20 December 2022

Recently at NBS we have made a full set of sample specifications available to download in both Uniclass and CAWS (Common Arrangement of Work Sections) format.

NBS Chorus allows the development of specification in either Uniclass or CAWS format. Our analytics show that on larger projects, the industry is broadly split between the two classification schemes.

This article focuses on the CAWS specifications and highlights some of the similarities and differences between preparing specifications in CAWS compared with Uniclass.

1. Introduction

CAWS was first published in 1987 as an industry-wide initiative to standardize construction documentation and allow contractors to more easily divide this information into work packages.

NBS uses CAWS to group specification clauses for similar products together in a single work section to allow the specification the quality of products and execution. Figure 1.1 below shows an example of two specification clauses from the CAWS work section M10 ‘Cement based levelling screeds’. Clause M10/305 allows the specification of the quality of the aggregates and clause M10/650 allows the specification of the execution requirements with respect to curing.

Figure 1.1 Example specification clauses in CAWS format.

When using the NBS CAWS libraries, the user is presented with a set of template clauses in each work section and the specification writer excludes clauses that are not relevant for the project and completes the clauses that are required. Interestingly, out of 74 potential clauses in the sample specification for M10, 43 were excluded and 31 were completed.

2. Developing project specifications in CAWS format

The CAWS sample specifications were developed by SpecStudio, a specification-writing consultancy with experience using NBS Chorus on large, complex projects. This article looks at some of the methods used by SpecStudio to produce robust specifications.

Cross-referencing related specification content

The products required to construct a masonry wall leaf in CAWS are situated in different work sections. To help the reader of the specification understand where to find related items an introduction clause was added to the top of each section. Figure 2.1 below shows that from F10 (bricks and blocks) it can be seen that F30 (wall ties, insulation, cavity trays), Z21 (mortar), etc. are referenced.

Figure 2.1 Related work sections.

Figure 2.2 shows that when the related clauses are in the same section, the precise clause references have been used. The clause H92/120 specifies the rainscreen cladding tile and also cross-references the clauses for sheathing board, insulation, breather membrane and the secondary support structure.

Figure 2.2 Related clauses within the same work section.

Design responsibility

Clarity on design responsibility was provided in the introduction clause. Figure 2.3 below shows an extract from a descriptive specification where the overall performance is specified to allow a specialist subcontractor to then complete the detailed design. Even though CAWS is not a ‘systems-based’ structure, the performance requirements were added to the most appropriate work section. SpecStudio indicated any user clauses they added with a ‘U’ suffix as shown in Figure 2.4.

Figure 2.3 – Clarity on design responsibility
Figure 2.4 – A descriptive specification as the brief for a specialist subcontractor.


Where one work section covered multiple trades/ subcontracts, then two versions of this work section were added to clearly separate the requirements.

Examples of this were with the work section L10. This was split into an L10A that covered the specification for the louvres and L10B for the windows. Similarly, L30 was split into two work sections – L30A that covered the specification for the balustrades and L30B for the staircase.

Figure 2.5 below shows a screenshot from L30A ‘Balustrades’ with the clauses for stairs, ladders, walkways and ramps all excluded.

Fig 2.5 – L30A, with clauses only relevant to balustrades remaining.

3. Similarities between CAWS and Uniclass

Many of the techniques used by SpecStudio were the same irrespective of what classification structure was used. Some examples of this are illustrated below.

Type codes

Specification clauses will be referenced in other project documentation such as models, drawings, schedules and spreadsheets. Classification on its own is not enough, as there may be many types of doorsets, beams or radiators. The approach taken by SpecStudio here was to use the ‘Suffix’ field in NBS Chorus to create a constant naming convention to logically help with any cross-referencing.

The short video below shows the same approach being taken for the type codes within the ’Suffix’ field for both CAWS and Uniclass specifications. The video then shows the classification code, clause title and ’Suffix’ field all being used in the drawing annotations.

Note – it’s worth mentioning that although this article is focused on UK specification writing practice, these techniques would work just as well for the NBS Australian, Canadian or US content to different international classification systems.

Fig 3.1 – The use of type codes in the ‘Suffix’ field of spec clauses.


When publishing PDF versions of the specifications, the naming conventions and the data fields described in BS EN 19650-2 were used. Figure 3.2 shows the approach to publication; it can be seen that the full specification was published initially. As this is quite a large document, containing 37 work sections and measuring 181 pages in length, small ‘packaged’ specifications were also published. The ‘(FCS) - Floor coverings’ specification, for example, includes only the work sections M10, M40, M50, Q23, Z20, Z21 and Z22 and is 32 pages in length.

Fig 3.2 – A publication history within NBS Chorus.

The full document set

The final observation is that the NBS Plug-in that coordinates the model and specification was used for both the Uniclass and CAWS content. Once the drawings were published, then the extranet folder containing these drawings was hyperlinked to from the introduction clause in each work section.

This same principle was followed for a summary of the prelims and a folder containing all of the specifications.

By cross-referencing this wider document set, then the recipient of the specification can easily find any related documentation that is required.

Fig 3.3 – Referencing and linking to prelims, other sections and the drawings from each work section.

4. Summary

Both the CAWS specifications and the Uniclass specifications are available to download for free. Hopefully, they help existing NBS users with their specification writing and help those unfamiliar with NBS understand the quality of output that is possible using NBS Chorus.