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Building Code Reference

Got a CRC R806.2 correction?

Attic ventilation ratio

For builders in California, Greater Seattle, the Las Vegas area, the Denver area and Kansas City · CrossBeam Permits

What this code requires, how to fix it, and how cities and counties review it

What this code requires

CRC R806.2 requires attic spaces to have cross-ventilation with a total net free ventilating area of at least 1/150 of the attic floor area. That ratio drops to 1/300 when 40 to 50 percent of the required vent area is placed in upper locations (within 3 feet of the ridge) and the remaining vent area is in lower positions at or near the eave line. Vent openings must clear any insulation so air can actually move through. Reroof, new construction, and ADU projects all trigger this requirement when the attic is part of the scope.

How to fix it

  1. Calculate required net free vent area

    Measure the total attic floor area in square feet from the architectural plans. Divide by 150 if you're using the standard ratio, or by 300 if 40 to 50 percent of the required area is in upper vents within 3 feet of the ridge and the rest is in the bottom third of the attic (in Climate Zones 6, 7 and 8, a ceiling vapor retarder is also required). Write both numbers on the roof plan or in a vent schedule table. Example: 1,200 sq ft attic floor ÷ 150 = 8 sq ft net free area required. At 1/300 with balanced vents: 1,200 ÷ 300 = 4 sq ft. Use the product's published net free area rating — not the rough opening size.

  2. Select vents and confirm net free area ratings

    Pull the specification sheet for each vent product you plan to install — ridge vent, soffit vent, gable vent, or power vent. Each product has a net free area expressed in square inches per linear foot (ridge vents) or square inches per unit (individual vents). Convert to square feet: divide by 144. List the product name, net free area per unit, quantity, and total net free area in a table on the plans. This is the table the plan-checker is looking for.

  3. Show vent locations on the roof plan

    Mark every vent location on the roof plan with a symbol, and include a legend. For ridge vents, dimension the continuous run length. For individual units, show placement with spacing noted. For soffit vents, dimension the perforated soffit panel runs. The checker needs to confirm that your calculation matches the installed count, and that low vents are at or near the eave line while high vents are at or near the ridge.

  4. Verify and show the upper/lower split if using 1/300

    If you're using the reduced 1/300 ratio, add a note or table to the plans that explicitly states: total required net free area, area provided in upper position (ridge, high gable), area provided in lower position (soffit, low eave), and the percentage each represents. The upper vents must provide 40 to 50 percent of the total and sit no more than 3 feet below the ridge, with the balance in the bottom third of the attic. If that math doesn't work with your chosen products, either add more ridge vent or fall back to the 1/150 calculation.

  5. Call out insulation baffles on the framing cross-section

    Add a wall-section or rafter-section detail showing insulation baffles at each rafter bay above the soffit vents. Baffles keep blown-in or batt insulation from covering the vent opening. Note the minimum 1-inch air gap between the baffle and the roof deck. If you're using spray foam in the attic, note whether the attic is unvented per R806.5 instead — don't try to apply R806.2 to a fully encapsulated spray-foam attic.

  6. Add a vent schedule to the plan set

    A one-page vent schedule is the cleanest way to pass this check. Include: attic area (sq ft), applicable ratio (1/150 or 1/300), required net free area (sq ft), vent product name and model, net free area per unit or per linear foot, installed quantity or linear footage, total net free area provided, and a pass/fail comparison. Keep it on the roof plan sheet or a separate details sheet. Checkers sign off on this in minutes when the table is complete.

  7. Resubmit with a response letter calling out where each item appears

    When you resubmit, write a response to each corrections-letter item by sheet and detail number. For R806.2: 'Response: Attic ventilation calculations added to Sheet A3.1, Vent Schedule. Net free area provided: X sq ft against X sq ft required per 1/150 ratio. Vent locations shown on Sheet A2.1, Roof Plan.' This format lets the checker verify your response in under a minute instead of hunting through the revised set.

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Why you got flagged

Here's why plan-checkers flag R806.2. The most common correction is no ventilation calculations on the roof plan at all. A sheet that shows sheathing, underlayment, and tile but has no vent schedule and no area calculation will get flagged every time. The checker can't verify compliance without numbers, so the correction letter goes out before anyone looks at whether you might actually be okay.

The second scenario is using the 1/300 ratio without showing the vent distribution. If you specify ridge vents and soffit vents, you can use the reduced ratio — but you must demonstrate that 40 to 50 percent of the required area is in the upper position and the rest is at the eave. A note that says "ridge vent with soffit vents" doesn't prove the upper/lower split. Checkers want a table or a labeled diagram.

Vent area calculations that use the gross opening size instead of the net free area will also fail. Every vent product has a net free area rating per square foot. Using the rough opening gives a larger number than the actual airflow area. Checkers know this, and a calculation using rough openings is usually caught.

The fourth trigger is insulation blocking the vent path. On dense-pack jobs or spray foam additions, the plan notes sometimes don't address how the vent pathway stays clear. R806.2 requires the vent opening itself to be clear of insulation, and the plans need to reflect that — baffles called out on the cross-section, or a note with a minimum clearance dimension.

The official code

California Residential Code (Title 24 Part 2.5) — Building Standards Commission

Read the source →

Common questions

What is the 1/150 rule for attic ventilation in California?
The 1/150 rule is the standard attic ventilation requirement in CRC R806.2. You need a total net free ventilating area equal to at least 1/150 of your attic floor area. So for a 900 sq ft attic, that's 6 sq ft of net free area. Net free area is the actual open airflow area of the vent product — not the rough opening size. Each product you specify should list this on the spec sheet. If you can't find it, call the manufacturer or pick a product that clearly states the rating.
Can I use 1/300 instead of 1/150 for attic vents?
Yes, if you can show 40 to 50 percent of the required vent area is in upper positions (within 3 feet of the ridge) and the remaining area is at the eave. With that balanced cross-ventilation arrangement, you only need half the total area — 1/300 of the attic floor instead of 1/150. The catch is you have to prove the split. Checkers won't assume you've hit 40 percent just because you have ridge and soffit vents. Add a table to your plans showing the upper and lower vent areas and what percentage each is.
Does R806.2 apply to a re-roof?
Yes. Any reroof that exposes the roof deck brings attic ventilation into scope. The plan-checker will want to see that existing ventilation meets the current R806.2 ratios, or that you're adding vents to get there. If the reroof is tear-off to deck, you have to address ventilation. If it's a recover (new layer over old), the requirements depend on whether the scope disturbs the existing vent penetrations. When in doubt, show a vent calculation — it takes 20 minutes and avoids a correction.
Does spray foam attic insulation need R806.2 ventilation?
No — a fully encapsulated spray foam attic is treated as unvented and follows R806.5 instead. R806.2 applies to vented attic assemblies. If you're using open-cell or closed-cell spray foam applied directly to the underside of the roof deck with no air gap, you're in unvented territory. The plans need to clearly identify the assembly type. Mixing the two approaches — spray foam to the deck plus soffit vents — is the scenario that confuses checkers most, so call it out explicitly in the notes.
Do solar panel installations trigger R806.2 corrections?
Sometimes. Solar racking systems don't require a full plan check in most OC jurisdictions, but when a solar permit is reviewed alongside a re-roof or addition, the building inspector may flag ventilation if existing vents are blocked or removed by the racking. If you're pulling a solar permit on a house that also needs re-roofing, coordinate the vent schedule with the roofing scope before submitting. A blocked ridge vent under a solar array is a real inspection failure point.
What counts as a net free area for attic vents?
Net free area is the actual open area through which air can flow, accounting for screens, louvers, and baffles. It's always smaller than the rough opening. A typical 16x8-inch soffit vent might have a rough opening of 0.89 sq ft but a net free area of 0.4–0.6 sq ft depending on the screen mesh. Use the manufacturer's published NFA rating. If you're specifying perforated soffit panels, the panel product should list NFA per square foot. Using rough opening numbers to meet the ratio will usually fail on plan check.

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