Net Free Area Calculations

GCManager

I'm currently bidding on a large residential rehab project. The project consists of 120 Units; 61 Buildings; with several configurations, some hip, some gable.

Need to accomodate for a required Net Free Area. Can anyone tell me some quick calculations for this to verify proper ventilation?

Ex. Drawings Read: Soffit Vent
328 SQ. IN.(MIN)
NET FREE AREA REQ'd

Ridge Vent
328 SQ. IN.(MIN)
NET FREE AREA REQ'd

A&E Exteriors

fully vent the soffit and the ridges, for maximum ventilation. I know this will cost a little more but you won't have to go over with your subs; "vent from here to there and over there to over there" and eliminates the possibility of "why in the hell did you vent that section?" just my opinion though.

mics_54

If I am understanding your question the net free area will be determined by specs in your ridge and soffit material manufacturers specifications. Other wise its a matter of area calculations. The size of vents is determined by the area/volume of attic space. The formula is in the IRC for residential applications and may be specific to geographic area. There may also be specific requirements by the oversite authority of your rehab project.
Section R806 of the 2006 IRC covers residential roof ventilation
basically roofs are required to have a ventilation equal to 1/150 or 1/300 of the attic space depending on how and where the ventilators are located.

Ed the Roofer

Applicable Code Sections

2006 International residential Code for One- and Two-Family Dwellings

• R806.1 Ventilation required
• R806.2 Minimum area
• R806.3 Vent and insulation clearance
• R806.4 Conditioned attic assemblies

2007 Supplement to the International Residential Code

• R806.4 Unvented attic assemblies
• Table R806.4 Insulation for condensation control

Ed

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mics_54

I think the formulation goes something like this.

for 1000sqft attic space

x/1000sqft=1/150
x=1000/150
x=6.66 sqft of free vent

so you need gable vents that equals 6.66 sqft
and 6.66 sqft of soffit vent vent.

or..if the vents are located in the ridge or upper 1/3 of the gables they allow venting equal to 1/2 that amount or 3.33 sqft. and 3.33 sqft of soffit vent (if my understanding is correct)