Taking Off Quantities From a Drawing Set: Sheet by Sheet to a Merchant Order

The plan is a set of rectangles that nobody drew as rectangles

Areas come first because so much else hangs off them, and the method is decomposition: cut the shape into rectangles and right-angled triangles along lines that already exist on the drawing, run each piece, and keep the pieces. Keeping the pieces is the part people skip and the part that pays. A total is a dead number — when the client moves the bifold two hundred millimetres you re-measure everything. A list of eleven pieces is a live document, and that same change touches one line of it.

Then decide, once and explicitly, which dimension line each area is measured to, because a room has at least three and they are all different. There is the structural line the plan is drawn to, the internal face of the blockwork, and the finished face after board and skim or after plaster. Screed follows one of them, floor covering follows another, and on a modest room the two can differ by thirty millimetres a side — nothing at all on a bag count, decisive on a tiled floor that has to set out symmetrically. That floor area is a defined term rather than an obvious one is worth taking seriously: the RICS Code of Measuring Practice and the International Property Measurement Standards define it one way, ANSI/BOMA Z65.1 another, and ASTM E1836 another again, and they produce different numbers from the same building on purpose.

Deductions are the last decision and the one most often made by reflex. If you are measuring to price against a contract, the method of measurement named in that contract also names the threshold below which a void is not deducted — RICS NRM 2 does it by work section, CESMM4 does it differently for civils work, and the older SMM7 differently again. If you are measuring to buy, the rule is physical rather than contractual: deduct an opening only where you cannot use the offcut it creates. A window hole in a plastered wall is a genuine deduction because the offcut is dust. The same hole in a brick elevation deducts far less than its area suggests, because the bricks that would have filled it are still in the pack.

The site plan is a different animal at a different scale

Everything outside the building line lives on one sheet drawn at a scale where a millimetre on the page stands for a fifth of a metre on the ground, and it carries quantities that dwarf the ones inside: topsoil strip, hardstanding, sub-base, membrane, turf, fencing run, drainage. The trap on this sheet is the unit rather than the measurement. Groundworks and landscape subcontractors quote by the hectare or the acre, the architect dimensions in metres, and the merchant sells geotextile by the roll in square metres — so one measured figure has to leave your sheet in three units without being re-typed into anything, because re-typing an area at half past four is how a factor of ten enters a job. Reconciling the areas a title deed, a mapping extract and a valuation give for the same parcel is a job of its own, and it has its own page here.

Two traps sit on this sheet. The red line boundary is a legal extent rather than a working area — it takes in ground you may not touch and ground an easement forbids you to load — so the strip area and the fence run come off the proposed works, not off the line. And plan area is not surface area on anything that slopes: ground on a slope is longer along its face than on plan, so turf area and topsoil volume both run over, and the spot levels and contours printed in small type are the only place on the set where that third dimension is recorded.

The foundation plan and the section make one volume between them

Volume is always two drawings. The plan gives you a footprint or a run, the section gives you a depth, and neither is a quantity on its own — which is why the concrete line is the one most often taken off a single sheet and most often wrong. On a raft or a slab the plan gives the area and the section gives the thickness plus every thickening and downstand hiding under it. On a strip footing the plan gives a run, and the run to take is the centre line rather than the external perimeter, for a reason that is worth deriving once so it stops being a rule to remember. Measure a rectangular footing around the outside and every corner gets counted twice, once from each direction. The centre-line length of a footing of width w around a rectangle is the external perimeter less four times w, exactly, and using that figure the corners look after themselves.

Then accept that the hole is not the drawing. Ground over-breaks where it is soft, machine-dug trenches are trimmed to a tolerance rather than to a line, and anything deeper than a shallow trench has to be battered back or supported before a person may enter it — OSHA's excavation rules in 29 CFR 1926 Subpart P set out the requirement in American practice and the equivalent obligation exists everywhere else. Every one of those makes the void larger than the section shows, and concrete fills the void rather than the drawing. ACI 117 is where the tolerances on formed and excavated surfaces are actually written down, and it is a better guide to how much extra to expect than a percentage carried over from the last job in different ground.

The order carries a specification alongside the volume, and the specification comes off the engineer's notes rather than off any dimension. Strength class, exposure class, consistence, maximum aggregate size and any requirement for sulfate resistance are all named there; BS 8500-1 and BS 8500-2 with BS EN 206 carry the British convention, ASTM C94/C94M with ACI 301 the American one, and ASTM C94/C94M is also where the way a delivered load's volume is determined and toleranced is set down. Ordering the right quantity of the wrong mix is by a wide margin the more expensive of the two available mistakes, because the wrong volume is a phone call and the wrong mix is a breakout.

Finally, decide the rounding deliberately. Ready-mixed concrete arrives by the load, and bringing a wagon back for the fraction of a cubic metre at the tail of a pour costs more than it can possibly be worth. Whole loads with a small bagged quantity held on site for the last of it is usually the cheaper arrangement, and that decision wants making before you ring the plant rather than after.

1. Take the trench run on the centre line of the footing, not around the outside, and check it against the external perimeter less four widths.
2. Read every depth off the section and the level notes, never off the plan, and treat a stepped footing as separate lengths at separate depths rather than as an average.
3. Add the thickenings, downstands, pile caps and pad bases as their own lines, because they are the volume the slab area does not contain.
4. Allow for over-break and trimming separately from any waste allowance; it is a property of the ground, not of your handling.
5. Copy the strength class, exposure class, consistence and aggregate size off the engineer's notes onto the order line before touching the volume.
6. Convert to whole loads plus a bagged tail, and confirm the access and the pour rate will support the number of loads you have just committed to.

The elevations are where the brickwork finally becomes a number

Masonry is the one quantity the plan cannot give you, because the plan shows a wall as a line of a certain thickness and says nothing at all about how tall it is. The elevations show the height, but here is the awkward part: elevations are frequently the least dimensioned sheet in the set. Take the horizontal runs off the plan where they are figured, take the vertical dimensions off the section where the levels are — top of foundation, damp-proof course, floor levels, head heights, eaves, verge — and use the elevation for the one thing only it can tell you, which is what interrupts the brickwork and where. Measure gross first, list the interruptions second, and settle the deduction question afterwards rather than in your head as you go.

The number of bricks in a square metre is a property of the coordinating size, which is the brick plus one joint, and not of the brick you were shown a sample of. A UK standard format unit at 215 by 102.5 by 65 millimetres laid with a 10 millimetre joint has a coordinating face of 225 by 75, so a half-brick wall takes one divided by 0.225 times 0.075, which is 59.3 and is conventionally called 60 to the square metre. An American modular brick with a 3/8 inch joint has a coordinating face of 8 inches by 2 2/3, which is 21.33 square inches, so 144 divided by that is 6.75 to the square foot — the figure the Brick Industry Association publishes in Technical Note 10. Both numbers move the moment the joint or the unit changes, and a 73 millimetre high brick or a queen size is a different arithmetic entirely. The count on the last job is a fact about the last job.

Two further things double quantities without looking like they do. Wall thickness is one: a cavity wall's outer leaf is what the elevation draws, while the inner leaf is a block on the plan bought by a different unit from a different supplier, and both of them sit on the same line of that elevation. Bond is the other: a one-brick solid wall is two leaves' worth of facework, and anything laid on edge or on end — soldier courses, plinths, cills, brick-on-edge copings, gauged arches — is counted as an item off the elevation rather than measured as area, because the unit rate and the wastage on a special have nothing to do with the field. Mortar is a separate order line in its own unit, governed by BS EN 998-2 or ASTM C270 depending on where you are standing, and it is the line most often forgotten by somebody who has just spent an hour counting bricks.

Partition types, and the wall that is thicker than its line

The internal plan almost always carries a partition type key, and the types are not interchangeable: a plain stud partition, a party or separating wall, an acoustic partition around a bathroom and a fire-rated enclosure look identical at 1:50 and buy completely different materials. Measure run by type rather than measuring internal walls as one figure, take the heights off the section from floor to underside of ceiling rather than floor to floor, and note whether the ceilings are in the same package, because a ceiling is an area from a different sheet arriving on the same lorry. Enclosures with only one accessible face — risers, shafts, bulkheads and soffits — are measured on a model of their own, and that is a different article on this site rather than a footnote here.

There is also a quiet consequence for everything measured earlier. The plan is drawn to a structural line; the room you will actually stand in is smaller than that by the linings on both faces, which for board and skim on each side of a stud is roughly thirty millimetres off each dimension. On the board quantity that is noise. On a floor covering bought to a measured size, on a fitted kitchen run, and on any tiled setting-out it is the whole problem, and it is why the floor area and the board area for the same room are legitimately two different numbers rather than one number entered twice. The application standards — ASTM C840 and Gypsum Association GA-216 in North America, BS EN 520 and BS 8000-8 in the UK — are also where the board thickness and layer count for a rated or acoustic type are pinned down, and the type key on the plan is usually pointing at one of them.

Net measured is not net ordered

Four different things sit between the quantity you measured and the quantity you buy, and collapsing them into a single percentage is how a job runs short of one product on the Friday while storing a pallet of another for two years. There is cut and offcut loss, which is pure geometry: your run against the module the product is made in. There is breakage and site damage, which is about handling, storage and how many times a pack gets moved. There is pack and stock-length rounding, which belongs to the merchant and is not negotiable. And there is contingency for the part of the drawing that turns out to be wrong, which is not waste at all but risk, and which has no business being hidden inside a waste figure where nobody can see it.

Cut loss is the one people guess at and the one that is entirely computable. A 3.55 metre run taken out of 4.8 metre stock uses one length and throws away 1.25 metres — twenty-six per cent on that line, unimprovable by care, because the loss lives in the ratio and not in the saw. The same run out of 3.6 metre stock wastes one per cent. That is a buying decision worth several hundred pounds on a full house of skirting, and it is only visible if the takeoff kept the runs as individual lines instead of adding them into a total first. Sheet goods behave the same way: a wall that is not a whole number of sheets high produces one offcut per sheet, and whether that offcut is usable depends on the wall next to it rather than on the wall you are measuring.

So round up line by line, at the point the pack or the stock ladder forces it, and only then work out what it costs. Rounding the job total is a different and wrong calculation, because a surplus pack of one product covers a shortfall of nothing. The allowance itself deserves a number built from your own last few jobs of the same kind, recorded next to the quantity it applies to so the next takeoff starts from evidence rather than from a rule of thumb.