Specifications are the written part of a design. Where drawings show where things go and how big they are, the specification says what they must be made of, how well, how the work is carried out and checked, and, on many civil projects, how it is measured and paid. Together, drawings and specification make up the design's instruction to the builder.
At a glance
| Says | Materials, products, workmanship, testing, measurement |
| Organised by | Numbered sections (MasterFormat, NBS, agency standards) |
| Typical section | General, Products (materials), Execution; often Measurement and Payment |
| Relationship to drawings | Complementary; the contract's order of precedence settles conflicts |
| Links to the model | Material classes and product data as element properties |
How a section is organised
| Part | Contents |
|---|---|
| General | Scope, related sections, referenced standards, submittals, quality assurance |
| Products / materials | Grades and classes (for example concrete strength and exposure classes), sources, certificates |
| Execution | Preparation, installation, tolerances, protection, curing, cleaning |
| Testing | What is tested, how often, and what passes |
| Measurement and payment | Units, what is included or deducted, pay items |
How specifications are used on projects
Specifications are prepared by each discipline in parallel with the drawings, often from a master library adapted for the project. Contractors price from them, suppliers deliver to them, and inspectors check against them. For a BIM coordinator they decide which properties model elements should carry (a concrete class, a fire rating, a finish), and how quantities taken from the model must be grouped and measured.
Highway agencies publish standard specifications that apply to all their projects, supplemented by project-specific provisions. The US Federal Highway Administration's FP series for federal highway projects is one example; state highway agencies publish their own.
Variations
- Prescriptive specifications state exactly what to use; performance specifications state what the result must achieve.
- Proprietary specifications name a product, often "or approved equivalent".
- Systems: MasterFormat (North America), NBS (UK), Uniclass-based structures, and national or agency standards.
Common mistakes
- Coordinating the model against the drawings only, and never reading the specification.
- Model elements that carry a generic material, so quantities can't be split by specified class.
- Copying a specification from another project with clauses that don't apply.
- Conflicts between specification and drawings left for the site to resolve.
Where to read more
CSI's MasterFormat and SectionFormat; NBS guidance in the UK; the specification standards used by your client or agency (for example FHWA's FP series for US federal highway projects); and EN 206 for concrete classes in Europe.