How to structure a technical specification: standard plan, requirements and pitfalls

Every professional specification follows the same eight-section structure, whatever the trade. That standard structure lets the contractor find what it needs quickly, and lets the design team check exhaustively that every requirement is covered.

Sections 4, 5 and 7 are the three critical ones. A weakness in any of them creates a direct contractual risk. Section 7, the limits of the package, is the most often forgotten and the most productive of interface disputes.

The limits section describes precisely what separates this package from the adjoining ones. For every technical interface it has to answer three questions: who supplies, who fixes, and how far does it run?

• The insulation contractor supplies and fixes: the insulation system up to 50 mm from the window reveal, with the starter profile and the corner profiles.

• The window contractor supplies and fixes: the window with its shutter box, the compressible foam seal on the window side, and the external sills.

• The interface belongs to: the insulation contractor, which has to lap onto the window frame to eliminate the thermal bridge. The window must be fixed before the insulation for that lap to be possible.

Without that precision in both specifications, each contractor reads the boundary in its own favour. The thermal bridge at the window reveal, one of the commonest heat losses of all, almost always comes from an interface that was never defined contractually.

ℹ The interface method: list every junction before you write Before drafting section 7 of each specification, produce an interface schedule listing every junction between the package concerned and the adjoining ones. For each interface: what package A does and up to where, what package B does and from where, and which package owns the junction zone. That schedule is produced once for the whole tender package and fed simultaneously into the specifications of every trade concerned. It is the most protective coordination document in the whole tender package.

Every trade has its fundamental technical requirements, the ones whose absence from the specification creates an immediate risk of non-compliance or dispute.

The interfaces that have to be dealt with explicitly are the most litigious zones on any site. The three most critical on a standard project are the insulation-to-window junction, which produces the thermal bridge; the openings through the structure for services, since drilling concrete afterwards is expensive; and the coordination of the ceiling void between mechanical and electrical services, where the clear height runs out.

On a private contract, naming a brand without allowing an equivalent is lawful but inadvisable: it restricts competition and invites extra cost. Good practice is to describe the product by its technical characteristics and add the words "brand X type Y or technically equivalent, subject to the prior approval of the design team".

Where a particular performance is required, acoustic, thermal or fire, the specification can require the contractor to submit its product data for approval before ordering. A clause requiring product data to be submitted for approval at least four weeks before installation allows substitutions to be controlled without holding up the site.

Cross-checking the specification against the drawings is one of the most demanding steps in producing a tender package. It consists in making sure that every requirement in the specification is represented on the drawings, and that every architectural detail has its technical requirement in the specification.

• Thicknesses: check that every thickness in the specification, 140 mm of insulation, a 100 mm partition, a 200 mm slab, matches the thickness drawn.

• Materials: check that the materials specified are consistent with the product data used for the engineers' thermal and acoustic calculations.

• Performance: check that the performance required is achievable with the technical solutions shown on the drawings.

ℹ The copied specification: the commonest trap of all The generic specification, copied from a previous project or downloaded without adaptation, is the main source of contradictions inside a tender package. One that calls for seismic resistance on a project in a non-seismic zone, enhanced insulation on a standard scheme, or acoustic testing on a project not subject to the requirement creates needless complexity and can lead the contractor to price work that is not actually required. Every specification has to be adapted to the project: climate zone, seismic zone, building type, level of specification, site constraints. That adaptation takes two to four hours per package for an experienced economist, and it is time well spent.

The most serious mistake, and the most frequent, is a missing limits section. It leads systematically to untreated interfaces: each contractor works up to the boundary it perceives, and the junctions between packages are never made. Those gaps come to light on site or after completion, when putting them right is expensive.

A professional technical specification is organised in eight sections: general provisions, reference documents, common requirements, material requirements, workmanship requirements, testing, limits of the package, documents to be supplied. The three critical sections are the material and workmanship requirements and above all the limits of the package, the section most often forgotten and the one that generates the most disputes. Every trade has its essential requirements: concrete class and cover for the frame, U-value and weather classification for windows, thermal resistance and junction detailing for façades. The interface method, a schedule listing every junction before any drafting begins, is the best available prevention against interface disputes. The three most serious mistakes are an untreated interface, a requirement inconsistent with the drawings, and a missing limits section.

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