Measurement rules by trade: the practical guide to French practice

1. The principle of deductions: the threshold rule

Most trades apply a threshold rule for deducting openings, windows, doors, bays, from the areas measured. That threshold varies with the applicable code.

A 0,50 m² threshold, the commonest: any opening larger than 0,50 m² is deducted from the area measured. Openings below that are counted in the gross area.
A 1 m² threshold, for external insulation and render: some trades apply a higher threshold of 1 m², so that a standard window of 0,80 × 1,20 m, at 0,96 m², is not deducted at all.
No deduction, for slabs and columns: some items carry no deduction whatever the size of the opening, and the gross area is counted.

Applying these thresholds incorrectly is one of the commonest sources of divergence between the economist's quantities and the contractors' own. An economist who applies the 0,50 m² threshold to external render, where the code sets 1 m², understates the deductions and overstates the quantities.

2. The reference table, trade by trade

The table below covers the commonest items across nine families of trades. For each: the correct unit, precisely what is measured, the deduction rules, the reference code, and the frequent error to avoid.

Trade and item	Unit	What is measured	Deductions	Reference code	Frequent error
CONCRETE FRAME
Trench excavation	m³	Volume excavated, length × width × depth	None	DTU 12	Forgetting the allowance for battering in loose ground
Reinforced concrete walls	m²	Developed area of each wall, height × length	Deduct openings over 0,50 m²	DTU 21	Measuring in m³ instead of m²; check which unit the rate is against
Reinforced concrete slabs	m²	Horizontal area per floor	No standard deduction; voids under 1 m² not deducted	DTU 21	Forgetting balconies and terraces
Reinforced concrete columns	lm	Clear height between floor faces	None	DTU 21	Measuring overall rather than clear, which overstates
MASONRY
Brick façade walls	m²	Developed area of the walls, perimeter × height	Deduct openings over 0,50 m²	DTU 20.1	Not deducting openings at all, overstating by 15 to 25 %
Single-coat external render	m²	Developed façade area including the window reveals	Deduct openings over 1 m²	DTU 26.1	Forgetting the reveals, which understates
ROOF STRUCTURE AND COVERING
Tiled roof covering	m²	Area on the slope, the real area of the pitch	Deduct rooflights over 0,50 m²	DTU 40.11	Measuring on plan, understating by the pitch factor; at 35° the real area is 22 % greater
Flat roof waterproofing	m²	Horizontal area including 200 mm of upstand	Deduct penetrations over 0,10 m²	DTU 43.1	Forgetting the upstands, understating by 8 to 12 %
Flashings, valleys, verges	lm	Real developed length of the profiles fixed	None	DTU 40.41	Measuring on plan, which understates
FAÇADES AND INSULATION
External insulation	m²	Developed façade area including the window reveals	Deduct openings over 1 m²	DTU 45.4	Leaving out the reveals, which both understates and leaves a thermal bridge
Rainscreen cladding	m²	Developed area of the external face	Deduct openings over 0,50 m²	DTU 41.2	Forgetting the soffits at the eaves
EXTERNAL JOINERY
Windows and French doors	nr	By number, with the full description of dimensions and type	None; each window is one unit	EN 14351	Measuring in m², which makes tenders impossible to compare
Roller shutters	nr	By number, tied to the corresponding window	None	EN 13659	Counting them separately from the windows, a frequent omission
PARTITIONS AND LININGS
Partitions	m²	Developed area of the partitions, height × length	Deduct openings over 0,50 m²	DTU 25.41	Measuring gross without deducting the doors, overstating by 5 to 10 %
Internal wall linings	m²	Area of the walls lined, the internal faces of the façades	Deduct openings over 0,50 m²	DTU 25.41	Confusing lining area with façade area; the lining is usually the smaller
Suspended ceilings	m²	Net horizontal area of the room	Deduct voids over 0,25 m², access hatches, recessed fittings	DTU 58.1	Measuring the whole floor without allowing for the areas with no ceiling
FLOOR FINISHES
Floor tiling	m²	Net area laid, the internal dimensions of the room	Deduct fixed elements over 0,10 m²	DTU 52.1	Measuring gross from centre line to centre line, overstating by 5 to 15 %
Timber flooring, glued or floating	m²	Net floor area, internal dimensions	Deduct fitted cupboards	DTU 51.1	Forgetting thresholds and transitions, a minor but recurrent item
Levelling screed	m²	Total floor area including circulation	Deduct plinths and services over 0,25 m²	DTU 26.2	Confusing screed area with tiled area; the screed is the larger
PAINTING
Internal wall painting	m²	Developed area of the walls including skirtings	Deduct openings over 0,50 m²	DTU 59.1	Measuring gross without deducting openings, overstating by 15 to 25 %
Ceiling painting	m²	Net horizontal area, the same as the suspended ceiling	Deduct voids over 0,25 m²	DTU 59.1	Lumping walls and ceilings into one item, distorting both
PLUMBING
Water supply pipework	lm	Real developed length of the pipework, fixing included	None	DTU 60.1	Forgetting the vertical risers and drops, understating by 20 to 40 %
Sanitary fittings	nr	By number: WC, basin, shower, bath, sink	None	DTU 60.11	Counting from the floor plans without checking the service ducts

3. The four most expensive confusions

One: roof area measured on plan instead of on the slope

Roof covering is measured on the slope, that is the real area of the pitch, not its projection on the ground. On a roof at 35°, the correction factor is the cosine of the pitch, 0,82, so the real area is 22 % greater than the horizontal projection. A roof take-off calculated on plan understates the quantities by 15 to 25 % depending on the pitch.

Two: tiling measured gross instead of net

Floor tiling is measured net, on the internal dimensions of the room, with fixed elements and thresholds deducted. A plan measured from centre line to centre line of the partitions overstates the tiled area by 5 to 15 %, depending on the partition thickness and the density of the plan. The rule is to deduct the partition thickness to obtain the real internal dimensions.

Three: internal lining confused with façade area

The internal lining is not equal to the external area of the façade. Linings are applied to the internal faces of the façade walls, with the same deduction rules as partitions. The window reveals, however, are generally not lined, which reduces the lining area against the façade area.

Four: pipework measured on plan instead of developed

Plumbing services are measured on their real developed length, including the vertical risers and drops. In a five-storey building the vertical supply and drainage stacks often represent 30 to 50 % of the total run. A plumbing take-off calculated only from the floor plans understates the quantities by 25 to 40 %.

4. The reference areas

Before any take-off, the project's reference areas have to be calculated or verified. They are what the quantities per trade are checked against.

Habitable area: the floor area of the rooms after deducting walls, partitions, stairs, service ducts, reveals and partition thicknesses. It is the area corresponding to real use, the one stated in sale contracts and leases.
Gross floor area: the reference area for planning purposes, calculated from the internal face of the façades, deducting voids over 0,30 m² and any area with a headroom below 1,80 m.
Gross usable area: the area used in commercial property for letting and for rates per m². It comprises the gross floor area plus the common private areas, halls, circulation, plant rooms.

ℹ Which area should the rates be based on? The choice of reference area determines whether rates can be compared at all. In housing, rates are expressed per m² of habitable area. In commercial property, per m² of gross usable area. In hotels, per room or per m² of floor area. A rate expressed per m² of gross floor area is not directly comparable with one per m² of habitable area: the ratio between the two varies from 0,75 to 0,90 with the compactness of the building. Always check which area is the denominator before comparing rates.

5. The reference codes by family

Concrete frame: DTU 12 (earthworks), DTU 13 (shallow foundations), DTU 13.2 (deep foundations), DTU 21 (reinforced concrete), EN 206 (concrete).
Masonry: DTU 20.1 (walls in small units), DTU 26.1 (cement renders).
Roof structure and covering: DTU 31.1 (timber framing), DTU 31.3 (engineered trusses), DTU 40.11 (slates), DTU 40.21 (clay tiles), DTU 43.1 (flat roof waterproofing).
Façades and insulation: DTU 45.4 (external insulation under render), DTU 41.2 (rainscreen cladding), DTU 26.1 (external render).
Joinery: EN 14351-1 (external windows and doors), DTU 36.1 (timber joinery), DTU 37.1 (metal joinery).
Partitions, linings, ceilings: DTU 25.41 (plasterboard), DTU 25.31 (internal plaster), DTU 58.1 (suspended ceilings).
Tiling and flooring: DTU 52.1 (bedded floor finishes), DTU 52.2 (adhesive-fixed finishes), DTU 51.1 (glued timber flooring), DTU 51.11 (floating flooring).
Plumbing: DTU 60.1 (sanitary plumbing), DTU 60.11 (copper pipework), DTU 60.32 (rainwater and foul drainage).
Heating and ventilation: DTU 65.11 (wet central heating), DTU 68.1 (mechanical ventilation), DTU 60.52 (hot and cold water distribution).

In summary

Measurement rules vary by trade and are codified in the French technical standards. The deduction threshold is the commonest source of error: 0,50 m² for most trades, 1 m² for external render and external insulation. The four most expensive confusions are roof area measured on plan (15 to 25 % understated), tiling measured gross (5 to 15 % overstated), lining confused with façade area, and pipework measured without the vertical runs (25 to 40 % understated). The reference areas are not interchangeable; always check the denominator before comparing rates. The main codes to keep to hand are DTU 21 for the concrete frame, DTU 25.41 for partitions and linings, DTU 40.11 and 40.21 for roof coverings, DTU 52.1 for tiling and NF C 15-100 for electrical installations.

The nature of the figures quoted

The percentages and amounts quoted on this page are orders of magnitude for framing, not measurements. They illustrate mechanisms and proportions, and they substitute neither for a tender exercise nor for an estimate prepared on drawings.

Articles liés

Sources: the BT construction cost indices published monthly by INSEE on its 2010 base, used to update the figures · construction cost statistics from SDES, the statistical service of the ministry responsible for construction · regulatory texts published in the Journal officiel and consolidated on Légifrance for the requirements cited.