how to determine NFA for soffit vent? (Net Free Area)

[WIHD]

NFA = Net Free Area see image

Working on correcting some attic ventilation but do not know the NFA of each soffit vent to complete the work.
Any idea how to find or at least estimate the NFA of each vent so I can complete vent calculations etc?
(home is not a new build etc)

Also unsure where to locate venting like this - its has 3 ribs/sections, locally most seem to have 4 ribs, are vinyl and only 2 middle ribs are vented.
These appear to be aluminum or galv steel. Image is 1 panel.

Thanks for any input.

​

 

[PCustoms]

Find a comparable vented soffit design and look at the specs, it will be listed.

For the most part this style will be within a few #s between brands.

 

[jack stand]

^^^ X2
Even though you have aluminum pictured a similarly "punched" vinyl panel should be very close.
Edit; watch the numbers that they give you. 95% of all the soffit that I've seen is a 12" panel.
Keep in mind that most soffit panels are only 12" vs your 16" for your math.

 

[WIHD]

Find a comparable vented soffit design and look at the specs, it will be listed.

For the most part this style will be within a few #s between brands.

thanks, I did think of this but holes in diff brands are all diff sizes/designs etc etc etc so.....though its prob what I'll have to do.

^^^ X2
Even though you have aluminum pictured a similarly "punched" vinyl panel should be very close.
Edit; watch the numbers that they give you. 95% of all the soffit that I've seen is a 12" panel.
Keep in mind that most soffit panels are only 12" vs your 16" for your math.

Thanks, yes I see 12" around town and online.

One issue...if I wanted to add another vent or two, its almost an all or nothing as I cannot match what is currently there (3-rib) so I'd have to put in a different style to add vents if needed etc. All is still in process...

 

[WIHD]

I found somewhat of a match to estimate - though what is the LENGTH of the panel in the video? See image and vid at :47

I assume the 12" width is as I marked but its unclear on every manufactures website what the LENGTH dimension is from the wall of the house (f-channel) out to the fascia. Is this also 12"?

He mentions "1 sq foot" in the video but wanted to make sure I was not misunderstanding.

Thanks for any insight.

see :47 in video link here on Youtube (click here or image below)

Link to product (sketches bottom of page 13)

https://qualityedge.com/wp-content/uploads/2025/02/SOFMINIBROCH24_2.pdf

 

[mm08822]

If you must know your existing soffit
Just measure an open rectangle, find its area and divide by 2. (2 is swag)

Multiple that result by 8.5. (Avg of 8 in one row and 9 in the next.)
Count the # rows in each section or per inch.
Keep going until the entire area of the desired section is accounted for.

Sounds tedious but it isnt really.

 

[WIHD]

? (L*W)/2 is the area of a triangle (?) then multiplying that by 8.5 (per row) means....what exactly

 

[mm08822]

? (L*W)/2 is the area of a triangle (?) then multiplying that by 8.5 (per row) means....what exactly

Use L xW of one punched rectangular opening. Since the punched opening is only pushed in deep, it is not fully open. Use 50% of that area (50% = 1/2).

One row has 8 punches. The next row has 9.
You could count all of the rows with 8 and then count all rows with 9. Or just simplify and say each row has 8.5.

So L x W x 0.5 x 8.5 equals the opening per row.
Now count how many rows per section.

 

[WIHD]

Use L xW of one punched rectangular opening. Since the punched opening is only pushed in deep, it is not fully open. Use 50% of that area (50% = 1/2).

One row has 8 punches. The next row has 9.
You could count all of the rows with 8 and then count all rows with 9. Or just simplify and say each row has 8.5.

So L x W x 0.5 x 8.5 equals the opening per row.
Now count how many rows per section.

Thanks, I see what you're saying. Its weird as all diff soffits online have diff holes etc so....was trying to match mine to something I could find like in the post above w/ video.

 

[jack stand]

What pattern did they use? Solid/vented panels alternating across the whole wall?

 

[WIHD]

What pattern did they use? Solid/vented panels alternating across the whole wall?

one panel is a vented triple-5 like in first photo, then two non-vented triple 5's that flavor. Again, 70s home, not new build.

Plan to pull some soffit this week to peek under and see what is going on, if stuff is blocked etc.

 

[oldcarpenter]

Just calculated net free area on a job this week.
I am in Florida, here all material subject to wind must have product approval for code.
My particular soffit is kaycan triple 16”
florida building code.org
product approval number FL 46503.3
Although i
this is may not be your exact profile, most of the slotted vents I have seen are very similar in size and spacing.

 

[WIHD]

Just calculated net free area on a job this week.
I am in Florida, here all material subject to wind must have product approval for code.
My particular soffit is kaycan triple 16”
florida building code.org
product approval number FL 46503.3
Although i
this is may not be your exact profile, most of the slotted vents I have seen are very similar in size and spacing.

thanks, though I found a 'similar' panel to mine and estimated my NFA.

lots of conflicting info online, or lack of info - many soffits are sold via "what they look like" and nowhere on their product page is the NFA listed.