Why the bill of quantities and the drawings stop agreeing at award
Two measurements of one building, taken at two different moments
A bill of quantities is measured from the tender drawings. The shop drawings are produced after the order, from the same building, at a level of detail nobody had at tender.
Those are two measurements of one object, taken months apart by two parties with two different jobs. They will not agree. The useful question is not whether they agree but whether the gap was visible before the order was placed, and who is carrying it.
What a BOQ cannot count, because nothing has been drawn yet
A BOQ can only measure what is on the drawing in front of the measurer. At tender, much of the building is not yet decided: connection design, bracing arrangement, purlin laps, opening framing, and every dimension that only becomes fixed once a particular structural system is chosen.
So the measurer does the only reasonable thing and measures the building as a shape — tonnage by member type, area by surface — with allowances for the parts that are not there yet. Those allowances are honest, and they are also a guess. The shop drawings replace the guess with an actual member list, and the distance between a guess and a list is the entire argument.
Where the drift actually comes from
Drift is rarely one large omission. It is a set of small, individually defensible measurement conventions, each of which moves the number in the same direction.
- Secondary members. Purlins, girts, sag rods, fly braces and eave stiffeners are often carried as a rate per square metre of roof or wall rather than as individual members. When they are detailed, the piece count is exact and the mass is not what the rate assumed.
- Connections. End plates, cleats, gussets and bolts are frequently allowed as a percentage rather than counted. On a heavily connected frame — crane brackets, mezzanines, moment joints — that allowance is thin.
- Cladding by developed area. A wall measured flat is not a wall measured with ribs, laps, corners and openings. The difference is real material, and it appears at detailing rather than at tender.
- Openings. Every door, window, louvre and penetration is framing that has to exist, and it is easy to leave out of a quantity measured from an elevation.
- Protection by surface area. A coating quantity follows surface area, and surface area follows the number of members rather than the tonnage. Heavy sections carry less surface per tonne; a light, highly divided frame carries much more.
- Fabrication allowance. Cutting, drilling and end losses are real, are small individually, and are cumulative across thousands of pieces.
Add them up and a building can differ from the tender measurement by a few per cent in mass with nobody having made a mistake. A few per cent of a large frame is not a rounding error. It is a commercial event.
Tonnage is a poor unit for a building
Tonnage measures mass. A project is delivered as pieces: cut, drilled, marked, packed, shipped and erected in a sequence. Two buildings of identical tonnage can differ by hundreds of pieces, and the piece count is what decides fabrication hours, packing volume, container count and erection time.
This is why a tonnage comparison between two offers is so often misleading. The heavier offer is not automatically the more complete one, and the lighter one is not automatically the cheaper one to build. It may simply have counted fewer connections, or measured the secondary members with a thinner allowance.
The register that makes the two reconcilable
The only reliable way to close the gap is to convert both documents into the same unit of account, and that unit is the piece.
A member register — one row per mark, with section, length, quantity and grid position — totals into tonnage at any time, but it can also be checked against the drawing, against the fabrication list, against the packing list and against the erection sequence. A BOQ cannot do any of that, because it was never a list of pieces.
Once the register exists, reconciliation becomes a mechanical operation: sum the register, compare it with the tender measurement, and explain every difference by cause rather than by total. A variance you can attribute is a variation. A variance you cannot attribute is a dispute waiting for a payment milestone.
Why a defined scope holds better than a measured quantity
A measured quantity ties the commercial outcome to a measurement taken before the building was designed. A defined scope ties it to a description of what will be supplied: the structural system, the span and bay grid, the eave height, the crane if there is one, the cladding, the openings, the protection system, the drawings and documents, and the exclusions.
The second form survives detailing, because detailing sits inside the definition instead of outside it. The first does not, because every sheet of shop drawings is an opportunity for the measured quantity to move — and the party holding the measurement is rarely the party doing the detailing.
Neither form removes the need to compare. Both decide where the argument happens: at the order, or at the end.
What to ask before you compare two offers
- Was this quantity measured from a drawing, or taken from a member register? Ask to see the register.
- How were secondary members counted — as pieces, or as an allowance by area?
- Are connections and bolts counted or allowed? Against what, and how thin is the allowance?
- Is cladding measured flat or by developed area, and are openings deducted?
- Is coating charged by surface area or by tonnage?
- What is excluded, in writing? The exclusions list is usually the most informative page in the document.
Two offers that cannot answer these questions in the same language are not comparable, whatever the totals say. The figure at the bottom of a BOQ is a measurement of a drawing. It is not yet a description of a building.
Written by Johnny Joe, who advises owners and project managers in Africa and South America on steel buildings. If a quantity is what worries you on a live project, send the BOQ summary page and the drawing register and I will tell you where the two would separate.
Related: what actually drives the cost of a steel building · what is in a shop drawing set · bay spacing trade-off · how to write a building brief
