Roofing (C-39) · Lesson 3 of 6

Attic Ventilation: Net Free Area (1/150 and 1/300)

"Attic Ventilation: Net Free Area (1/150 and 1/300)" is lesson 3 of 6 in the Roofing (C-39) Learning Hub. It opens with "Why Roofers Care About Attic Vents" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Why Roofers Care About Attic Vents
  2. Net Free Area Is What Counts
  3. The Base Rule Is 1/150
  4. The 1/300 Exception Has Strings Attached
  5. Keep the Air Path Open and Ember-Resistant

The worked example in the course applies this lesson to a realistic job in 6 steps, starting with "Attic floor area".

What you'll be able to do

  • Explain the main idea of "Attic Ventilation: Net Free Area (1/150 and 1/300)" in plain language you can use on the exam.
  • Apply this step with confidence: "The Base Rule Is 1/150".
  • Apply this step with confidence: "The 1/300 Exception Has Strings Attached".
  • Apply this step with confidence: "Keep the Air Path Open and Ember-Resistant".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Attic Ventilation: Net Free AreaATTIC CROSS-SECTIONRIDGE VENT (upper)SOFFITSOFFITATTIC FLOOR: 60 x 30 = 1,800 sq ftNFA = air-passing area printed on the ventSIZE THE VENTS1/150: 1,800 / 150 = 12 sq ft12 x 144 = 1,728 sq in NFA1/300 exception: 1,728 / 2 = 864Upper 40% to 50%: 346 to 432 sq inTake 50%: 432 upper, 432 lowerRidge vent 18 sq in/ft: 432/18 = 24 ftSoffit vent 9 sq in/ft: 432/9 = 48 ftFor comparison, 1/150 needs:864 upper = 48 ft ridge vent864 lower = 96 ft soffit vent1,800 sq ft attic: 1,728 sq in NFA at 1/150, or 864 sq in at 1/3001/300 is allowed only with 40% to 50% of the vents high, near the ridgeVent ratings of 18 and 9 sq in per ft are example label values; read the real NFA.CRC R806.2, R806.3 and CBC 1202.2.1: 1/150 of the vented area; 1/300 only if both exception conditions are met.Upper vents within 3 ft below the ridge; 1 in airspace between insulation and roof sheathing at the vent.CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Why Roofers Care About Attic Vents

Enclosed attics and enclosed rafter spaces must have cross ventilation through openings protected against rain and snow (2025 California Residential Code R806.1; 2025 California Building Code 1202.2.1, with 1503.4 sending roof vent intake and exhaust to 1202.2). The roofer usually installs the ridge, box, and soffit vents, so the roofer is the one who has to size and count them. Unvented attics are a separate design allowed under CRC R806.5 and CBC 1202.3 when the attic is inside the thermal envelope and the listed conditions are met.

Step 2: Net Free Area Is What Counts

The code sizes vents by net free ventilating area (NFA), the area air can actually pass through after louvers and screens are subtracted. It is printed on the vent label in square inches per vent or per lineal foot, so never use the outside size of the vent. Openings must have a least dimension between 1/16 inch and 1/4 inch, or be covered with corrosion-resistant wire cloth screening, hardware cloth, or perforated vinyl with 1/16 to 1/4 inch openings, so birds, rodents, and snakes cannot get in (CRC R806.1; CBC 1202.2.2).

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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