Structural work take-off: foundations, reinforced concrete and masonry

This guide follows French practice: French standards (NF DTU), French regulations and French bill conventions. Most sources are in French.

Structural items and their units

The structural work package brings together very different items: concrete volumes, wall areas, lengths and counted items. In a priced bill of quantities (DPGF) or a schedule of rates, each one has its unit, and the specification (CCTP) sets it, together with what the rate includes: formwork, reinforcement, blinding.

The French workmanship standards, NF DTU 13.1 for shallow foundations, NF DTU 21 for concrete work and NF DTU 20.1 for small-unit masonry, say how to build. They do not say how to measure: the method of measurement is the one in your contract documents, to be read before drawing the first line.

Reading the structural drawings

Structural work is measured on the structural engineer's drawings rather than the architect's: foundation plan, formwork drawings for each floor, sections. Each element carries a reference that gives its section, for instance “SF1 60x30” for a strip footing 60 cm wide and 30 cm deep, and the levels (formation level, top of footing, top of slab) give the heights.

Before measuring, check the scale against a known dimension (see the scale of a drawing) and list the references: one line per reference in the take-off avoids mixing two sections under the same name.

Foundations: trenches, footings and ground beams

The take-off follows the order of the works:

• Trench excavation. Trench length × width × depth, from the reduced level dig down to formation level. The volume is measured in situ; bulking is dealt with separately, according to the contract.
• Blinding. Paid per m² of bearing area or per m³: its area follows from that of the footings.
• Strip footings. Length × width × depth, one item per section: an SF 50x25 and an SF 60x30 are not added together.
• Pad footings and bases. Counted reference by reference, then unit volume × number.
• Ground beams. They tie the pad footings together. Their length is taken between the faces of the footings, plus the bearing the document requires: an exam paper listed in the sources adds 5 cm at each end.

Junction rules

Differences between two structural take-offs come mainly from junctions, counted twice or missed. Four rules cover most cases:

• A closed outline is measured on the centre line. For a footing or a wall running round the building, centre-line length × section gives the exact volume: what is counted in excess outside the corners makes up for what is missing inside. Measured on the outer face, each corner is counted twice.
• At a T-junction, the incoming element stops at the face of the one that continues. An internal footing drawn from centre line to centre line counts part of the external footing's concrete a second time.
• A beam is measured below the slab. Its depth is the downstand, overall depth minus slab thickness, and its length stops at the faces of the columns; otherwise the concrete at the intersections is paid twice.
• A wall starts at the top of its footing, and a slab cast between walls is measured between their inner faces. If the contract runs the slab over the walls, the wall stops below the slab.

Walls, columns, beams and slabs

Concrete walls are measured like any wall: length × height, openings deducted or not according to the contract's rule, then × thickness for the concrete. If formwork is paid separately, it is measured on both faces, plus the reveals of openings; for a closed outline, twice the centre-line length gives the sum of both faces.

Columns and beams are counted per item, per metre or per m³ according to the bill, with the downstand rule. Slabs are measured as areas, voids deducted according to the contract, then × thickness; slab edges, box-outs and slab joints often have their own line.

Blockwork and render

A blockwork wall is measured in m², length × height, with the same question about openings as for an elevation, and one item per thickness: a 15, 20 or 25 cm wall does not have the same rate. A 50 × 20 cm block covers 0.10 m² of wall: ten blocks per m². Ring beams, lintels and sills are measured per metre or per item, render face by face.

Reinforcement, according to the contract

Reinforcement is paid per kilogram, or included in the reinforced concrete rate. The contract chooses between two methods:

• From a bar schedule: developed length of each bar × mass per metre of its diameter (0.395 kg/m for an 8 mm high-yield bar, 0.617 kg/m for a 10 mm one), from the reinforcement drawings.
• By ratio: kilograms of steel per m³ of concrete, given by the structural engineer for each family of elements, multiplied by the measured volume. There is no universal ratio: the engineer's figure prevails.

Worked example

A small single-storey building. Perimeter strip footing SF1, 0.50 × 0.25 m, whose centre line forms a 12.00 × 8.00 m rectangle. In the middle, a column on a pad footing SI, 1.20 × 1.20 × 0.30 m, tied to the two long elevations by two ground beams L1, 0.20 × 0.40 m. On the strip footing, a plinth wall 0.20 m thick and 0.80 m high, on the same centre line. Inside, a 0.12 m ground slab cast between the inner faces of the wall, with a 1.00 × 0.80 m void deducted. Ground beams measured between faces with no bearing, volumes written to three decimal places (rules taken for the example).

Foundations

Plinth wall

Ground slab

Footing measured on the outer face, 2 × (12.50 + 8.50) = 42.00 ml: 5.250 m³, i.e. 0.250 m³ too much. Ground beams drawn from centre to centre, 8.00 ml: 0.640 m³ instead of 0.504 m³, with 0.136 m³ counted a second time in the footings. Slab measured on the wall centre line, 12.00 × 8.00 = 96.00 m²: 3.96 m² and 0.475 m³ too much, already counted in the wall.

The most common mistakes

• Different sections added together under a single reference.
• Junctions counted twice: footings at T-junctions, ground beams from centre to centre, beams over their full depth below the slab, a closed outline measured on the outer face.
• Heights read from the architect's section instead of the levels on the structural drawings.
• In-situ and bulked volumes mixed together.
• A steel ratio taken “from habit” instead of the engineer's figure.
• A take-off with no record: impossible to trace where a cubic metre came from when the concrete ordered falls short. Take-off calculations written line by line avoid this dead end.

Doing it in Kotelys

In Kotelys, each reference becomes a type, with its section in its name (“SF1 60x30ht”, “M1 120x120x40ht”), its colour on the drawing and its total.

• Strip footings, ground beams and ring beams are drawn with the Length tool: the total is in ml, and the volume follows from the section in your estimate. For a direct volume, draw the footprint of the footing with the Area tool and give its depth as the thickness.
• Walls and blockwork are drawn as Lengths with a height given to the type: the wall area is written out in full (“40.00 ml × 0.80 m = 32.00 m²”), each opening is taken off with its own height, and voids not deducted are set type by type.
• Slabs, ground slabs and rafts are drawn as Areas, voids as deductions (S key), with a thickness that gives the volume: “91.24 m² × 0.12 m = 10.95 m³”.
• Pad footings and bases are counted with the Count tool in the rectangle shape, stretched to cover the drawing of the reference.
• One zone per building or per floor breaks the reports down; the take-off calculations write out every calculation, and the Excel workbook gives the summary and the object-by-object detail.

Kotelys does not calculate reinforcement and does not find junctions for you: it is your line, stopped at the right face, that makes the take-off accurate.

Further reading: structural work take-off in Kotelys, and the other guides.

Sources

1. AFNOR, NF DTU 13.1 P1-1 « Travaux de bâtiment – Fondations superficielles » (September 2019, replaces DTU 13.11)
2. AFNOR, NF DTU 21 P1-1 « Exécution des ouvrages en béton »
3. FFB, « Mise à jour du NF DTU 20.1 pour maçonnerie de petits éléments »
4. Éduscol STI, vocational baccalaureate TB ORGO 2020, E21 paper: footing volumes and steel ratio per m³ of concrete
5. Éduscol STI, vocational baccalaureate TB ORGO 2022, E21 paper: ground beam volumes (5 cm bearing) and mass per metre of high-yield bars

Thresholds and values described as “taken for the example” are not rules: those of your contract (specification, bill of quantities) always prevail.