Wood framing and structural steel takeoff

Wood and steel frame most low-rise buildings and many larger ones. Their takeoffs look different, because wood framing is built from many small pieces cut and assembled on site while structural steel arrives as fabricated members bolted together, but both start from the same principle: measure each member from the drawings, organize the members by size and type, and convert the counts and lengths into the units in which they are bought. This lesson covers light wood framing and sheathing, and the tonnage takeoff of structural steel.

Light wood framing

Light wood framing uses dimension lumber for studs, plates, joists and rafters, with structural panels for sheathing and subfloors, and engineered products such as I-joists, laminated veneer lumber and trusses for longer spans. The takeoff separates walls, floors and roofs, and within each, the members by size, grade and length, because lumber is bought by the piece in standard lengths or by volume in board feet.

In Canada, lumber is dimensioned by its nominal imperial size, such as 2 × 4 or 2 × 6, but drawings often give actual metric sizes: a 2 × 4 is 38 mm × 89 mm and a 2 × 6 is 38 mm × 140 mm. Lengths are sold in 2-foot increments, commonly 8, 10, 12, 14 and 16 ft, which are 2.44, 3.05, 3.66, 4.27 and 4.88 m. Stud spacing on metric drawings is usually 400 mm or 600 mm, close to the traditional 16 in and 24 in, and sheathing panels are 1220 mm × 2440 mm. Prices are often quoted per piece or per thousand board feet (MFBM).

A board foot is the volume of a piece of lumber 1 inch thick, 12 inches wide and 1 foot long, using nominal dimensions. It is the traditional unit for pricing lumber in both countries.

Wall framing takeoff

Wall framing is taken off wall by wall, or by linear length of each wall type. For each wall the estimator counts the studs, the plates and the extra members at corners, intersections and openings.

• Studs: the wall length divided by the spacing, plus one, plus extra studs at corners, intersections and openings.
• Plates: usually one bottom plate and two top plates, so three times the wall length, converted to stock lengths.
• Openings: each window and door adds king studs and jack (trimmer) studs, a header sized from the header schedule, and cripples above and below.
• Blocking and backing: fire blocking, backing for cabinets, grab bars and fixtures, measured by length or allowed as a percentage.
• Sheathing: wall area divided by panel area, with waste for openings and cutting.

Floors, roofs and sheathing

Floor framing is taken off by counting joists at their spacing across each bay, with rim joists, blocking or bridging, and beams. Engineered I-joists and trusses are usually bought from a supplier who designs and prices them from the drawings, so the estimator's takeoff becomes a check on the supplier's quote and a count for installation labour. Roof framing with rafters needs the rafter length from the span and pitch, as in CON-130, plus ridge boards, collar ties and blocking.

Sheathing and subfloor are measured by area and divided by the panel area. Waste varies with the layout: a simple rectangular roof may waste 5 %, while a cut-up roof with hips, valleys and dormers may waste 15 % or more. Walls with many openings waste more than plain walls, even though the openings are deducted, because pieces cut out of openings are rarely reusable.

Framing labour is commonly priced per unit of floor area for houses and small buildings, or by labour-hours per member or per board foot for larger work. Panelized and prefabricated wall systems shift labour from the site to a factory and change both the takeoff and the pricing: the estimator counts panels and prices them as a supply item, with a smaller site installation crew.

Mass timber, such as cross-laminated timber panels and glued-laminated beams and columns, is a growing alternative to light framing and steel for mid-rise buildings. It behaves more like structural steel than like light framing for estimating purposes: members are fabricated off site from the supplier's shop drawings, delivered in an erection sequence and lifted into place by crane, so the takeoff counts panels and members by size and volume to check the supplier's package and to plan crane time, connections, moisture protection during construction and any encapsulation required by the building code.

Structural steel

Structural steel is bought and priced by mass or weight: tonnes in Canada, tons of 2,000 lb in the United States. The takeoff lists each member by size and length from the structural drawings, multiplies the total length of each size by its mass or weight per unit length, adds an allowance for connections, and totals the tonnage. Members are grouped into beams, columns, bracing and miscellaneous, because each has a different fabrication and erection cost per tonne or ton.

Canadian wide-flange shapes are designated by nominal depth in millimetres and mass in kilograms per metre, such as W310×39, which is about 310 mm deep and weighs 39 kg per metre. Hollow structural sections are designated by outside dimensions and wall thickness, such as HSS152×152×6.4. Steel design follows CSA S16, and the Canadian Institute of Steel Construction publishes the handbook of member properties.

Connections, the plates, angles, bolts and welds that join members, add mass that does not appear in a simple member list. Estimators add a percentage for connections, commonly 5 to 15 % of member mass depending on the framing type, with moment frames and heavily braced frames at the high end. A detailed estimate may take off connections individually from typical details. The fabricator's own estimate and later its shop drawings are the definitive quantity.

Steel is usually bought as a subcontract from a fabricator, often with erection by the fabricator or a separate erector, priced as a lump sum or per tonne or ton. The general contractor's takeoff serves to check the subcontract prices, to level them against each other and to price changes. Items that are often missed or disputed include steel deck, which is measured by area and may be a separate subcontract; miscellaneous metals such as lintels, embedded plates, stairs and railings, which belong to Division 05 50 00 rather than structural steel; shop and field painting or fireproofing; and anchor bolts, which are usually supplied by the steel fabricator but set by the concrete contractor.

• Steel deck: measured by area of floor or roof, by gauge and profile, plus closures and edge forms.
• Miscellaneous metals: lintels, stairs, ladders, railings, embedded plates, counted or weighed by item.
• Coatings: primer, finish paint, galvanizing or fireproofing, measured by tonnage or surface area.
• Erection: crane time, erection crew, bolting and welding, plumbing and safety equipment, often priced per piece rather than per tonne or ton, since light members take nearly as long to erect as heavy ones.

Checking framing and steel quantities

Ratio checks work for framing and steel as they do for concrete. Board feet of framing per unit of floor area, sheathing area per unit of wall or roof area, and steel mass or weight per unit of floor area are well known for common building types. A one-storey steel-framed retail building with a steel deck roof might use roughly 30 to 50 kg of structural steel per square metre of floor area, or roughly 6 to 10 lb per square foot, while a multistorey office with moment frames uses much more. A takeoff that falls far outside the expected range should be rechecked before pricing.