Measurement rules for construction works

Every prezzario contains a section devoted to measurement criteria and technical specifications. It is the part that establishes, for each work category, the unit in which measurement is taken, what the rate covers, which deductions apply and which additions are permitted.

That section is consistently the least read and the most decisive. Applying the correct item with the wrong measurement basis produces an error that no check on the total will reveal, because the bill remains internally consistent: what breaks is its correspondence with what happens on site.

The first operational rule is therefore methodological before it is technical: before measuring a category, read the criterion the adopted prezzario assigns to it. The price book criterion always prevails over personal convention.

Unit	Applies to	Method note
Linear metre	Elements where one dimension dominates: kerbs, skirtings, pipework, containment	Whether the measurement follows the centreline or the external girth must be clarified
Square metre	Surfaces: plaster, floor and wall finishes, waterproofing, scaffolding	The unit most exposed to the question of deductions
Cubic metre	Volumes: excavation, concrete, thicker masonry, bulk demolition	Requires the reference cross section to be defined
Kilogramme	Metalwork: reinforcement, structural steel, sections	Depends on the assumed unit weight, which should be stated
Number	Discrete elements: fittings, luminaires, joinery, outlet points	What constitutes one element must be defined
Lump	Work that cannot be broken into homogeneous units	To be used sparingly. Makes the item impossible to value by quantity

The lump item warrants a caution. It is convenient at estimating stage and problematic at valuation stage, because it removes the work from the measurement mechanism. Under a measured contract a lump item is an almost guaranteed point of friction.

Measuring vuoto per pieno, literally void as solid, means measuring a surface as though the openings were not there, without deducting them. It is not a shortcut: it reflects the fact that for some operations the presence of an opening does not reduce the effort in proportion to the area removed, and sometimes increases it, because reveals, heads and sills have to be worked.

The rules vary from one prezzario to another and from one operation to another, and there is no single national convention. Broadly, price books set dimensional thresholds below which an opening is not deducted, and provide extras for working the surrounds. Both the thresholds and the treatment of surrounds must be read in the prezzario adopted: assuming them from memory or by analogy with another list is one of the most frequent errors.

The principle to hold onto is that deductions and extras form a system. Deducting an opening while also applying the extra for reveals intended for the case where no deduction is made means either counting twice or not counting at all, depending on the circumstances.

Excavation. The distinction that matters is between actual and theoretical cross section. Measuring to theoretical section values the geometric volume designed, regardless of the widening the site requires for operational reasons. The applicable criterion and the treatment of batters, timbering and over excavation are defined by the prezzario.

Masonry. Thickness determines whether measurement is by area or by volume. The treatment of junctions, reveals and lintels needs clarifying, as does the distinction between loadbearing and infill masonry, which carry separate items.

Concrete and reinforcement. Concrete is measured by volume, steel by weight. Reinforcement weight derives from the diameters and lengths shown on the structural drawings, including laps where the prezzario includes them, and should be taken from the drawings rather than estimated as an incidence per cubic metre other than at preliminary stage.

Plaster and finishes. These are the categories where deductions weigh most, given the frequency of openings and the significance of surrounds. Whether the prezzario treats curved, inclined and high level surfaces separately should be verified.

Waterproofing. Upstands, laps between sheets and junctions with penetrating elements need defining.

Services. Measurement may be by network length, by outlet point or by fitting, and the three bases are not equivalent. The outlet point basis includes a notional allowance of upstream network, which must be identified so as not to count it again in the distribution item.

Demolition. The choice between volume and area depends on the nature of the element removed. Whether the rate covers loading, haulage and disposal should be verified, as these are often separate items.

Scaffolding. Measurement is normally taken on the area of elevation served, not on the girth of the structure. The time criterion, meaning the hire period included in the rate, is always an element to check.

The practical effect of the convention is clearest on a case. Consider an internal wall 12.40 metres long and 2.90 metres high, with two openings of 1.40 by 1.30 metres, to be plastered on one face.

Criterion assumed	Resulting quantity	Additional items
Full deduction of openings	32.32 m2	Extra for working reveals, measured per linear metre
Measurement without deductions	35.96 m2	None. Working the surrounds is included in the rate

The divergence in quantity is about eleven per cent, but the final cost of the two assumptions can be very close, because the extra for reveals recovers part of the area deducted. The point is therefore not which criterion produces the higher figure, but that the two must not be mixed: applying the deduction and omitting the extra understates the work, applying no deduction and adding the extra overstates it.

The same reasoning applies to the length of reveals, which must be measured according to the prezzario convention: some lists consider only the perimeter of the opening, others separate vertical reveals from the head and the sill, with different items and rates. Some lists cover the plaster alone, others include the finish.

When reviewing a bill prepared by others, checking the measurement criteria is more effective if done by sampling rather than exhaustively. The sample should be drawn from the categories carrying most weight.

The method is to select the three or four categories with the greatest incidence on the total, identify the prezzario item applied to each, read the corresponding criterion and reconstruct the quantity for one significant dimension line. If the reconstruction reconciles, the criterion has been applied correctly and it is reasonable to assume the same for the other lines in that category. If it does not, the error is almost always systematic and affects the whole category rather than the single line.

The categories warranting priority attention are those where the convention matters more than the geometry: plaster and finishes for deductions, excavation for the actual versus theoretical section, services for the choice between length and outlet point, scaffolding for the hire period. Plaster, finishes and demolition work are the typical examples.

Price books are drawn up under common criteria but remain distinct documents, and measurement conventions also reflect local construction practice. A rule valid in one regional prezzario may not hold in another, and the same operation may be measured in different units in two lists.

Two operational consequences follow. First, the prezzario adopted must be stated in the bill together with its edition, because without that reference the quantities cannot be interpreted. Second, a bill drawing on more than one source must be checked item by item against the measurement criteria, because consistency of rates does not imply consistency of conventions.