Taking Off Board for Shafts, Soffits and One-Sided Walls

A bulkhead is a girth, not an area

Everything a bulkhead needs comes off two drawings that are rarely open at the same time. The run lengths are on the reflected ceiling plan, where the boxing shows as a band with a note. The drop and the underside width are on the section or on a ceiling detail, and they change more often than anyone expects — a soffit chasing a duct that reduces on its way across a floor plate steps down twice, and each step is a separate entry with its own dimensions rather than an average that splits the difference.

Which faces you count is the decision that goes wrong most often, and it is binary. An island bulkhead standing clear of the walls shows two vertical sides and an underside: three faces, and a girth of two drops plus the width. A bulkhead running tight against a wall shows one side and an underside, so its girth is one drop plus the width — near enough half the material for the same length of run. The top of either is closed against the structure it hangs from and takes no board, which is the one simplification in this whole page that is safe to make. Where a run ends in the open rather than dying into a wall, that returned end is a further small face of drop by width, negligible as area and thoroughly disproportionate as labour.

There is a coincidence in these numbers worth knowing about because it doubles as a sanity check. A 300 mm drop over a 400 mm underside gives a girth of exactly one metre, so on an island bulkhead of those proportions the run length in metres and the board area in square metres are the same number. That makes the total easy to check at a glance and easy to under-respect at the same glance: 24 m of bulkhead reads as 24 m² of board, which is less board than a single decent partition, and it will absorb several times that partition's hours before it is finished.

1. Take each run separately off the reflected ceiling plan — a soffit that changes direction is two runs, and a soffit that steps is two runs with different sections.
2. Read the drop and the underside width off the detail for that run, not off the first detail on the sheet.
3. Decide the face count: two sides for an island bulkhead, one for a run held against a wall.
4. Add a return face wherever the boxing stops in the open instead of meeting a wall.
5. Count the external arrises as their own quantity — two per metre of run on an island, one on a wall-side run, plus a vertical pair at every end and every change of direction.
6. Mark any run that crosses a rated wall or encloses a rated duct, because the girth stays the same and the build-up on it does not.

Two layers on one face weighs the same as one layer on two

Here is why the riser error survives so much review. Take a shaft 2.4 m square, so 9.6 m round, at a 3.0 m storey height: 28.8 m² of face. The two-sided model returns 57.6 m². A one-hour or two-hour listed design that puts two layers of Type X on the single finished face also comes to 57.6 m² of board. The wrong method and the right method have produced the same square metrage, and a checker comparing totals sees agreement. Change the design to one layer, or to three, and the agreement evaporates — but it evaporates quietly, in a direction nobody is looking, because the last job it was checked against happened to be a two-layer wall.

The board total is only ever half the item anyway. What the two-sided model is really claiming is that the shaft side gets sheet, and it does not: it gets a 25 mm (1 in) liner panel, typically 610 mm (24 in) wide, held in the webs of the studs and installed from the corridor. That is a separate product on a separate delivery, counted along the run instead of over the area, and 9.6 m of perimeter at 610 mm centres is sixteen panels per floor before any allowance for openings or breakage. A takeoff that has doubled the sheet quantity and omitted the liner is not one error cancelling another; it is over-ordering one item, missing a second, and pricing a rate against neither.

The layer count itself comes from the listed design and from nowhere else. A fire-resistance rating is the result of a full-scale test to ASTM E119 or UL 263 on one specific assembly, and the listing — in a UL certification directory, or in the Gypsum Association's GA-600 Fire Resistance and Sound Control Design Manual — names the stud, the liner, the board type and thickness, the number of layers and the fastener schedule together. Two designs with the same hourly rating routinely differ in layer count because they differ everywhere else. Reading a layer count off the rating is a guess, and it is a guess that lands squarely on the largest quantity in the package.

Deductions need a colder eye on a shaft than on a partition. Riser enclosures are punched with access panels and louvred doors, and the instinct is to deduct all of them. Deduct only the openings big enough that what comes out of the sheet is a piece somebody will actually use. A run of small penetrations does not save board — it generates offcuts and consumes labour, and deducting their area makes the order short at exactly the moment the wall is being closed up. Where the enclosure crosses a compartment floor, the fire-stopping at the slab and the head detail are their own quantities and belong nowhere near the board line.

The stud does not know where the floors are

The elevation says the storey height is 3.0 m, so the wall is 3.0 m, so the stud is a 3.0 m stud and the framing rate is the framing rate. On a partition between two rooms that chain holds, because both ends of the stud are in track against structure and the floor slab is continuous through the line of the wall. On a shaft it can be nonsense. There is no floor inside a lift shaft. A smoke shaft is a clear vertical route by definition. A services riser with a grated or partial floor at each level gives you nothing to brace against even where a slab edge is drawn on the plan. The dimension the framing has to satisfy is the distance between real lateral supports, and on a shaft that can be two, three or a dozen storeys.

The second dimension is the pressure the enclosure is designed against, and it is not a property of the wall. The International Building Code sets a minimum lateral load for interior walls and partitions in its structural design chapter — 5 psf, 0.24 kPa — and shaft enclosures are routinely specified well above it. The figure originates with the lift supplier, who knows what piston effect a car of that speed and size generates in a hoistway of those dimensions, or with the smoke control design, which imposes a pressure difference on purpose. Where the shaft is on the building perimeter it has to be checked against wind as well, at whichever governs. None of these are numbers to assume; all of them are numbers that exist somewhere in the design team's documents before the drywall package is priced.

The deflection limit is the third input and it tracks the finish rather than the fire rating. A shaft face left as liner and a corridor face taking paint are commonly held to L/240; a corridor face carrying tile, stone or a rigid coating goes tighter, because those finishes crack while the framing is still perfectly comfortable. The specification says which, and the difference between L/240 and L/360 on the same stud is a substantial slice of allowable height.

Put those three together and what falls out is a stud — a depth and a gauge — and that is the estimating consequence. Framing is priced per square metre against an assumed member, and a package priced on a light gauge that turns out to need a heavier one is not a small adjustment. It is a rate change on the largest linear quantity in the enclosure, discovered by the framing foreman after award, at the point where the argument is about who should have known. Non-structural framing members are covered by ASTM C645 and installed to ASTM C754, with AISI S220 the North American standard for cold-formed steel framing non-structural members; the section properties that decide height come from the manufacturer's own tables, because the effective stiffness of a thin cold-formed section is not derivable from its outside dimensions.

Treat the height check as a screening test on the takeoff rather than as design. It tells you whether the wall your rate assumes is anywhere near the wall the drawing needs, and which enclosures to raise as a question before the price goes in. The manufacturer's tabulated limiting height is the lesser of a deflection check and a bending strength check, and it is tied to a tested assembly with its own track, fastening and liner. Where the table is lower than a deflection calculation, strength is governing and the table is the answer.