One Loop, Two Openings
Air crosses an attic for two reasons: warm air stands up and leaves at the highest opening it can find, and wind moving over a roof drops the pressure along the ridge line. Both effects need somewhere for replacement air to enter. The low opening and the high opening are not two products bolted onto the same roof — they are the two ends of one pressure loop, and that loop moves only as much air as the smaller end permits. Forty feet of ridge vent above a starved soffit does not ventilate forty feet of roof. It ventilates whatever the soffit hands it, then goes looking for the rest of its air somewhere else.
Pricing habits push the imbalance in one direction almost every time. Exhaust is the roofer's line item: visible, countable, installed the same afternoon as the cap shingles. Intake belongs to whoever hung the soffit, blew the insulation or wrapped the eave — and on a re-roof, often to nobody at all. Most of the existing housing stock therefore carries more exhaust than intake, which happens to be the more destructive of the two failures. The reverse case is rarer and quieter, though far from harmless, and a crew that recognises only one failure mode will misread the other.
Every defect below is the same imbalance viewed from alternating sides. Read them as a pair, because the fix for one is usually the diagnosis for the other.
Setting the Pair Before the First Cut
Net free area is the working currency, and it is not the size of the hole. A vent's listed NFA comes from the manufacturer's published data with the baffle, louvre and insect screen already accounted for; the gross opening is always larger and always irrelevant. Start from attic floor area, apply the ratio your adopted code requires, then split that total between low and high. Jurisdictions vary: many US jurisdictions adopting the International Residential Code work from an attic-floor ratio that may be reduced where the code's stated conditions are met — a vapour retarder in climates that require one, or a defined share of the total placed in the upper portion of the space. Canadian work is governed by the National Building Code of Canada and its provincial adoptions instead, and local amendments override both. Confirm the ratio and the reduction conditions with the authority having jurisdiction before you commit to a product count.
Convert the split into things a crew can actually install: linear feet of continuous ridge or hip vent, linear feet of strip intake, or a count of soffit panels at their listed NFA per panel. Two numbers survive to the job sheet — required intake NFA and required exhaust NFA, per compartment. Everything after that is verification.
Sequence matters more than the arithmetic. Intake corrections made from the ladder while staging is standing are trivial: cut the sheathing behind the vented panel, set baffles, clear the eave. The identical corrections after the scaffold comes down are a return trip with a second mobilisation. Price the pair together or you will price it twice.
The intake and exhaust halves have to exist as two separate numbers before anyone touches the deck, and this is the point in the sequence where that split gets fixed.
The length of the attic floor, measured along the ridge; the page takes each eave and the ridge to be this long.
The width of the attic floor, wall plate to wall plate across the building.
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 intake product, so the page can count it; the slot is a real product rated by its maker.
The exhaust product high on the roof: a ridge vent, or individual roof vents.
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.
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.
Exhaust-Heavy: The Ridge Outruns the Eave
An attic with more exhaust than intake sits negative against the space below it. Air has to come from somewhere, and the easiest openings on most jobs are in the ceiling plane rather than at the eave: recessed light housings, top plates, the attic hatch, plumbing and flue chases, bath fan boxes. The roof stops ventilating the attic and starts ventilating the house.
Winter turns that into damage. Indoor air carries the household's moisture up through those leaks and deposits it on the coldest surface available, which is the underside of the sheathing and the shanks of every nail poking through it. Frost forms, accumulates through a cold spell, then melts during the first mild spell and drips. The homeowner reports a leak; the roofer chases flashing that was never wrong. Repeated cycles delaminate OSB, stain rafters, and grow mould on the north slope first because that slope stays coldest longest.
Combustion safety belongs in this conversation and does not belong to the roofer alone. Where atmospherically vented appliances share a leaky ceiling plane with a depressurised attic, spillage and backdrafting become possible. That is a gas-safety question requiring a combustion-safety test by someone qualified to run one — flag it, do not judge it from the roof.
Reading it on site is straightforward once you know the pattern. Loose-fill insulation drawn into ridges around ceiling penetrations, ghosting lines on the ceiling below that trace the framing, a far corner of the attic that feels dead still on a windy day while the ridge whistles — all of it says the same thing. The exhaust is drawing, and it is not drawing from the soffit.
Intake-Heavy: The Quiet Half of the Imbalance
Flip the ratio and the attic sits slightly positive against outdoors. Flow is still capped by the smaller opening, so a roof with generous intake and stingy exhaust simply moves less air than the design assumed: deck temperatures run higher through summer, and winter moisture leaves slower than it arrives. That part is underperformance rather than damage, and it is easy to overlook because nothing looks wrong from the ground.
Damage in this direction starts at the eave. Wind entering a large, low opening does not travel politely up the rafter bay to the ridge — it washes sideways across the top of the insulation at the perimeter, drags loose fill back off the top plate and strips effective R-value exactly where the ceiling assembly is already thinnest. The perimeter runs cold, the field runs warm, meltwater refreezes at the overhang, and the ice dam that follows gets blamed on gutters.
Weather ingestion is the second cost. Oversized or unbaffled intake takes in driven rain and dry powder snow; snow that melts on the first warm afternoon reads to everyone as a roof leak. Some coastal and high-wind jurisdictions require vents tested for resistance to wind-driven rain under a local product-approval scheme, and a generic vented panel will not satisfy it.
Fire exposure sets a hard limit on the intake-heavy approach. In wildland-urban interface areas, vents may need to resist ember entry and flame impingement — the relevant test method is ASTM E2886/E2886M, Standard Test Method for Evaluating the Ability of Exterior Vents to Resist the Entry of Embers and Direct Flame Impingement — and the local WUI requirements govern which products qualify. Adding raw open area at the eave to hit a ventilation ratio can put a compliant house out of compliance.
The Ridge as Metering Device
Continuous ridge vent is the only exhaust that spreads its draw evenly along the entire upper edge, which is why it pairs cleanly with continuous soffit intake: both ends of the loop are distributed the same way, so no section of attic is left to be ventilated by a neighbour twenty feet away. Its capacity is running length multiplied by the listed NFA per foot, so length is the whole design.
The length you get is never the length you measure off the plan. End caps sit back from the terminations, the cut slot stops short of each end, hip ridges count only where the product is listed for hip application, and any run of ridge sitting above a framed compartment that does not connect to the space below contributes nothing at all. Slot width comes from the vent's published installation instructions — cutting wider does not buy net free area, because the vent's own baffle geometry sets it — and cutting a slot through a structural ridge beam is a framing decision, not a roofing one.
Exhaust capacity stands or falls on usable ridge length rather than drawn ridge length, so the deduction for caps, terminations and dead runs belongs right here, before the slot is marked.
SettingsSettings for this calculation
Waste is set to 5% by hand. Pick a tier above to replace it, or keep your own figure.
The combined length of all ridge lines to receive ridge vent.
Extra material to allow for cut waste at ridge ends and intersections.
Ridge vent needed
51.45 ft
They open the calculator with your figures already in it
Ridge Vent Linear Footage Calculator: 51.45 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Buys vent, not the slot it sits over. A continuous ridge vent only works if the deck is cut back each side of the ridge board — typically 25 to 38 mm (1.5 in) a side — and that slot stops short of the ridge ends, of hips, and of any wall the ridge dies into, with the ridge board itself left uncut. Vent laid over unbroken decking looks exactly like vent laid over a slot and moves no air at all.
- Leaves out everything that finishes the run. Hip-and-ridge cap runs over the top of the vent as a separate quantity, and because the vent lifts the cap, coverage per bundle is a little less than over a bare ridge. End plugs, the connectors between vent sections and the longer nails that reach through the vent into the deck are all separate line items.
The Eave Fails First
Insulation packed hard into the eave, baffles missing or crushed, baffles stopped short of the top plate, perforated soffit hung over solid sheathing with no opening cut behind it, vented panel painted until the perforations closed, screening blocked with dust and insect nests: each of these turns listed net free area into decorative pattern while the ridge above keeps drawing at full strength.
Checking takes ten minutes. From inside the attic on a bright day, kill your light and look down the line of the eave — daylight should show in every bay, not every third bay. From below at dusk with a work light lying in the attic, the same test runs the other way and needs no crawl. Where you find blockage, count it as zero rather than reduced; a screen packed with paint or nest material is not a partly open vent.
Cathedral and vaulted assemblies raise the stakes because each rafter bay is its own independent loop. One crushed baffle in one bay does not degrade the roof by a few percent — it shuts that bay down completely, and the wet sheathing shows up later as a single dark stripe on an infrared scan while every bay beside it reads dry.
One chute per bay is what the eave keeps failing on, so count bays off the eave run instead of eyeballing it — a bay left out is that bay closed, not a few percent shaved off the roof.
The total length of the eave run where baffles are needed.
The on-center spacing between rafters.
Baffles needed
25 baffles
They open the calculator with your figures already in it
Attic Rafter Baffle/Chute Count Calculator: 25 baffles — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Putting a baffle in every rafter bay is not the same as ventilating the roof. The count says nothing about whether the soffit intake and the ridge or high-level outlets together provide enough net free vent area for the insulated ceiling area below, a ratio fixed by your local code, nor whether the soffit vents behind the baffles are actually open. A baffle keeps a path clear; it cannot create intake.
- The count assumes a baffle physically fits at the wall plate. It does not check that the rafter depth there leaves room for the full insulation thickness plus the clear air channel required above it, which is the pinch point on a standard-heel truss. Where it does not fit, the fix is a raised heel, a deeper rafter or a tapered insulation edge, not more baffles.
- The result is a count of bays, not of packs or of running length. Baffles are sold as fixed pieces, commonly around 1.2 m long and pre-sized for 400 mm or 600 mm bays, so any bay baffled further up the slope, such as deep loft insulation or a vaulted section vented to the ridge, takes two or more pieces. No allowance is added here for cut-offs, staples or the edge sealing that stops blown insulation being driven behind the baffle.
- The figure covers one straight eave run. A gable roof has two eaves and has to be worked out and added run by run, and every bay in the run entered is counted whether or not it can be ventilated, including bays closed off by a chimney, a party or fire wall, or a stretch of eave with no soffit vent beneath it.
- Every bay is assumed identical and the spacing box stops at 0.61 m (2 ft), so roofs framed at wider centres cannot be entered, and the trimmed short bays around a rooflight or a hip are not represented — split such a roof into uniform runs and count the odd bays by hand.
One Roof, Several Pairs
Balance is a per-compartment property, never a whole-roof average. Hip returns, dormers, knee-wall attics, dropped soffits over cabinet runs, tray ceilings and any framed wall left standing inside the attic all cut the space into cells that may not exchange air with each other. Totalling intake and exhaust across the whole roof and declaring the result balanced will hide a starved cell perfectly.
Additions are the usual offender. An original gable end gets framed over, the new section gets its own ridge, and the old attic is left with exhaust it can no longer feed. Walk the attic before you specify anything and mark the cells on the sketch — a hip roof in particular gives you very little ridge, so the cells nearest the hips need hip-ridge vent, a listed alternative, or an intake ratio matched to whatever exhaust they actually have.
Two Exhausts Cancel Each Other
Ridge vent plus open gable louvres is the classic short circuit. The louvre is a large opening high in the same cell, much closer to the ridge than the soffit is, so the ridge draws from the gable and the soffits go idle. The upper attic exchanges air busily with itself while the space above the ceiling perimeter — the part that actually needs drying — stagnates.
Powered fans deserve their own caution. A fan pulls from the largest, nearest opening available to it, and on a ridge-vented roof that is the ridge itself, drawing weather backward through a vent designed to exhaust. A fan's intake requirement is set by its airflow rather than by the passive ratio, which produces a different and considerably larger intake number. Pick one exhaust strategy per cell, block or remove the others, and re-check intake against whichever one survives.
A powered unit answers to its own airflow rather than to the passive ratio, and knowing that figure is what tells you whether the soffits it needs are anywhere near what the cell already has.
The total floor area of the attic space.
Minimum fan CFM
756 CFM (minimum)
Dark roofs, steep pitches, and hot climates all increase attic heat load beyond this baseline rule of thumb — size toward the higher end of available fan models if any of those apply.
- Attic area
- 1,080 sq ft
- Same airflow in L/s
- 356.79 L/s
They open the calculator with your figures already in it
Attic Fan CFM Calculator: 756 CFM (minimum) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 756 CFM (minimum) — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Floor area is the only geometry this uses. Roof pitch, ridge height and attic depth never enter the formula, so a shallow hip and a steep gable over the same footprint return the same CFM even though the steep one holds far more air to turn over.
- It does not check that the attic has enough intake. A fan sized from this number still needs matching net free intake area at the soffits; starved of intake it pulls make-up air out of the house through ceiling leaks, which can depressurise the top floor and backdraft an atmospherically vented water heater or furnace. Size the intake separately before you buy the fan.
- Nothing adjusts for roof colour, climate, insulation level, radiant barrier, or existing ridge and gable venting — the same divisor is applied to every attic. Dark roofs, steep pitches and hot climates all put the real heat load above this baseline.
- This is a rule-of-thumb equipment sizing figure, not a code calculation. Codes govern attic ventilation as net free vent area against attic floor area, and fitting a powered fan does not remove that requirement.
- Rated CFM is not delivered CFM. Shutters, insect screens, a clogged louver and static pressure at the opening all cut what the fan actually moves, and the figure here makes no allowance for that, for the thermostat or humidistat setpoint, or for splitting the airflow across more than one fan.
Penetrations Sit on Both Sides of the Ledger
Every pipe boot, exhaust cap and vent hood is a penetration that must shed water and, in ember-exposed areas, resist entry. Count them at estimating time alongside the vent take-off so nothing gets improvised on the day the deck is open and the crew is waiting.
The ceiling plane carries more weight than the roof plane here. Bath and kitchen fans terminating into the attic instead of through the roof or wall load the space with the wettest air in the building, and no ventilation ratio anywhere is sized to absorb a family's daily shower vapour. Air-seal the ceiling at the same time as you balance the vents. Balancing an unsealed ceiling only changes which failure you get.
Boots are what gets improvised on the morning the deck is open, so put a count and a spare allowance on the same take-off sheet as the vent quantities rather than sending somebody to the merchant.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
The total number of plumbing or exhaust vent pipes penetrating the roof.
Extra boots to allow for installation damage or sizing mismatches.
Vent pipe boots needed
9 boots
They open the calculator with your figures already in it
Roof Vent Pipe Boot Count Calculator: 9 boots — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Counts penetrations, not the flashing they take. What a "boot" is depends on the roof covering and the slope: a shingle roof takes a base flashing woven into the courses, a single-ply membrane takes a heat-welded or adhered pipe flashing over a target sheet, a standing-seam roof takes a formed collar bedded in sealant. Same count, three different products at three different prices, none of them interchangeable.
- Vent pipes only. Flue and chimney penetrations, exhaust duct terminations, conduit and cable, lightning protection, satellite and solar mounts and equipment supports all pass through the same deck and each takes a detail of its own. None are in this number, and on most roofs they outnumber the plumbing vents.
- Assumes every pipe can be flashed on its own. A boot needs clear roof around it for its flange or target sheet, so two pipes closer together than that flange — or a pipe sitting in a valley, tight against a wall, or just under a ridge — cannot each take a separate boot. Those get a shared curb or a pitch pan instead, which lowers the boot count and adds a fabricated detail nobody priced.
Proving the Pair Afterwards
Verification is a short list and it is worth doing before the invoice. Hold a smoke pencil at a soffit opening on a breezy day: air should move in steadily, not out and not stall. Measure the attic-to-house pressure difference — it should be small. In a cold snap, look at nail points and sheathing for frost. Take pin moisture readings on the north slope. Run an infrared scan across the ceiling perimeter for the cold band that says insulation has been wind-washed off the top plate.
Record what you installed: required NFA by cell, listed NFA per product, quantities, and the intake-to-exhaust split as built. When somebody investigates a ceiling stain three years later, that record separates a design question from an installation question, and it settles the argument in minutes rather than by opening the roof.
What Governs, and Where It Varies
Adopted code governs everything above, and the adoption is local. Ratios, the conditions permitting a reduced ratio, minimum baffle clearance above insulation and acceptable vent locations all differ between jurisdictions and between code cycles. Unvented roof assemblies are a separate compliance path with their own insulation and vapour-control conditions; a roof designed unvented must not be half-vented in the field because someone assumed all attics need air.
Product qualification runs on a parallel track. Use the manufacturer's listed net free area, not a measured opening; check for ember-resistance listing where wildland-urban interface requirements apply; check for wind-driven-rain product approval in coastal jurisdictions. Note also that ASHRAE Standard 62.2, Ventilation and Acceptable Indoor Air Quality in Residential Buildings, addresses dwelling ventilation and not attic ventilation — the two get conflated in conversation with clients, and separating them early avoids a pointless argument about fan sizing.
Take-off and check list
Carry these off the roof as numbers and conditions, one set per attic cell rather than one set per building.
- Listed net free area, per vent unit and per linear foot — Manufacturer's published figure only; a measured opening overstates it every time.
- Usable ridge run, after caps, terminations and dead sections — Deduct set-backs and any ridge above a compartment that does not open to the space below.
- Continuous eave baffles, one per rafter bay — Set them past the top plate and confirm none were crushed by the insulation crew.
- Openings cut in the soffit sheathing behind vented panel — Vented panel over solid plywood is the most common zero-intake defect on re-roofs.
- One exhaust type per cell — Block or remove gable louvres and powered units left in a ridge-vented compartment.
- Ceiling-plane air sealing and fan terminations — Chases, hatch, light housings; bath and kitchen fans must discharge outdoors, not into the attic.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.