Taking On a Cat A Shell and Scoping the Cat B

Three Grids, and Whether They Agree

A fitted office sits on three grids drawn by three different people. The planning grid comes off the facade — 1.5 m between mullion centres in most UK base builds, 5 ft in North American practice — and partitions have to land on it, because a partition meeting glass anywhere other than a mullion needs a bespoke junction and generally leaks both light and sound. The ceiling module is 600 mm or 24 in. The access floor panel module is nominally the same figure, which is precisely why everyone assumes the two coincide.

They often do not. The ceiling was set out by the ceiling subcontractor from a datum that suited the ceiling, usually the facade, which puts the border tile at the core. The access floor was set out by a different subcontractor working from the core, which puts the border panel at the facade. Both are correct in isolation. Together they mean the tile joints and the panel joints run parallel with a permanent offset between them, so a partition line sitting cleanly on a tile joint sits somewhere in the middle of a floor panel, and the reverse.

That offset is charged three times. At the floor, every partition line becomes a cut line through panels needing edge support along it rather than at the module. At the ceiling, the head detail lands mid-tile and the tile is cut both sides of it, which is the junction people look straight at walking into a meeting room. At the glass, a door set dimensioned to the planning grid stops fitting between two mullions once the partition has been nudged to catch a panel joint.

Half a day with a tape settles it, and it is the most valuable half day in the whole exercise. Measure the border tile at all four walls, then the border panel at all four walls, then the mullion centres along both glazed elevations, and write the three sets of numbers on the survey drawing as offsets from one agreed corner. If the borders are equal and both fall a whole number of modules off the mullion line, the partitions can be set out once. If not, you are choosing which grid they follow and budgeting for the other two to be cut.

Count What You Were Handed Before Anyone Lifts a Panel

The inherited count is one set of numbers doing three jobs, which is the argument for spending a day on it. As a reinstatement liability it is the schedule you may have to restore at lease end. As a disturbance count it tells the ceiling contractor how many tiles come down and go back, which is a labour figure and not a materials one. As a waste stream it is the skip count and the line in a sustainability report saying how much of a six-week-old ceiling went into a grinder.

Tile is where the count and the batch matter together. Mineral fibre ages, and a tile pulled from a store two years later reads as a different colour under the same luminaire even carrying the same product code. Establish what attic stock came with the demise, where it is, whether it is contractually yours and which batch it is from — a floor with no attic stock and a reinstatement obligation is a floor where the whole field may have to be replaced to match. Several ceiling manufacturers run take-back schemes for mineral fibre tile, and a strip-out is the moment to use one, but they want clean, dry, unpainted product and a count agreed in advance.

The survey also has to become evidence, not just a spreadsheet. Photograph the floor before anybody touches it, with the date embedded, and get the schedule of condition agreed with the landlord's surveyor rather than filed unilaterally. Six years later the argument is not about what you did; it is about what was there before you arrived, and the party holding a dated, agreed record wins it cheaply.

1. Walk the plate with the reflected ceiling plan and mark every tile that is not a plain tile: luminaires, diffusers, return grilles, sprinkler heads, detectors, speakers, access hatches, sensors.
2. Count the field by zone rather than by floor plate, because the zones are what you will later disturb, reinstate and hand back separately.
3. Lift a panel in four widely separated places and record the pedestal type, the void depth, whether stringers are present, and what already runs through the void.
4. Record the ceiling void depth at the worst point rather than the typical one — usually under a duct crossing or a sprinkler main — because that decides whether a partition can reach the soffit at all.
5. Take the access floor grade and panel reference from the operation and maintenance manual rather than from the look of the panel, and check it against the heaviest thing your layout puts on it.
6. Photograph every riser, every landlord's board inside the demise, the label on every isolator, and the meter positions that establish which supply is measuring your demise.
7. Agree the whole record with the landlord's surveyor and date it before the first panel is lifted.

One Perimeter Becomes Fifteen

An open floor plate has one ceiling perimeter. A 30 by 24 m plate has 108 m of it, and the landlord bought that trim once. Put fourteen meeting rooms of roughly 4 by 3.6 m into the same plate and each brings its own closed perimeter of about 15 m, so the layout has added over 200 m of ceiling perimeter to a floor that had 108 — and the open area left over still needs its own edge. Perimeter trim moves more violently than any other line between an open plan and a cellular layout, and it is the one most often carried forward from a previous job as a rate per square metre of floor.

The piece count moves faster than the metreage, because trim is bought in stock lengths and a closed perimeter breaks into as many runs as it has corners. Fourteen rectangular rooms are fifty-six internal returns, none of which can be made from less than one length however short the wall is. Add the external returns that columns, risers and recesses generate and the offcut pile on a cellular fit-out is a genuine line rather than a rounding.

That line is never only trim, which is why pricing it as wall angle understates it. Whatever runs along the head of each new partition is at once a ceiling edge, a head track, an acoustic seal, a movement allowance and, on any wall with a fire function, a rated junction — and choosing between an exposed angle, a shadow gap and a proper deflection head changes all five together. Price the metre with everything that follows it along the wall, and take the corner count off the layout rather than from a factor.

The Grid Was Hung for an Empty Floor

The landlord's suspension was counted for a continuous open ceiling, and a partition line crossing it breaks that assumption. Wherever a main tee is cut to let a partition head through, or a run is removed to form a bulkhead, the cut end is unsupported and needs a wire inside whatever end distance the governing document allows — the ASTM C636 family with ASTM E580 layered on in seismic regions, or the BS EN 13964 family in European practice. Tying, anchorage and end distances are a trade in their own right, and this site treats them in the suspended ceiling guide rather than here.

The bigger decision is whether the partitions stop at the tile or continue to the soffit, and it is an acoustic question before it is a cost one. A partition that dies at the ceiling relies on the tile and the void to stop sound crossing above it, and that performance has a measured value — Ceiling Attenuation Class under ASTM E1414 in North American practice, and the flanking transmission methods of the BS EN ISO 10848 series in Europe. Cat A tile is very often selected for absorption and appearance, which are different properties, so a meeting room specified for confidentiality and built to the underside of a base-build tile can measure poorly while looking exactly like the drawing. Taking the partition to the soffit fixes it and brings its own bill: grid removed and re-hung, cavity barriers where the wall has a fire function, and every service crossing the line needing a seal.

Count the added wires per room rather than across the plate. That is the honest unit anyway — the room is where the grid gets cut, where a bulkhead lands and where somebody later hangs a screen — and it keeps each number small enough to check. Then add for what clean grid arithmetic never sees: wires at every cut end, at every new opening, and wherever a duct or sprinkler main already occupies the plumb line the wire wanted.

The wires already up there belong to the landlord's contractor and terminate on the landlord's anchors, and a Cat B fitter tying a new main to an existing one is the classic defect on this kind of job, particularly when the existing wire turns out to be carrying somebody's duct. Grid you take down is worth keeping: tee sections are the cheapest reinstatement material you will ever own, and the store costs nothing while the floor is empty.

Lighting and Power on Somebody Else's Board

The Cat A lighting is a uniform field on the ceiling module, switched or dimmed in zones the size of the open floor it was designed for. Your cellular layout wants a room to be controlled as a room, which is a controls problem rather than a photometric one; the illuminance and glare side of an office lighting design is dealt with in the workspace lighting guide on this site. What matters at scoping stage is who holds the commissioning software, whether the system is addressable at all, and whether reprogramming it is something your contractor may do or something the landlord's framework supplier must be paid to do.

Escape lighting is where a cellular layout does the most damage quietly. The landlord's scheme was designed for an open plate with a defined route to each core, and partitions create rooms, corridors and dead ends that did not exist when it was drawn. BS EN 1838 sets the minima the revised scheme must still meet — 1 lux along the centre line of an escape route, 0.5 lux across the core of an open area, the open-area provision applying above the standard's 60 square metre threshold — with BS 5266-1 covering design, testing and records, and NFPA 101 reaching the same place in North American work. Whoever signs it off needs the layout, not the lighting schedule.

Then there is the board itself. Whatever sits inside the demise usually has some spare ways, and how many decides whether your feature lighting, tea point, comms room and workstation power are an addition to an existing installation or the reason a new board and a new submain appear in the cost plan. Get the schedule, count the spare ways, and confirm what may be worked on and when — a board in a landlord's riser typically means a permit, an out-of-hours isolation window and a witnessed re-energisation, all of which are programme rather than cost until somebody forgets them.

Do the load arithmetic at scoping stage rather than design stage, because it is what tells you whether the answer is ways or a board. Take driver input wattage from the product data rather than the nominal lamp figure, add your fittings to the landlord's on each existing way, and check the total against the way the local wiring rules load a circuit — the eighty per cent continuous-load rule under NFPA 70 in the United States, and design current against the protective device and cable rating, with diversity applied honestly, under BS 7671 in the UK. Where it comes out marginal, it is a board, and a board has a lead time.

The Rate the Board Will Approve

Eventually somebody asks what the fit-out costs per square foot, and the answer is worthless unless two things travel with it. The first is which area it was divided by: a lease quotes net internal area, a modern measurement instruction produces IPMS 3 for offices under RICS Property Measurement, and the two are not the same number for the same floor. Name the area on the same line as the rate, every time, or two floors get compared on a difference that is pure measurement.

The second is what the numerator held. A rate quoted at a pitch is usually construction only, and what sits outside it is what most reliably arrives: furniture, audiovisual, the IT active kit, professional and project management fees, the landlord's consent costs, the reinstatement provision, contingency and tax. A cost plan structured on the RICS New Rules of Measurement keeps those as separate lines rather than folding them into one figure, which is what makes the eventual variance readable.

Handle the landlord's contribution as its own line rather than by netting it off the rate. A contribution, a rent-free period taken in lieu of works, and a capital payment on completion are three different things to a finance function, and only one of them is cash at the time you are spending it. A rate that has already had an incentive deducted from it cannot be compared with anything and cannot be benchmarked against the next floor.

Then use the rate for what it is good for, which is a sanity check and not a budget. Against a published benchmark it tells you whether your specification is where you think it is; it cannot tell you what this floor should cost, because the benchmark carries another building's grid, another building's Cat A and another market's labour. A rate sitting well under a benchmark is rarely efficiency. It is usually scope that has not been described yet, most often the ceiling and floor disturbance the delta take-off exists to find.