Soffit ventilation question

Can anyone give me an idea of what the free air ventilation of this type of soffit would flow? I have no idea who manufactured it, it was here when I bought the house 16 years ago, and the house had been up about 10 years prior.

I’m trying to make sure I keep the ventilation in balance when I install a ridge vent. The total sq ft of the attic area is about 1100 sq ft. I have about 32 linear feet of this stuff installed. Any and all help is appreciated.



Anyone? How about a rough guess?

I was thinking of 6 in per linear foot…eh?

That would be 9 inches per foot.

Cool, so if I aim for 1/300, given the area is 1100 sq ft, that gives me 528 sq inches of ventilation, correct? Split that in half between the soffit and the ridge, and I need at least 264 inches at each.

Using 9 inches per linear foot, I get 288 inches at the soffit. I’ll need as close to that number at the top. Given that the SnowCountry Ridge vent flows at 18 sq inches per linear foot, I’ll need 16 feet of ridge vent, correct?

The house is 24 x 40, but the roof hangs over by 4 ft at the front, and 16 inches at the back. I assume I calculate on the roof, not just the house, so it looks like the ventilation is balanced and correct using the numbers here.

Anyone see anything I’m missing?

Here is a link
to an Alcoa preforated vinyl soffit that does 8" per lineal ft., but I’m pretty sure you would need to have every panel be perforated material to achieve this much ventilation (note the panels are only 10" wide). In your picture you have unvented panels in there, and depending on how many unvented panels you have, you may only have half as much or less ventilation than you are guessing. In my experience, the only way to get close to enough ventilation with perforated vinyl to properly balance out a continuous ridge vent is to have your entire overhang done with perforated. It’s OK to have more venting in your soffits than at the ridge, you just don’t want to have less.

Thanks Steve,

I have a total of 32 linear feet of this panel installed between the front and back of the house. That should scope out to 288 sq inches using 9" per linear foot. If I use 8", it’s 256 sq inches.

For an attic space of 1100 sq ft, I need a min of 527 sq inches. I don’t plan on running a full ridge vent. If I run 16 feet, that will balance the soffit vent with no modifications at all (pretty close anyway).

I’m using 1/300 for my calc. The house was built in 1980’s and has a vapor barrier in the ceiling.

That being said, I do have some of the original soffit vent the guy left behind, at least another 10 linear feet or so. Perhaps I should splice it in just to be safe? That would of course change the ridge length, but that doesn’t bother me.

Any thoughts?

Usually what I tell people when they call in here at Cor-A-Vent is that you can’t have too much soffit vent. That being said, there are some older, more arcane building codes still out there that basically say you can’t have any more than 50% of your total roof ventilation in the lower portion of the roof, and as much as 60% in the upper portion, which would obviously make your vent system unbalanced and void any claims you may have down the road (for both your vents and your shingles - read your shingle warranties very closely).
At the very least, make sure you’re balanced, and if there’s any way at all, try to figure out what you may have installed up there and what the actual NFVA might be. I don’t know if you can track down the original builder, or maybe go to the local lumberyard or siding supply house and see what they have - you never know, you might find it there and can continue your search.

Thank you Steve, I appreciate the feedback. Hopefully the original builder was hung for crimes against construction. I think the material was bought from Wick’s Lumber which went out of business around here in the mid 90’s.

Now, could I measure a hole using a drill bit, count the total in a linear foot and calculate the total open space provided by the holes and get the number I’m looking for? That should work, eh?

Teadious, yes but at least then I’ll get to the bottom of it.

That would be the simple, crude way of doing it, but that is essentially what Net Free Vent Area is - the measured opening of a vent, extrapolated over one ft.
One other thing to make sure of is that your insulation in the attic isn’t covering those vents or blocking the air flow into the atic. Won’t do you much good if the air can’t even get to the ridge vent.

I’ve been up there two dozen times in the past 15 years…they’re open. That was one of the first things I checked.



OK, in case anyone cares…it flows 7.75 inches per linear foot.

The holes are 1/8". There are 480 in a 12x12 section. multiplying 480 by .125 gives 60. The square root is 7.745 blah blah blah

This gives me actually less ventilation than I want, so I will be cutting in a few more pieces in the back of the house since the front has more linear feet installed than the back, and that should help the balance a little more.

Thanks to all who commented.


Sounds like the right thing to do, always make sure you at least meet minimum code requirements for your area and that your ventilation is balanced. Out of curiosity, are you re-roofing right now as well or just re-doing your ventilation situation. If a re-roof, it would be a good idea to read over the warranty for the shingles you’re intalling to make sure you’re providing enough ventilation to meet your warranty requirements as well.

I’m re-roofing in a couple of weeks. The shingles that are on now are curling under:

I was told that the roof got too hot, had too much moisture trapped (the idiots who built it vented the plumbing into the attic area…I’ve since fixed that) and that’s why they’re curling.

GAF’s warranty should be satisfied when I have a balanced, 1/300 or better ratio vent solution.

If you notice the white line along the roof’s rake edge, that’s the drip edge they put ON TOP OF THE SHINGLES.

See why I hate the guy who did this work?

:shock: That is the most ridiculous roof I have ever seen, your shingles look like ash! Do you know what kind of shingles those are (or I guess ‘were’ would be more appropriate)? I’ve seen some bad shingles before, but that roof looks like the aftermath of a volcanic eruption. Instead of doing a tear-off, you could just do a sweep-off.
I don’t mean to make light of your situation, but that is bad. Good luck with your repairs and exacting any revenge on the “builder” that did that.

Besides the curling and the plumbing issue, what they did was:

Install the rake drip edge ontop of the shingles

Failed to offset the starter course so all the butt joints line up perfectly.

Becuase of that, the only thing protecting the decking on the first 5 or 6 inches was the felt. There is no ice and water dam installed. To be fair, I don’t know that it was widely used in the 80’s. So the first foot or two I’m just going to remove and patch in new plywood for the fix. I hope none of the trusses are damanged.

There is no overhang of the shingles over the drip edge, so when all the granuals come off, they stain the front facia.

They had an “afterthought” metal chimney on the back of the house that was punched through the wall and run outsode in a chase way that was half-assed. They flashed the chimney to the roof with a gallon of cement, layed a bed like they were laying bricks, set 2x4’s into it and spiked it to the roof. That lasted until we moved in, then it started leaking. I removed that chimney this year and will be going with a direct vent stove, so no need to disturb the roof.

Nice eh?

That’s unconscionable. Maybe the roofer that did that was just out sick the day they taught roofing at roofing school. Any of you other guys see this type workmanship much, or is this an extreme case?

That’s just the roof.

I could go into the hidden behind a false wall electrical panel, the “I ran out of junction boxes, so I’ll just use a roll of electrical tape” over a wiring splice, or the drain at one end of the tub, spigot and handles at the other…subfloor not aligned on the joists so the ends flex.

Oh yeah, what fun I’m having.

That’s why as I redo one of his F-up’s, I try to get the best information and materials I can afford and do it right the first time. I usually succeed :slight_smile:

Thanks again,


I’m just jumping in for one quick comment.

Why in the heck would you limit yourself to only 16 feet of ridge ventilation.

At 18 sq inches per lineal foot, that ONLY gives you a total of 288 square inches of exhaust ventilation.

Man, you are re-roofing your home so do it right and ensure the most ventilation as possible.

For your ventilation, you can improve your intake ventilation, either with a product from the company Steve works for, called, IN-VENT from cor-a-vent or use SMART VENT from DCI Products, Inc.

Exceed the guidelines to ensure the most long term solution and enhance your interior climate comfort.


there is no need for specialized intake vents. the house is 26 years old with a big soffitt. so there is no kneewall. all he has to do to boost his intake is…make sure he has insualtion baffles and change out the soffitt panels to fully vented panels.

It would be less expensive for him to add the under shingle style of ventilation instead of replacing the aluminum soffit panels.

If he is handy and could do the soffit panels himself, then he could save some money doing as you suggest.