Taking Off a Whole House: Which Length Each Trade Is Bought From

One Plan, Four Different Lengths of Wall

Ask four trades how long the walls of a 12 × 9 m (39 × 30 ft) house are and you can get four answers, all of them right. The framer measures along the centre of the wall, because studs stand on the plate's centre line. The sheathing and cladding crew measure the outside face, which runs past the centre line by half a wall at every outside corner. The insulation and drywall crews measure the inside face, which stops short by the same half wall. And whoever buys the block, the concrete or the bricks of a core wants the core length with each corner counted once, so the corner square is not bought twice.

On a rectangle of 200 mm (8 in) walls the outside faces are 0.8 m (about 2 ft 7 in) longer in total than the centre lines and the inside faces 0.8 m shorter. That is a few sheets and a few packs, small on one house and large across a year of houses, and it is exactly the kind of error that is never noticed because every line looks plausible. The habit that prevents it is to write down, for every line of the takeoff, which of the four lengths it was taken from.

Interior partitions add their own wrinkle. Where a partition meets an outside wall it interrupts that wall's inside face for its own thickness, so the inside face is shorter again, and the partition itself stops at the outside wall's inside face rather than its centre line. Each partition has two faces to line, and both of them are bought.

Where Rounding Is Allowed to Happen

Every calculator rounds up to something you can buy: whole studs, whole sheets, whole packs, whole rolls. Round too early and too often and the order swells; round too late and a long wall breaks a calculator's own limits. The working rule has two halves.

Pieces counted along a run are rounded wall by wall. A stud count along a 12 m wall is one answer and along a 9 m wall another; adding the lengths and counting once would lose the stud at each wall end. Calculators that count along a run often state a ceiling on what one run may be, and a whole house entered as one wall can exceed it.

Material bought by area is rounded once for everything that shares a build-up, a height and a storey. Sheathing for four walls of the same type is one order of sheets, not four orders each rounded up. When the lines of a project are later merged into a bill of materials by product and unit, pack-size rounding is applied once more at the end, and that is the last rounding the order should see.

1. Count run-based pieces (studs, posts, joints along a wall) one straight wall at a time.
2. Total the area-based lines across walls of the same build-up, height and storey before rounding.
3. Merge identical products at the end and round to the pack once.
4. Add any contingency on the merged bill, never on top of a calculator's own waste on each line.

Waste Belongs in One Place

Most quantity calculators already include a waste or cutting allowance in their answer, set to a figure appropriate to the material: modest for vinyl siding, higher for brittle fibre-cement, a stated lap for membranes. Adding a house-wide percentage on top of those figures counts the same waste twice. If a particular site warrants more, change that line's own allowance where the calculator offers one, so the working still shows where the extra came from.

A separate contingency for the unknown is legitimate, but it belongs on the merged bill as a single, visible line, and it should be zero by default when every line already carries its own allowance.

The Ground and the Roof Are Taken From the Footprint

Below and above the walls, the measurements change again. A strip footing runs under the centre line of the outside walls, so its length is the centre-line perimeter; its width and depth come from the design and the ground. The backfill is the trench above the footing less the wall standing in it, a different volume from the dig and a different one again from the spoil once it is bulked. A slab on grade is its plan area by its thickness, with edge formwork from its perimeter.

A gable roof is measured from the drip edge, not the wall: the footprint plus the eave overhang at each eave and the rake overhang at each gable end. The slope adds area by a factor that depends only on the pitch, about 1.12 at 6 in 12 and 1.41 at 12 in 12, and the ridge runs drip edge to drip edge. Where winters freeze, the ice-dam membrane at the eaves comes off the underlayment area rather than being added to it.

What a Takeoff Never Decides

A takeoff counts what a design asks for; it does not make the design safe. Stud size and spacing for a wall's height and load, the depth and width of a footing, the thickness and reinforcement of a slab, the size of every header and beam: these are taken as given and should come from the framing plan, the structural drawings and the building official. When the estimate moves one of them to save material, it has made a structural decision that belongs to somebody else.

Equally, a takeoff is silent where the site has no calculator yet. Floor and roof framing members counted from a span, plates and headers as a schedule, and service runs measured along a route are all real quantities that a complete house needs, and an honest estimate lists them as not yet counted rather than leaving them out without a word.