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The length of the attic floor, measured along the ridge; the page takes each eave and the ridge to be this long.
Measure the vented space's floor along the line of the ridge. On a simple gable roof both eaves and the ridge run this whole length, which is how the page checks whether a continuous vent fits. A hipped roof has a shorter ridge and longer eaves; run its vent counts by hand from the areas the page gives.
Tools needed: Tape measure
The width of the attic floor, wall plate to wall plate across the building.
Length times width is the area the US and Canadian rules divide. Under BS 5250 the width is the roof's span, and a span of 10 m (32.8 ft) or more brings in a high-level vent at any pitch. Australia's NCC states its openings per metre of the roof's longest horizontal dimension, so where the width is the longer, it is the one the page multiplies.
Tools needed: Tape measure
The rule the vent area comes from: the IRC's ratio, Canada's, or the UK's and Australia's openings along the eaves and ridge.
The IRC asks 1/150 of the attic floor, or 1/300 when 40 to 50% of it sits in upper vents, not more than 3 ft (914 mm) below the ridge, and, in climate zones 6, 7 and 8, a Class I or II vapor retarder lines the warm side of the ceiling. Canada asks 1/300 of the insulated ceiling, 1/150 for a low slope or roof joists. In the UK, BS 5250 sets gaps along the eaves instead of a ratio, for cold roofs with a high-resistance underlay; a breathable underlay follows its own certificate. Australia's NCC 2022 sets open area per metre of the roof's longest side, by pitch, in climate zones 6, 7 and 8 only; elsewhere in Australia that clause asks for none.
The intake product, so the page can count it; the slot is a real product rated by its maker.
A continuous soffit or eaves vent is rated by its net free area per length, an individual vent by its area each. The page counts the pieces it takes and, for a continuous vent, checks that the length it needs fits along the two eaves. The slot follows the unit system, not the country: a metric reader is offered Marley's 25 mm eaves vent, made for pitches of 12.5° to 55°. A Canadian reader enters the rating of a vent complying with CAN3-A93-M, which NBC 9.19.1.2.(5) asks for, and an Australian reader a local product's.
The exhaust product high on the roof: a ridge vent, or individual roof vents.
A ridge vent is rated per length and runs along the ridge; box vents, roof louvres and gable vents are rated each. The page counts the pieces and, for a ridge vent, checks that the length it needs fits along the ridge. The slot is GAF's RidgeRunner in both unit systems, a vent for a shingled roof: a UK reader at 35° or more enters a dry-ridge rating, and a Canadian reader a CAN3-A93-M vent's (the RidgeRunner's data sheet lists a CCMC evaluation).
Net free vent area needed
6.5 sq ft (net free area)
R806.2 of the 2021 IRC: 1/150 of the attic floor. The code sets no split at this ratio; the page divides it evenly between intake low on the roof and exhaust high up, and keeps the intake at least equal to the exhaust bought, as GAF's balanced-ventilation guidance asks.
- Attic floor area
- 975 sq ft
- Net free area needed
- 936 in²
- Intake, low on the roof (eaves or soffit)
- 475 in²
- Exhaust, high on the roof (ridge or upper vents)
- 468 in²
- Intake as a continuous gap along both eaves
- 0.51 in
- Exhaust as a continuous gap along the ridge
- 1 in
- Intake vent to fit
- 6 vents (8 ft)
- Exhaust vent to fit
- 38 ft
- High-level share of the total
- 50 %
They open the calculator with your figures already in it
Attic and Loft Ventilation Calculator (Intake, Exhaust and Vent Count): 6.5 sq ft (net free area) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- International Code Council, 2021 International Residential Code, as adopted without amendment in the Colorado Division of Fire Prevention and Control Residential Code 2021: R806.2, net free ventilating area 1/150 of the vented space; exception, 1/300 where both conditions are met: a Class I or II vapor retarder on the warm-in-winter side of the ceiling in climate zones 6, 7 and 8, and 40 to 50 percent of the required area in ventilators in the upper portion of the attic or rafter space
- Residential Code of Ohio, Ohio Adm. Code 4101:8-8-01 (effective 1 July 2019), sections 806.1–806.3 on the 2018 IRC text: upper ventilators not more than 3 ft (914 mm) below the ridge or highest point, measured vertically; the balance in the bottom one-third of the attic space; installation lower where framing conflicts; not less than 1 in (25 mm) between insulation and roof sheathing (Ohio deletes the exception's vapor-retarder condition)
- National Research Council of Canada, National Building Code of Canada 2020, Division B, 9.19.1.1–9.19.1.3: vent area not less than 1/300 of the insulated ceiling area, or 1/150 where the roof slope is less than 1 in 6 or the roof has roof joists; vents distributed uniformly on opposite sides, at least 25% at the top and at least 25% at the bottom of the space; 63 mm between insulation and sheathing
- Marley, 'How do I ventilate a cold roof?' (technical support guidance, citing BS 5250, Management of moisture in buildings): with a high-resistance underlay, 25 mm along the eaves for pitches of 15° or less, 10 mm for pitches above 15°, and 5 mm at high level where the pitch exceeds 35°, the span exceeds 10 m, or the roof is lean-to or mono-pitch; Wienerberger, 'Roof space ventilation' (2013): a continuous 10 mm gap gives 10,000 mm² per metre run, and the 5 mm high-level vent where a building is 10 metres wide or more or the pitch 35 degrees or above (the page takes this, the stricter reading at the boundaries). Approved Document C (2013) 6.10–6.11 refers roof condensation to BS 5250
- Vent ratings and the balance rule: GAF Cobra RidgeRunner data sheet RESCB171 (2020), 12.5 sq in per lineal foot (26,458 mm² per lineal metre), exhaust never to exceed intake, and the chart 1,600 sq ft to 31 ft with 388 sq in of intake; Lomanco 140 soffit vent product specification (2012), 88 sq in per 8 ft unit; Marley Ventilated Dry Ridge System, 5 mm continuous
- Marley, Universal 25mm Eaves Vent System: 25,000 mm² free area of ventilation per linear metre, for roof pitches of 12.5° to 55° (product page; Marley's NBS Source data, ventilation opening 25 000 mm²/m, 6000 mm packs; fixing instructions, December 2018, a continuous 25 mm free ventilation path, pack code 46351)
- Australian Building Codes Board, National Construction Code 2022 (as adopted), Housing Provisions 10.8.3, ventilation of roof spaces: in climate zones 6, 7 and 8, Table 10.8.3, under 10° pitch 25,000 mm²/m at each of two opposing ends; 10° to under 15°, 25,000 mm²/m at the eaves and 5,000 mm²/m at high level; 15° to under 75°, 7,000 mm²/m at the eaves and 5,000 mm²/m at high level, plus 18,000 mm²/m at the eaves for a cathedral ceiling; areas per metre of the roof's longest horizontal dimension; high level within 900 mm below the ridge; not for concrete roofs, structural insulated panels or BAL-FZ. Applied along both eaves as the Australian Steel Institute's 'Ventilation in Steel Roofing' (VSR V27, 2024) works the table for a gable or hipped roof
Inputs used
- Attic or Loft Length (along the ridge)
- 39 ft
- Attic or Loft Width (the span, eaves to eaves)
- 25 ft
- Rule
- IRC R806.2: 1/150 of the attic floor (United States)
- Share of the Vent Area High Up (%)
- 50
- Intake Vent (eaves or soffit)
- Lomanco 140 soffit vent, 88 sq in per 8 ft length
- Net Free Area of One Intake Vent
- 88 in²
- Exhaust Vent (ridge or upper)
- GAF Cobra RidgeRunner ridge vent, 12.5 sq in per foot
- Net Free Area of One Exhaust Vent
- 12.5 in²
Intermediate steps
- Attic floor area
- 975 sq ft
- Net free area needed
- 936 in²
- Intake, low on the roof (eaves or soffit)
- 475 in²
- Exhaust, high on the roof (ridge or upper vents)
- 468 in²
- Intake as a continuous gap along both eaves
- 0.51 in
- Exhaust as a continuous gap along the ridge
- 1 in
- Intake vent to fit
- 6 vents (8 ft)
- Exhaust vent to fit
- 38 ft
- High-level share of the total
- 50 %
Confidence note: R806.2 of the 2021 IRC: 1/150 of the attic floor. The code sets no split at this ratio; the page divides it evenly between intake low on the roof and exhaust high up, and keeps the intake at least equal to the exhaust bought, as GAF's balanced-ventilation guidance asks.
What this calculation does not cover
- Net free area is a rating, not a hole size. The counts divide by the rating printed on the product after its louvres, mesh and screens, and assume every vent is open: intake vents painted over, blocked by insulation slumped into the eaves with no baffle, or fitted over solid sheathing count for nothing.
- The shape is a simple gable roof with the ridge along the length. A hip roof has longer eaves and a shorter ridge, a roof with dormers, valleys or a firewall is several vented spaces, and each compartment is worked on its own; the fit checks along the eaves and ridge then need doing by hand.
- The US exception's vapor-retarder condition in climate zones 6, 7 and 8 is taken on trust: nothing here knows the climate zone or whether a Class I or II retarder is in place and continuous. Where either is missing, the 1/300 figure is not available and the 1/150 one applies.
- UK openings are BS 5250's for a cold roof with a high-resistance underlay, as makers restate them, taking the stricter maker at the 35° and 10 m (32.8 ft) boundaries; a breathable underlay, a warm roof, a lean-to or mono-pitch roof, and roofs of buildings other than dwellings each have their own provisions, and the underlay's certificate governs a breathable one.
- Australian openings are Table 10.8.3's, which applies in climate zones 6, 7 and 8 only, applied along both eaves of a dual-pitched roof. The roof space's 20 mm (0.8 in) minimum height, the cathedral ceiling's extra eaves opening, the unsarked tiled roof that needs no openings, and the exemptions for concrete roofs, structural insulated panels and BAL-FZ are the reader's to check against the clause itself.
- This restates a ratio or an opening size; it is not a code check, a moisture assessment or a performance calculation. Air leaking up from the house is usually the bigger moisture source, ember-resistant vents are required in some wildfire areas, and the authority having jurisdiction or building control decides what applies to a given roof.
Part of a bigger job
Or plan the space itself: measure it once, doors and windows included, and this figure comes back worked out on that space, with what goes with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-10-05 · v1.1.0
Regulatory standards & verification citations7
- International Code Council, 2021 International Residential Code, as adopted without amendment in the Colorado Division of Fire Prevention and Control Residential Code 2021: R806.2, net free ventilating area 1/150 of the vented space; exception, 1/300 where both conditions are met: a Class I or II vapor retarder on the warm-in-winter side of the ceiling in climate zones 6, 7 and 8, and 40 to 50 percent of the required area in ventilators in the upper portion of the attic or rafter space
- Residential Code of Ohio, Ohio Adm. Code 4101:8-8-01 (effective 1 July 2019), sections 806.1–806.3 on the 2018 IRC text: upper ventilators not more than 3 ft (914 mm) below the ridge or highest point, measured vertically; the balance in the bottom one-third of the attic space; installation lower where framing conflicts; not less than 1 in (25 mm) between insulation and roof sheathing (Ohio deletes the exception's vapor-retarder condition)
- National Research Council of Canada, National Building Code of Canada 2020, Division B, 9.19.1.1–9.19.1.3: vent area not less than 1/300 of the insulated ceiling area, or 1/150 where the roof slope is less than 1 in 6 or the roof has roof joists; vents distributed uniformly on opposite sides, at least 25% at the top and at least 25% at the bottom of the space; 63 mm between insulation and sheathing
- Marley, 'How do I ventilate a cold roof?' (technical support guidance, citing BS 5250, Management of moisture in buildings): with a high-resistance underlay, 25 mm along the eaves for pitches of 15° or less, 10 mm for pitches above 15°, and 5 mm at high level where the pitch exceeds 35°, the span exceeds 10 m, or the roof is lean-to or mono-pitch; Wienerberger, 'Roof space ventilation' (2013): a continuous 10 mm gap gives 10,000 mm² per metre run, and the 5 mm high-level vent where a building is 10 metres wide or more or the pitch 35 degrees or above (the page takes this, the stricter reading at the boundaries). Approved Document C (2013) 6.10–6.11 refers roof condensation to BS 5250
- Vent ratings and the balance rule: GAF Cobra RidgeRunner data sheet RESCB171 (2020), 12.5 sq in per lineal foot (26,458 mm² per lineal metre), exhaust never to exceed intake, and the chart 1,600 sq ft to 31 ft with 388 sq in of intake; Lomanco 140 soffit vent product specification (2012), 88 sq in per 8 ft unit; Marley Ventilated Dry Ridge System, 5 mm continuous
- Marley, Universal 25mm Eaves Vent System: 25,000 mm² free area of ventilation per linear metre, for roof pitches of 12.5° to 55° (product page; Marley's NBS Source data, ventilation opening 25 000 mm²/m, 6000 mm packs; fixing instructions, December 2018, a continuous 25 mm free ventilation path, pack code 46351)
- Australian Building Codes Board, National Construction Code 2022 (as adopted), Housing Provisions 10.8.3, ventilation of roof spaces: in climate zones 6, 7 and 8, Table 10.8.3, under 10° pitch 25,000 mm²/m at each of two opposing ends; 10° to under 15°, 25,000 mm²/m at the eaves and 5,000 mm²/m at high level; 15° to under 75°, 7,000 mm²/m at the eaves and 5,000 mm²/m at high level, plus 18,000 mm²/m at the eaves for a cathedral ceiling; areas per metre of the roof's longest horizontal dimension; high level within 900 mm below the ridge; not for concrete roofs, structural insulated panels or BAL-FZ. Applied along both eaves as the Australian Steel Institute's 'Ventilation in Steel Roofing' (VSR V27, 2024) works the table for a gable or hipped roof
Which documents these citations point at
- International Residential Code (United States)One- and two-family dwellings and townhouses up to three storeys — structure, fire safety, energy, plumbing, mechanical and electrical in a single document.
- Approved Documents to the Building Regulations (England) — C (United Kingdom)Practical guidance on meeting the Building Regulations, one lettered part per subject — Part A structure, Part B fire, Part K protection from falling, Part L conservation of fuel and power, and the rest.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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