16 March 2022

The specification libraries in NBS Chorus are structured either by Common Arrangement of Work Sections (CAWS) or Uniclass. Understandably, one of the most common questions that we receive at NBS is “Which structure should be used?”

CAWS has the advantages of both (a) being familiar and (b) presenting specifiers with a checklist of clauses which can be worked through on a project. Each set of clauses is grouped in a work section covering similar building materials and requirements for execution.

However, structuring a specification by CAWS also has its disadvantages:

  1. As work sections are mainly structured around materials, there is no natural location where systems can be specified. This presents a problem when system performance, system execution or the associated tasks at completion need to be specified.
  2. The products that comprise a system are located in different work sections, which means lots of manual cross-referencing.
  3. Specification clauses often describe a combination of products (for example, brickwork is a combination of brick and mortar products, and this makes linking to associated information less precise).

The Uniclass structure for specifications was introduced in an attempt to address all three of these challenges.

1. Specifying systems

Figure 1.1 below shows that, in CAWS, there isn’t a single classification reference for metal stud partition systems. Clause K10/115 is to be used for performance-based specification with clause items for overall structural, fire and acoustic requirements. Clause K10/125 is to be used if the partition is to be specified by the products that it comprises. In this example, there are clause items for references to products for insulation, lining, plaster and jointing. 

Figure 1.1 – Two alternative clauses in CAWS for specifying metal stud partitions

This is not just a challenge for specifying walls and partitions: there is no natural home for systems across CAWS, whether they are roof coverings, floor coverings or ceilings.

When the latest version of Uniclass (Uniclass 2015) was being developed, the need for a logical location for these system requirements was one of the main drivers. The rise of design and build contracts, and specialist contractor design portions of traditional contracts, had led to an increased demand for this.

Figure 1.2 below shows how systems can now be specified in the Uniclass library structure. Early on in a project, specific systems can be added and given a type reference (for example, for partitions, type references such as IWS-001, IWS-002, IWS-003 may be used). A summary description can then be added to the system (see Summary Schedules article) to allow an overall report summarizing all systems to be generated. As the project progresses, linked clauses can be added as instructions for those completing the final design and completing the work. The screenshots below show requirements for design standards and performance; they also show system execution and system completion requirements.

Figure 1.2 – Specifying system requirements when using the Uniclass structure

With the Uniclass libraries, it is possible to specify performance requirements even earlier on in the project timeline. Section PM_35 in NBS can be used by the client team at the briefing stage, before specific systems are being considered. Figure 1.3 shows how performance requirements can be specified for types of spaces (for example, classrooms or washrooms) or the elements that form these spaces (walls and floors). Section PM_35 covers high-level project performance for requirements such as structure, sustainability, fire, acoustics, durability, heating and cooling. 

Within the ‘Notes’ feature of Chorus, it is possible to link back to Section PM_35 from the systems as a cross reference so that checks can be put in place to ensure that the designed system performance meets (or exceeds) the initially planned whole building performance.

This can form a thread of requirements that can be tracked back from a specific manufacturer product that is installed. For example, an insulation product that is installed because it meets the product specification, which provides a designed system performance requirement, which meets the needs of the acoustic and fire performance of the element which forms the space. And the space meets the client requirements for the activities that are required in that building.

Figure 1.3 – Specifying performance very early on in a project against spaces and elements

2. Linking systems to products

The second major benefit of the Uniclass structure is the ability to easily add products to the system and maintain a hyperlink between them.

In NBS, when reading the general guidance for the CAWS work section K10, the specifier is informed that to fully specify a partition, relevant clauses should be considered in other work sections. For example, metal framing is in section G10; timber-framing is in section G20; and insulation and fire barriers are in section P10. Working this way results in a lot of manual cross-referencing for the specifier, both when preparing the specification and when checking that all clauses are coordinated at publication. From the point of view of the contractor receiving the specification, even if the specification is fully coordinated, there is still a need to flick back and forward between referenced work sections, which is not ideal.

In the Uniclass structure, the potential products that form the system are suggested as drop-down values to the specifier. Once these are added, they remain as hyperlinks in both the Chorus platform and the published PDF.

Figure 2.1 below shows a system outline clause for a partition system; the products for the frame, the insulation, the linings and the fire separation are all linked. The guidance provided by NBS to the right of the screen lists potential products – in this example, for the insulation there are options such as expanded polystyrene boards, mineral wool insulation and phenolic foam boards.

Figure 2.1 – A system outline clause with linked products

Although this article looks at architectural examples, these principles are the same for all disciplines. The table below shows example system-to-product relationships for other disciplines.

Discipline Typical system Example product groupings 
Asphalt concrete paving systems  Geotextile membranes
    Base course

    Surface course
    Surface features

Electrical engineering Hardwired general lighting systems  Cabling


Landscape architecture Board fencing systems  Posts 


Mechanical engineering  Low-temperature hot water heating systems Heat Source 


    Heat emitters 
Structural engineering  Timber-framing systems Beams 



3. Associating products with information external to the specification

In CAWS, there are many clauses that cover multiple products in their scope. For example, the clause F10/110 covers the requirements for both the clay bricks and the mortar. This makes linking to information external to the specification more difficult. It also does not give sufficient granularity for more detailed reporting. Examples of the benefits of having individual clauses for products in the Uniclass structure are shown below.

Figure 3.1 below shows that the mineral wool insulation is specified in a single clause. In this example, partition systems IWS-001 and IWS-002 are linked to it to avoid duplication of requirements. Furthermore, through NBS Source, almost 100 mineral wool insulation products from around ten manufacturers are positioned against this clause so that the specifier can either select a product to specify or research what specification is possible to achieve in the market.

Figure 3.1 – Positioning products against single clauses in the Uniclass library structure

Information will also be associated with the specification outside of NBS. In particular, any schedules or drawings that are developed will cross-reference the specification. Figure 3.2 is a screenshot from a recent customer success story from David Miller Architects: a drawing showing different wall constructions with each of the individual products annotated against the NBS project specification. Further efficiencies can be gained here by generating drawings and schedules using parametric design tools that use objects. NBS Chorus has plug-ins for the market-leading tools Autodesk Revit, Archicad and Vectorworks to help designers associate digital objects with their specifications and then keep these associations synchronized so that all documentation is coordinated.

Figure 3.2 – A drawing sheet showing annotations to the NBS project specification


Although a large number of practices in the UK still use CAWS for their specification structure, an increasing number are now using Uniclass.

Uniclass is particularly useful for specifications that are developed throughout the project timeline – and specifically for projects where aspects of the work will be contractor design portions. Uniclass also has benefits, as shown, for those practices that are modelling information digitally and are moving away from paper-based processes.

To learn more about Uniclass, visit our homepage. To find a code, visit the Uniclass website.

Learn more:
Learn about NBS Chorus.
• Read our customer success stories.
Join our Uniclass launch webinar.