Quantity takeoff for the capstone building

Owen had done takeoff before, but never for a whole building on his own. On the first evening he opened the foundation plan, picked a colour for footings and started clicking. By midnight he had measured every footing on the plan and had no idea whether the total was right. The next morning Ruth asked him three questions: what units he was measuring in, what he would compare the total against, and who else would check it. 'Takeoff is not clicking,' she said. 'It is a method. If you cannot explain how you got a number, nobody can trust it.'

This lesson covers the fourth deliverable: the quantity takeoff that supports your estimate. It builds directly on construction mathematics (CON-130) and quantity takeoff and estimating (CON-200). In a capstone you will not measure every screw. You will measure the work your contractor self-performs and the major quantities that drive cost and schedule, and you will check subtrade scopes against the documents rather than re-measuring every trade.

What to measure and what to leave to subtrades

A general contractor's estimator does not usually measure all of the work in detail. Mechanical, electrical, roofing, doors and many finishes are priced by subtrades, who do their own takeoff. The general contractor's job is to make sure those prices cover the full scope, and to measure enough to judge whether they are reasonable. The work the contractor self-performs, and the work for which it must check or carry quantities, is measured in detail.

This division also shapes the capstone deliverable. Your takeoff should show detailed measurement for at least the structure and envelope, and key quantities for the systems, so a reviewer can see that you understand the building and can check the subtrade prices you use in the estimate.

Units, conventions and waste

Every quantity needs a unit and a measurement rule. Concrete is measured in cubic metres, formwork and masonry in square metres of contact or wall face, reinforcing steel in tonnes, structural steel in tonnes, roofing in square metres, and linear items such as flashings and curbs in metres. Most measurement standards measure the net quantity in place and price waste separately, so that the quantity can be checked against the drawings. The estimator then adds a waste or loss factor based on experience: perhaps 3 % to 5 % for ready-mix concrete, 5 % for block, 10 % for drywall and more for materials cut to fit irregular shapes.

In Canada, the Canadian Institute of Quantity Surveyors publishes guidance on measurement, and many owners and consultants use its methods. Contractors often use their own company conventions as long as they are consistent. The capstone rule is simple: state your units and conventions at the start of the takeoff, and follow them everywhere.

Key quantities for the structure and envelope

The station is an L-shaped plan. The apparatus bay measures 30 m east to west by 22 m north to south. The support wing measures 36 m by 15 m and attaches to the west side of the bay, leaving 7 m of the bay's west wall exposed. The building perimeter is therefore 30 + 22 + 30 + 7 + 36 + 15 + 36 = 176 m. Owen used these dimensions for a first pass at the main structural and envelope quantities, before refining them with the detailed footing and wall layouts on the drawings.

Owen then turned the dimensions into a short program, so that when addendum 2 changed the bay slab thickness at the trench drains, he could update every related quantity in one place rather than hunting through a spreadsheet.

# Key quantities for Pembrook Fire Station No. 3 (metric)
bay = (30, 22)          # apparatus bay length and width
wing = (36, 15)        # support wing length and width
perimeter = 176
bay_slab_t, wing_slab_t = 0.2, 0.125
waste = 1.05
bay_area, wing_area = bay[0] * bay[1], wing[0] * wing[1]
slab = (bay_area * bay_slab_t + wing_area * wing_slab_t)
footing = perimeter * 0.9 * 0.3
frost = perimeter * 0.25 * 1.4
print(f'Ground floor area: {bay_area + wing_area:,.0f} m2; gross floor area with level 2: {bay_area + 2 * wing_area:,.0f} m2')
print(f'Slabs {slab:,.1f}, footings {footing:,.1f}, frost walls {frost:,.1f} m3 (net)')
total = slab + footing + frost
print(f'Concrete to order with 5% waste: {total * waste:,.0f} m3')
# Masonry: wall face less openings, then block count
gross_wall = 89 * 7.5 + 87 * 8.0
openings = 6 * 4.3 * 4.9 + 140
net_wall = gross_wall - openings
blocks = net_wall * 12.5 * 1.05
print(f'Masonry wall face: gross {gross_wall:,.0f}, openings {openings:,.0f}, net {net_wall:,.0f} m2')
print(f'Concrete blocks with 5% waste: {blocks:,.0f}')

Ground floor area: 1,200 m2; gross floor area with level 2: 1,740 m2
Slabs 199.5, footings 47.5, frost walls 61.6 m3 (net)
Concrete to order with 5% waste: 324 m3
Masonry wall face: gross 1,364, openings 266, net 1,097 m2
Concrete blocks with 5% waste: 14,399

The block count assumes 12.5 standard blocks per square metre of wall, which follows from a 400 mm by 200 mm module including mortar joints. The masonry subtrade would do its own count, but Owen's number let him judge whether the quotes on bid day covered the full wall area.

Structural steel and other weight-based items

Structural steel is priced by weight. The steel fabricator does a detailed takeoff from the structural drawings, but the general contractor should know roughly what tonnage to expect. Owen used the steel schedule on the drawings for the columns and main beams and an allowance per square metre for joists, deck and bracing. His estimate came to about 128 tonnes of structural steel, joists and bracing, or roughly 74 kg per square metre of gross floor area. That is high for an office building but normal for a station with a 22 m clear span over the apparatus bay and a heavy roof carrying rooftop units. When the steel quotes arrived on bid day, they ranged from 124 to 136 tonnes, which told Owen that all bidders were reading the same scope.

Earthwork and site work

Earthwork quantities come from comparing the existing ground surface with the finished subgrade, plus excavation for footings, trenches and the trench drain sumps. Owen used the civil drawings' cut and fill table as a starting point, then added footing excavation and backfill. The site needed about 4,800 m³ of cut and 3,900 m³ of fill, with the surplus taken off site, and about 6,200 m² of asphalt paving plus 1,100 m² of reinforced concrete apron in front of the bay doors. The geotechnical report showed till over bedrock, with rock at 2.4 m to 4.0 m below existing grade. Footings would be above rock in most places, but the deepest trench drain sump and part of the storm sewer might hit it. Owen measured the volume of rock that could be affected and passed it to the risk register rather than pricing it as certain.

Earthwork is a good example of why quantities must carry their assumptions. Swell and shrinkage change volumes when soil is dug, hauled and compacted. A cubic metre of rock in the ground can become well over a cubic metre in a truck. Owen stated the swell and compaction factors he used and where they came from, so that the reviewer could see that the number of truckloads in the logistics plan matched the earthwork quantities.

Digital takeoff and model-based quantities

Most estimators now measure on screen with digital takeoff software, which records each measurement against the drawing, colours what has been counted and exports totals to the estimate. The software is fast, but it does not understand the building. It will happily measure a wall twice on two overlapping sheets, or miss a footing shown only on a detail. The team set three rules: every measurement is tagged with its WBS code from lesson 2, every drawing sheet is marked complete only after a second person has looked at it, and every addendum is overlaid on the earlier sheets so changed areas are re-measured rather than patched.

Larkin Hale had also issued a design model for information only. As CON-260 explained, model quantities can be a useful cross-check, but a design model is built to show design intent, not to be measured. Walls may be modelled to the wrong layer, slabs may run through columns, and items such as reinforcing, blocking and temporary works are not modelled at all. Owen exported the model's concrete and wall quantities and compared them with his own. They agreed within 4 % for slabs and 7 % for masonry, which gave him confidence without replacing his measured takeoff. Because the contract stated that the model was for information only, any difference would be settled by the drawings, not the model.

Checking the takeoff

A takeoff is only useful if it is right, and the most effective checks are simple. The Tamarack team used four.

• Ratios: compare quantities with the floor area. Concrete per square metre, steel per square metre, wall area per floor area and similar ratios should fall within ranges from similar past projects.
• Independent measurement: a second person measures a sample of items, such as one wall elevation or one footing grid, and compares.
• Colour-coded drawings: every item measured is marked on the drawings, so unmarked items show up.
• Subtrade comparison: when quotes arrive, their quantities are compared with the team's key quantities, and large differences are questioned.

Aaliyah did the independent check on the masonry. Her measurement of the north elevation differed from Owen's by 9 %. The difference turned out to be a set of louvres that Owen had deducted as openings and Aaliyah had treated as wall. The specification said the louvres were set in the masonry, so the opening deduction was correct. The check took an hour and confirmed the method.

Your capstone deliverable

• A statement of units, measurement conventions and waste factors.
• Detailed takeoff for the structure and envelope, with sketches or marked-up drawings showing what was measured.
• Key quantities for systems and finishes that subtrades will price, so their quotes can be checked.
• A summary sheet listing each major quantity, its unit and its source drawing.
• Ratio checks against benchmarks, and a record of an independent check of at least one part of the takeoff.