# Construction Specifications Writing The construction specification is the written document that describes the quality, performance, and workmanship standards for every material, product, and process in a building project. If drawings show *what* to build and *where*, the specification defines *how well* and *to what standard*. For the practicing architect, specification writing is a core professional skill — inadequate specifications lead to disputes, defective work, cost claims, and professional negligence exposure. A well-written specification protects the client, the contractor, and the architect. --- ## Table of Contents - [Purpose of Specifications](#purpose-of-specifications) - [Types of Specification](#types-of-specification) - [Specification Formats](#specification-formats) - [NBS and Uniclass](#nbs-and-uniclass) - [Specification Structure](#specification-structure) - [Writing Effective Clauses](#writing-effective-clauses) - [Proprietary vs Performance Specifications](#proprietary-vs-performance-specifications) - [Coordination with Drawings](#coordination-with-drawings) - [Common Specification Errors](#common-specification-errors) - [See Also](#see-also) --- ## Purpose of Specifications The specification serves multiple simultaneous purposes: | Purpose | How | |---------|-----| | **Quality definition** | Defines acceptable materials, products, and standards of workmanship | | **Contractual obligation** | Forms part of the construction contract; legally binding | | **Tender comparison** | Enables like-for-like pricing by all tenderers | | **Construction guidance** | Tells the contractor what standard is expected | | **Record** | Documents the design intent for future reference, maintenance, and disputes | | **Coordination** | Links to drawings, schedules, and other contract documents | --- ## Types of Specification ### Prescriptive Specification Specifies the exact materials, products, methods, and standards to be used. The architect takes full responsibility for the specification's suitability. **Example**: > External walls to be constructed in Ibstock Rutland Red Multi facing bricks, laid in stretcher bond with 10mm flush mortar joints using Cemex designation (iii) mortar (1:1:6 cement:lime:sand) to BS EN 998-2. Brickwork to be protected from rain during construction and cleaned with clean water only on completion. ### Performance Specification Specifies the required performance outcome, leaving the contractor to select materials and methods to achieve it. Responsibility for product selection transfers to the contractor. **Example**: > External wall cladding system to achieve: U-value ≤0.18 W/m²K; air permeability ≤3.0 m³/h/m² @ 50Pa; weather resistance to BS EN 12865 method A; fire classification A2-s1,d0 to BS EN 13501-1. System to be from a manufacturer with a minimum 10-year product warranty and BBA certification. ### Proprietary Specification Names a specific manufacturer's product, with or without "or approved equivalent": **Example**: > Roof waterproofing: Sika Sarnafil TS 77-18 single-ply membrane, mechanically fixed, in accordance with manufacturer's installation instructions. Installer to hold current Sika Sarnafil Registered Contractor status. --- ## Specification Formats ### Work Section Format Organised by trades or building elements, aligned with standard classification systems: | System | Jurisdiction | Structure | |--------|-------------|-----------| | **NBS** (National Building Specification) | UK | Uniclass-aligned work sections | | **CSI MasterFormat** | US/Canada | 50 divisions (00-49) | | **NATSPEC** | Australia | Trade-based sections | | **CPWD** | India | Central Public Works Department format | ### NBS Work Sections (Commonly Used) | Section | Content | |---------|---------| | **Preliminaries** | Project details; contractor obligations; site management | | **Substructure** | Excavation; foundations; ground slabs | | **Masonry** | Brickwork; blockwork; stonework | | **Structural steelwork** | Fabrication; erection; fire protection | | **Concrete** | In-situ concrete; formwork; reinforcement | | **Roofing** | Flat roofing; pitched roofing; rainwater | | **Cladding** | Curtain walling; rainscreen; render | | **Windows and doors** | Frames; ironmongery; glazing | | **Internal finishes** | Plaster; screeds; tiling; painting | | **Mechanical services** | HVAC; plumbing; sprinklers | | **Electrical services** | Power; lighting; fire alarm; data | | **External works** | Paving; drainage; landscaping | ### MasterFormat Divisions (CSI, US) | Division | Content | |----------|---------| | 03 | Concrete | | 04 | Masonry | | 05 | Metals | | 06 | Wood, Plastics, Composites | | 07 | Thermal and Moisture Protection | | 08 | Openings (doors, windows, glazing) | | 09 | Finishes | | 21 | Fire Suppression | | 22 | Plumbing | | 23 | HVAC | | 26 | Electrical | --- ## NBS and Uniclass **NBS** is the UK's primary specification system, providing standardised clause libraries: | Product | Description | Format | |---------|------------|--------| | **NBS Chorus** | Cloud-based specification platform; Uniclass-aligned | Digital; collaborative | | **NBS Create** | Desktop specification tool | Legacy; being replaced by Chorus | | **NBS Source** | Product selection platform linked to BIM objects | Digital | **Uniclass 2015**: The UK's unified classification system for the construction industry. Provides a hierarchical code for every element, system, and product — used for specifications, BIM, cost planning, and facilities management. Example Uniclass codes: - Ss_25_10 — Wall completion systems - Ss_25_10_30 — Framed wall completion systems - Pr_25_71_14_09 — Clay facing bricks **BIM integration**: NBS specifications can be linked to BIM model elements, ensuring that the 3D model and the written specification describe the same thing. See [[BIM Fundamentals and LOD]] for information management framework. --- ## Specification Structure A typical specification clause contains: ### 1. General Requirements - Applicable standards (BS, EN, ASTM, IS) - Definitions and abbreviations - Submittals required (samples, test certificates, shop drawings, method statements) - Storage and handling requirements ### 2. Products/Materials - Material properties and grades - Manufacturer and product (if proprietary) or performance criteria - Accessories and ancillary materials - Prohibited materials ### 3. Execution/Workmanship - Preparation of substrate - Installation method and sequence - Tolerances - Protection of completed work - Testing and inspection requirements - Remedial work for non-conforming work ### 4. Completion - Cleaning and finishing - Documentation and certification - Warranty and guarantee requirements --- ## Writing Effective Clauses ### Language Principles | Principle | Good | Bad | |-----------|------|-----| | **Imperative mood** | "Apply primer to all surfaces" | "The primer should be applied" | | **Specific standards** | "To BS EN 998-2, designation M4" | "To the relevant British Standard" | | **Measurable criteria** | "Flatness tolerance ±3mm in 2m" | "Surface to be reasonably flat" | | **Unambiguous** | "Three coats of paint, each 50μm DFT" | "Multiple coats of paint" | | **Avoid weasel words** | "Submit samples for approval before ordering" | "Samples may be requested" | | **Single requirement per clause** | One testable requirement | Multiple requirements bundled together | ### Key Drafting Rules 1. **Be specific**: Name standards with their full designation, date, and clause where relevant 2. **Define quality**: Every material must have a measurable quality criterion — grade, class, standard, or tested performance 3. **Define workmanship**: Every process must have a measurable standard of completion 4. **Avoid contradictions**: The specification must not contradict the drawings; establish a priority hierarchy in the preliminaries 5. **Cross-reference**: Link to drawings, schedules, and other specification sections rather than duplicating information 6. **Update for each project**: Never re-use specifications without reviewing and updating for the specific project conditions 7. **Prohibited materials**: Explicitly list any materials that must not be used (e.g., ACMs with combustible cores for buildings >11m; high-GWP refrigerants; materials containing asbestos) --- ## Proprietary vs Performance Specifications | Aspect | Proprietary | Performance | |--------|------------|-------------| | **Risk** | Architect bears selection risk | Contractor bears selection risk | | **Competition** | May limit competition; "or approved equivalent" mitigates | Maximum competition | | **Quality control** | Known product; predictable result | Outcome uncertain until submitted/tested | | **Design-build** | Less appropriate (contractor selects) | Preferred for D&B | | **Innovation** | Limited | Encourages alternative solutions | | **Procurement** | Public procurement may restrict proprietary naming | Performance-based preferred for public | **Best practice**: Use performance specification as the primary approach, supplemented by named products where specific quality or compatibility is essential. For critical building elements (waterproofing, structural connections, fire-rated systems), proprietary specification with named products provides greater certainty. --- ## Coordination with Drawings | Principle | Application | |-----------|------------| | **No contradictions** | If the drawing shows brick and the spec says stone, there is a contractual dispute | | **Hierarchy of documents** | Establish in the contract which document prevails in case of conflict (typically: specification > drawings > schedules) | | **Cross-referencing** | Specifications reference drawings by number; drawings reference specification sections | | **Schedules** | Door schedules, window schedules, finish schedules, and ironmongery schedules bridge the gap between drawings and specifications | | **Avoid duplication** | Specify a requirement in one place only; other documents reference it | --- ## Common Specification Errors | Error | Consequence | Prevention | |-------|------------|-----------| | **Copy-paste from previous project** | Irrelevant clauses; wrong standards; contradictions | Review every clause for project relevance | | **Specifying obsolete standards** | Non-compliant; unenforceable | Check current standard editions | | **Over-specification** | Unnecessary cost; unrealistic quality demands | Specify to the level the project actually requires | | **Under-specification** | Substandard work; no contractual remedy | Define measurable quality criteria | | **Contradicting the drawings** | Disputes; claims; delays | Coordinate spec and drawings simultaneously | | **Missing items** | Unpriced work; variations; cost overruns | Use systematic checklists; NBS clause libraries | | **Ambiguous "or equivalent"** | Substandard substitutions | Define equivalency criteria: "equivalent in performance to [standard], fire class, warranty, and colour range" | | **No submittal requirements** | Contractor installs without approval | Require samples, test certificates, and approval before procurement | --- ## See Also - [[Architectural Detailing Principles]] - [[BIM Fundamentals and LOD]] - [[RIBA Plan of Work]] - [[Traditional Design Bid Build]] - [[Building Codes and Regulatory Framework]] - [[Material Weight Tables]] --- #specifications #nbs #writing #contracts #quality #detailing