GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Total Net Free Vent Area Question

stephenr | Posted in GBA Pro Help on

So, i am trying to better understand this calculation as it applies to this client’s house for the purposes of sizing ridge vents and soffit vents.  Its a 12/12 single gable cathedral roof over a 250 square foot, 2 floor vaulted space.  The top floor is a loft that partially covers the first.  I have read several articles on calculating the NFVA and have come up with this logic.  Feedback please.

The total roof covers a floor area of 10×20 for 200 square feet.  There is a 35 square foot bathroom off the first floor that the roof covers, so that is a vented area of 235 square feet.  I then divide this number by (and this is where I am a little confused) 300 to get the nfva.  Then I put it into inches and divide by a ratio of 60 (soffit)/ 40 (ridge) to get the number for each end of the vent.  I then divide it by the number of rafter bays to see how big a venting gap i need on each end, for each bay.

But wait, don’t they say that one should actually divide by 95 to get a more correct number for a steep cathedral roof (12/12) like this one. ?? 

I don’t want to create a situation where i pull moisture through my roof assembly by miscalculating the venting needs and ratios for this assembly. Any insight would be helpful.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member


    This is how our code puts it:
    "... the unobstructed vent area shall be not less than 1/300 of the insulated ceiling area."

    So you need more venting because the ceiling follows the slope than you would if that same 12/12 pitched roof had an attic.

    Maybe it's different in hot/humid climates, but I don't think you can over ventilate a roof space.

  2. rockies63 | | #2

    Notice that he said "unobstructed" vent area - which means the area of the holes in the vent mesh, not the vent size itself. In other words, you have to deduct about 20% from the vents size (due to the presence of the metal mesh) to properly size the vent. Thus a two foot square vent actually has 1.6 square feet of open, unobstructed air flow area.

  3. stephenr | | #3

    ok. So the roof area I am working with is 8' x20'. So, to double it would be 320 square feet. So its 320/300 so basically 1 square foot. That's 144 square inches divided by 30 (top and bottom of 15 bays) and I get almost 5 inches per bay. That's a 10 inch 1/2 inch vent area, maybe a little bigger on the soffit vent. That's not much. If I had a 1 inch strip at the soffit and the ridge, that would be more than enough. and over venting is never a problem. Sound about right?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |