Residential Building Inspector B1Roof-Ceiling ConstructionMedium
An inspector is evaluating the installation of a new roof covering. The roofing contractor has installed a continuous ridge vent system. What is the minimum net free ventilating area required for attic ventilation, expressed as a ratio of ceiling area to vent area, when both soffit and ridge vents are provided?
- A1:300
- B1:600
- C1:100
- D1:150
Show answer & explanationAnswer & explanation
Correct answer: A. 1:300
When both soffit (eave) and ridge vents are provided, and the ventilation opening area is split between the soffit and ridge, the minimum net free ventilating area can be reduced to 1:300 of the attic floor area.
Why the other options are wrong
- B. 1:600 is not a standard IRC ventilation ratio.
- C. 1:100 is not a standard IRC ventilation ratio for attics.
- D. 1:150 is the general requirement when only one type of vent is used or when the 1:300 exception is not met.
Attic Ventilation Ratio (Balanced)
The minimum ratio of net free ventilating area to the area of the vented space (attic floor) when a balanced system of continuous soffit and ridge vents is used.
- Applies when both soffit and ridge vents are present.
- Requires the ventilation area to be split between the two types of vents.
- Reduces the overall required ventilation area compared to unbalanced systems.
Memory trick: Balanced flow gets the 300 deal, unbalanced needs 150 for real.