Deduction rules for quantity take-off of ductwork fabrication and installation: the developed area deducts no area of inspection holes, measuring holes, supply outlets or extract inlets; duct length follows the centreline shown on the design (elbows, tees and other fittings included) minus the length occupied by dampers and other components, which are listed and measured separately under code 030703 (conventions of GB/T 50856-2024 and local consumption standards)

Topic Positioning

The core deduction rules of ductwork (ventilation duct) fabrication and installation quantities in HVAC works pricing: does the developed area deduct the openings of inspection holes and supply outlets? Does the duct length deduct the length occupied by dampers and other components? This card consolidates the conventions of the national BOQ measurement standard GB/T 50856-2024 and local consumption standards (DB11/T 2382-2024, SJG 111.3-2022), for use in BOQ preparation, quota pricing and quantity checking.

Core Conclusions

Overall rule: no deduction of openings from the area, no component length in the length - the developed area of ducts deducts no area of inspection holes, measuring holes, supply outlets or extract inlets; the duct length excludes (deducts) the length occupied by dampers and other components, which are listed and measured separately by number.

1. National BOQ measurement (GB/T 50856-2024 Annex G, HVAC works) Table G.2.1 fabrication and installation of ventilation ducts (code 030702), explanatory notes and measurement rules:

• The developed area of ducts deducts no area of inspection holes, measuring holes, supply outlets or extract inlets; duct length follows the centreline length shown on the design (main and branch ducts divided at the intersection of their centrelines), including the length of elbows, tees, reducers and round-to-rectangular transitions but excluding the length occupied by components. The developed area of ducts excludes overlapping areas of ducts and duct mouths.
• Tapering ducts: circular ducts by average diameter; rectangular ducts by average perimeter.
• Wall-penetrating sleeves are computed by developed area and included in the corresponding ductwork quantities per material and thickness requirements.
• Factory-finished ducts are listed under the corresponding ductwork codes; stainless steel smoke-exhaust ducts under the corresponding stainless steel sheet duct codes.
• The ductwork items include fabrication and installation of supports and hangers; where supports and hangers are to be listed separately, the corresponding items of Annex N (other and ancillary works) of this standard apply.
• Where galvanizing of the shaped steel used in clean-room ductwork is required, the work content shall note the galvanizing of the shaped steel.
• Table G.2.1 itself: unit of measurement sq m, computed by developed area of the internal diameter shown on the design; work content: fabrication and installation of ducts, fittings, flanges and parts. Supporting conventions of Table G.3.1 fabrication and installation of duct components (code 030703): carbon steel dampers (duct butterfly dampers, non-return dampers, fire dampers, sealed inclined slide-plate dampers, opposed multi-blade control dampers, tee control dampers etc.), grilles / diffusers / louvres, hoods, flexible connections, silencers etc. are all component-type BOQ items listed separately, not merged into the duct area or length; plenum boxes are computed by developed area of the dimensions shown on the design, without deducting opening areas.

2. Beijing consumption standard (DB11/T 2382-2024, section 6.7 HVAC installation works)

• 6.7.29 ductwork fabrication and installation quantities are computed by developed area of the internal diameter shown on the design, without deducting the opening areas of inspection holes, measuring holes, supply / return outlets and the like.
• 6.7.30 duct length follows the centreline length of the ducts shown on the design, without deducting elbows, tees, reducers and other transition fittings, and deducting the length occupied by dampers and components; the start and end points of the centreline are the intersections of the duct centrelines.
• 6.7.32 installation of wall-penetrating duct sleeves is computed by the developed area of the sleeve and priced under the corresponding duct fabrication and installation items; installation of duct end caps is computed by the cross-sectional area of the duct blanked off, under the corresponding duct fabrication and installation items.
• 6.7.36 plenum box fabrication and installation quantities are distinguished by the long side and computed by developed area, without deducting the opening areas of the connecting ducts.
• 6.7.39 and 6.7.41 installation of GRP (fibreglass) ventilation ducts and composite ventilation ducts is computed by developed area of the external diameter shown on the design, without deducting the opening areas of inspection holes, measuring holes, supply / return outlets and the like.
• 6.7.50 where grilles are installed in the duct wall, the stub piece connecting grille and duct is measured under the duct rules (clauses 6.7.29 and 6.7.30).
• Component measurement: dampers, smoke-exhaust outlets (6.7.45), grilles and diffusers (6.7.51), silencers and silenced bends (6.7.60) are all distinguished by size and counted by the number shown on the design - on a separate track from ducts measured by area.

3. Reference component deduction lengths (SJG 111.3-2022 consumption standard for urban utility tunnels) Duct length is the centreline length shown on the design (main and branch ducts divided at centreline intersections), including elbows, reducers, round-to-rectangular transitions and other fittings, and deducting the length occupied by components; where the design is silent, deduct the following: butterfly damper L = 150 mm; non-return damper L = 300 mm; fire damper on circular ducts L = 220 mm; fire damper on rectangular ducts L = 210 mm; sealed opposed multi-blade control damper L = 210 mm; sealed inclined slide-plate damper L = 220 mm. Under the same standard, thin steel sheet ducts are computed in square metres by developed area of the external diameter shown on the design, without deducting the area of inspection holes, measuring holes, supply outlets or extract inlets; overlapping areas of ducts and duct mouths are already included in the items and must not be counted again.

4. Practical convention BOQ quantity of ducts = developed area by internal / external diameter per the applicable standard: length by centreline (including elbows, tees, reducers and other fittings) minus the length occupied by dampers and components; area without deducting openings, and overlapping duct-mouth parts not counted. Dampers, grilles, diffusers, silencers and the like run as component items (030703), listed separately, counted by number and priced under their own quota items, avoiding duplication with the duct quantities.

Applicability Boundaries

• Scope: BOQ preparation, budget take-off and settlement checking of fabrication and installation of HVAC ducts (thin steel sheet, stainless steel sheet, GRP, composite etc.); ventilation works of tunnel ancillary facilities may refer to SJG 111.3-2022.
• The "no area deduction" applies only to openings such as inspection holes, measuring holes, supply / return outlets and extract inlets; overlapping areas of ducts and duct mouths are not counted additionally under any of the standards.
• The internal / external diameter basis of the developed area varies by standard and material: metal ducts mostly by internal diameter (GB/T 50856-2024 Table G.2.1; DB11/T 2382-2024 clause 6.7.29); GRP and composite ducts by external diameter (clauses 6.7.39 and 6.7.41); thin steel sheet ducts under SJG 111.3-2022 by external diameter - whichever standard / quota applies, its basis governs; do not mix.
• "Excluding component length" only means the duct length deducts the components; the components themselves must be listed and measured under component items - neither omit the component items because of the deduction, nor merge the components back into the duct length and count them twice.
• Specific component deduction lengths: GB/T 50856-2024 gives no figures; where the design is silent, local standard figures may be referenced (e.g. butterfly damper 150 mm per SJG 111.3-2022); where the locality has current provisions or quota notes, those provisions govern.
• Local consumption standards are used with the local quota (DB11/T 2382-2024 is a Beijing regional standard; SJG 111.3-2022 is for urban utility tunnel works); verify the local current edition and pricing basis before cross-regional use.

Further Reading

• GB/T 50856-2024 "Standard for Method of Measurement of General Building Services Installation Works", Annex G HVAC works: G.5.8 (Table G.2.1 fabrication and installation of ventilation ducts, code 030702, explanatory notes and measurement rules), G.5.9 (Table G.3.1 fabrication and installation of duct components, code 030703, notes) and the unit of measurement and work content of Table G.2.1.
• DB11/T 2382-2024 "Consumption Standard for Construction Works", section 6.7 HVAC installation works: clauses 6.7.29, 6.7.30, 6.7.32, 6.7.36, 6.7.39, 6.7.41, 6.7.45, 6.7.50, 6.7.51 and 6.7.60.
• SJG 111.3-2022 "Consumption Standard for Urban Utility Tunnel Works (Volume 3, Ancillary Facility Installation Works)", ventilation works 4.2.2: component deduction length table for duct length, splitter vane computation tables 4.2.2-1 and 4.2.2-2, and the supporting rules for flexible ducts, canvas connections, inspection holes and measuring holes.