to behave in the same manner as the actual product
•    Visualisation data so that it can be easily recognised in the model

Generic and Manufacturers’ Objects

The objects can be generic examples of a particular type of feature such as a generic insulation layer or created by manufacturers with detailed specifications of the exact product and its performance. Generic products are more likely to be used in the earlier design stages before the actual solutions are established, when they will be replaced with the manufacturer’s objects. The level of detail available in the model will grow as the project develops from initial designs through to more detailed final solutions which will be in place when the project moves into the build stages. This information will then remain in the model for the lifecycle of the building and could be updated as alterations or additions are proposed and made to the building.


Assemblies are made up of many layers. Each layer represents different objects which are then sandwiched together, for example on a flat roofing system which may comprise of a waterproofing layer, insulation, a vapour control layer and concrete. Each object in the layer can have details about material, thickness and performance which are all combined to form the model of the complete roofing system. The dimensions of the resultant assembly are dependent on the design of the building, as the layered objects can be any size.

Finding Objects and Layers

Finding a BIM object is easy – there are a number of libraries of objects online such as the NBS National BIM library. These can range from specific objects provided by a manufacturer to generic objects which are created to allow designs to be created before exact solutions have been decided. As a design is evolved and matured the generic objects are replaced by more specific objects that detail the exact product or solution chosen. Links to BIM objects can often also be found on manufacturer’s websites for specific products.

So as an example the Kooltherm K103 Floorboard (a layered object) is available on the National BIM library and this contains basic product information, from the name and brand name for the product as well as the physical sizes and thermal properties of the product and can be downloaded into different CAD programs. Because this is a layered object it will be used to build up an assembly in the model with the other floor components. But you can also find it by looking at the product specification where there is a link to the object. 

Related articles