SettingsSettings for this calculationUS
The total length of the eave run where baffles are needed.
Measure along the eave the full distance over which rafter bays need ventilation baffles installed.
The on-center spacing between rafters.
Common spacings are 0.4 m (16 in) or 0.6 m (24 in) on center — check your framing plan or measure in the attic.
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.
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How this was calculated
Formula source(s)
- Baffle count = ceil(eave run length ÷ rafter spacing), since one baffle is installed in each rafter bay to maintain a clear soffit-to-attic airflow path over the insulation; a run holds one fewer bay than it holds rafters, so no member-count adjustment applies
Inputs used
- Eave Run Length
- 49 ft
- Rafter Spacing
- 2 ft
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.
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.
Part of bigger jobs
This trade is one line of a job takeoff. Run the whole job and every other trade comes back with it, off the same measurements.
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.
also measures10 other trades
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 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Baffle count = ceil(eave run length ÷ rafter spacing), since one baffle is installed in each rafter bay to maintain a clear soffit-to-attic airflow path over the insulation; a run holds one fewer bay than it holds rafters, so no member-count adjustment applies
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Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.
Tools and safety for this job
To fit the eaves and ridge ventilation. Generic types, no brands, no prices.
Protection this work requires
- Mineral and glass wool irritate skin, eyes and airways: cover your arms, wear a P2/N95 respirator while cutting, and rinse in cold water — hot water drives fibres in.
- Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.
- Knives cause more site injuries than any power tool: cut away from your body, change blades often, and wear cut-resistant gloves to EN 388 level C for repeated cutting.
Fit the eaves and ridge ventilation: Fitting vents and trays is roofing. How much free vent area a roof needs at the eaves and the ridge is set by the regulations or code the work falls under and by the underlay's maker, and this site counts the parts rather than judging whether a roof is ventilated enough.
Show the 5 tools this job needsHide tools
Essential
Staple gun
Tape measure
Utility knife
Recommended
Cordless drill/driver
Crawl boards
Also needed as materials: drill and driver bits, replacement blades, staples.
what each roofing tool is for, and the spec that decides which to buy where one does.