Most buyers assume structural weight is one number calculated once. In reality it changes at least twice: once in the structural study, and once after final detailing and section selection. The gap between them is normal and familiar to engineers, but it is rarely explained to the buyer — and that is where avoidable disputes begin.
Note: The figures here come from one specific project with its own dimensions and conditions, and are not a general ratio to apply elsewhere. The size of the gap varies with span, structural system, and how far detailing is optimised.
1. What is the difference between design weight and as-built weight?
Design weight is the output of the structural calculation: loads are combined, the structure is analysed, and sections are chosen that satisfy safety with a margin. At this stage the engineer picks from standard section tables and usually takes the section slightly larger than needed — because sections do not come in continuous sizes.
As-built weight is what actually leaves the shop after detailing: sections are reviewed against real connections, members are combined, tapered sections are optimised, and purlin spacing is revisited. The result is usually lighter.
The gap is not an error in the study. The study was correct; the optimisation came from detailing that was not available before it.
2. How large was the gap on a real project?
From our projects: the Sham View sales hall — 1,200 m², 30 m clear span, 5 m eaves height.
Design study weight: 49 tonnes.
As-built weight: 42 tonnes.
Gap: 7 tonnes, or 14%.
Seven tonnes is not a rounding difference. At prevailing steel prices it is a tangible difference in material cost alone, before counting its effect on foundations, transport and lifting.
Which is why it matters to know which weight the quotation in front of you is based on.
3. Where does the gap come from?
From four places, in order of impact:
- Tapered sections: instead of a constant section along a column or rafter, the section follows the moment curve — deep where the moment is large, slim where it is small. This alone accounts for most of the saving.
- Standard section rounding: the study takes the larger section as a margin; detailing reviews each member individually and finds that some accept the smaller one.
- Frame and purlin spacing: adjusting spacing by half a metre can remove an entire frame from a long hangar.
- Connection consolidation: cleaner connection design reduces stiffener plates and bolts.
Note what is not on the list: reducing loads. That is not optimisation, it is changing the assumptions — and it must not happen between study and execution without a signed recalculation.
4. Does the gap always work in your favour?
No. And this is the part nobody mentions.
The gap can reverse, making as-built weight higher than the study, for entirely legitimate reasons: a requirement that was not calculated (an extra opening, a suspended service), changed site conditions, or a review that raises a load value. In that case, who pays for the extra tonnes?
The answer lives in the contract, not in goodwill. A quotation that says "estimated weight 49 tonnes, price adjusted against final weight" is honest — but know that the ceiling is open. One that says "lump sum for the attached specification" moves the risk to the supplier, and its price includes that risk.
Both are acceptable. What is not acceptable is a quotation that says neither.
5. Which weight belongs in a quotation?
The design weight — or a preliminary estimate declared as an estimate.
Because as-built weight cannot be known before detailing, and detailing comes after contract. Anyone promising you the as-built figure at quotation stage is either guessing or will invoice you the difference later.
Sound wording states three things: the weight, its source (study or estimate), and the adjustment mechanism if it changes. Three lines that prevent an entire dispute.
6. How do you protect yourself if the gap goes the other way?
With four items written down before signing:
- Fix the load values in the contract. If loads do not change, weight does not rise except by a change from you.
- Fix the specification, not a description. "Sandwich panel roof" is a description; "sandwich panel of thickness X and density Y" is a specification.
- Ask for shop drawings for approval before cutting. This is your last chance to see what you are paying for.
- Agree the adjustment mechanism. If weight rises because of a change you requested, at what price per tonne? Write it now, not later.
For our part: we state the design weight in the quotation and call it by its name, and we show the gap after detailing rather than passing it through silently. The two figures in this article — 49 and 42 — are published for that reason.
- Design weight
- As-built weight
- Tapered sections
- Contract scope
Have a project like this?
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