In construction, specs (short for specifications) are written documents that describe exactly what materials to use, how to install them, and what level of quality the finished work must meet. While drawings show where things go and what they look like, specs explain the details that drawings can’t convey: the grade of steel, the type of paint finish, the performance rating of an exhaust fan, or the standard a concrete mix must meet. Together with drawings, they form the contract documents that legally define what a contractor is expected to deliver.
What Specs Actually Cover
A construction specification spells out four things: the scope of work, the materials to be used, the methods of installation, and the quality of workmanship expected. A set of drawings might show a wall in a particular location with certain dimensions, but the specs will tell you what type of insulation goes inside it, what fire rating it needs, what fasteners to use, and how the joints should be sealed.
Specs matter most in three situations. During bidding, contractors rely on them to price a project accurately. If the specs are vague, you get wildly different bids because each contractor is guessing at a different level of quality. During construction, specs give the project team a shared standard to measure work against. And if a dispute ends up in court or arbitration, specs serve as the primary written record of what was contractually required.
How Specs Differ From Drawings
Drawings and specs work as a pair, but they do different jobs. Drawings communicate visually: floor plans, elevations, structural details, mechanical layouts. Specs communicate in words: material properties, testing requirements, manufacturer standards, warranty terms. Anything mentioned in one but not the other is treated as though it appears in both.
When the two documents conflict, specs typically take legal precedence. The Federal Acquisition Regulation, which governs U.S. government construction contracts, states this explicitly: “In case of difference between drawings and specifications, the specifications shall govern.” Most private contracts follow the same convention, though you should always check the specific contract language on your project.
Types of Construction Specifications
Not all specs are written the same way. The approach a specifier chooses depends on how much flexibility they want to give the contractor and how tightly they need to control the end result.
Descriptive Specifications
A descriptive spec details the physical properties of a product without naming a brand. For example, a spec for sand might read: “Clean, washed, sharp, durable natural particles, free from soluble salts or organic impurities. Sand for grouting shall be screened to pass a 30 mesh sieve with not more than 5% passing a 100 mesh screen.” Any manufacturer’s product that meets that description is acceptable. This keeps competition open and often lowers costs. However, if the description is written so narrowly that only one brand could possibly match, it effectively becomes a closed spec even without naming anyone.
Performance Specifications
Performance specs define the end result rather than the materials or methods used to get there. Instead of telling a contractor which brand of exhaust fan to install, a performance spec might require a fan “rated for continuous operation at 500 CFM against a static pressure of 0.5 inches of water column at 50 degrees Celsius.” The contractor picks whatever product meets those numbers. Performance specs also typically require a specific way to prove the product qualifies, such as a named test method or calculation, along with a deadline for submitting that proof. They give contractors the most freedom but require more careful review during submittals.
Reference Standard Specifications
These specs point to an established industry standard instead of writing out every requirement from scratch. A reference standard spec for portland cement might simply read: “Conform to ASTM C150, Type I or Type II, low alkali.” The standard itself contains all the detailed requirements. This approach saves pages of writing and ties the project to widely recognized benchmarks. It works well for commodity materials like concrete, steel, and lumber where industry standards are mature and well understood.
Proprietary Specifications
A proprietary spec names a specific product or manufacturer. This is the most restrictive approach, and it’s common when the architect or engineer has a particular product in mind for design, aesthetic, or compatibility reasons. Many proprietary specs include the phrase “or equal,” which allows the contractor to propose a substitute as long as it matches the named product’s characteristics. Without that language, the contractor has no choice in the matter.
How Specs Are Organized
In the United States and Canada, most specs follow the CSI MasterFormat system, published by the Construction Specifications Institute. MasterFormat divides all construction work into 50 numbered divisions. Division 03 covers concrete, Division 07 covers thermal and moisture protection, Division 09 covers finishes, and so on. Each division breaks down further into sections. A section might cover a single product type, like gypsum board assemblies, with its own scope, materials list, execution requirements, and quality standards.
This standardized numbering system means that anyone in the industry can quickly find the specification section they need, regardless of the project or the firm that wrote the documents. Internationally, other classification systems serve a similar purpose. Uniclass 2015 is widely used in the United Kingdom, CoClass in Sweden, and OmniClass in broader international contexts. There is no single global standard, and most countries still rely on their own national system.
Who Writes Them
Specs are typically written by the architect or engineer of record, often with help from a dedicated specification writer or consultant. On large projects, each discipline (structural, mechanical, electrical, plumbing) writes the spec sections that fall under its scope, and a project manager or design lead coordinates the full package. The work happens during the design phase, with specs evolving through schematic design, design development, and construction documents. By the time a project goes out to bid, the specs should be complete and coordinated with the drawings.
Some firms and owners use master specification templates like MasterSpec, which provide pre-written sections that the design team edits to fit the project. This speeds up the process and reduces the risk of leaving something out, but it still requires careful editing. A master spec section for roofing, for instance, might include options for dozens of membrane types. The specifier has to delete what doesn’t apply and tailor what remains to the actual design.
Why Specs Go Wrong
The most common problems with specs come down to conflicts, ambiguity, and copy-paste errors. When a specifier reuses sections from a previous project without fully updating them, the specs can reference products or standards that don’t match the current design. Conflicting requirements between spec sections, or between the specs and drawings, create confusion on the job site and often lead to change orders or claims.
Vague language causes similar issues. A spec that calls for “high-quality paint” without defining what that means gives the contractor room to use the cheapest product they can defend. A spec that requires “paint with a minimum of 45% solids by volume, satin sheen between 20 and 35 units at 60 degrees, and a minimum dry film thickness of 1.5 mils per coat” leaves far less room for interpretation. The clearer the description, the more likely the finished work matches what the designer intended.