Roof venting PSA

If you own or are planning to buy a home, and that home has a roof, you might want to read this. Or, at the very least, go to the end of the post for the summary, observations, and cautions.

So, I need a new roof.

Sad, that. The roof is only 17 years old. Most manufacturers warrant their asphalt shingles to 30 years, but realistically, most roofs will only make it to 25 years, so I’m a bit shy of that (40%).

Twenty-five years if you are lucky, if the roof was installed correctly, if you don’t live through extreme weather, and if your roof has proper and balanced ventilation. Even then, you need the original paperwork and receipts, neither of which we have (there were two owners before us). But, even if I had all the receipts and the installers did a bang-up job, you’d only get a pro-rated amount for the material replacement and not the labor (although some include labor).

. . . let’s step back . . . proper and balanced ventilation.

Hmm . . .

The main culprit for my roof having degraded at an accelerated rate (especially these last three years) is heat build-up due to insufficient ventilation. That causes heat to be trapped, which causes high temperatures, which causes the granules to loosen, the fiberglass to be exposed, and the roof to prematurely fail in serving its main function, namely to protect what’s under it.

Now, most people wait until there’s a problem with a roof (for example, a leak). That don’t make no sense to me, but it’s par for the course because I also notice people don’t take care of their cars until there’s a problem, or their health until there’s a problem, or their kids until there’s a problem, and so on.

Anyway, all this started innocently enough last week . . . I called four roof installers and asked them to come out and give me a quote.

Here’s a sample of my conversations with them. This is an amalgamation of four different conversations spanning hours and days.

“Hi. I’m interested in a new roof.”

“Wow! What a coincidence! We happen to do roofs!”

“…eh … yeah, I know; it’s why I called you.”

“We have a deal with the shingles manufacturers (one of the three major brands), and we can offer ten extra years atop the normal thirty years warranty.”

“Yes, about that . . . I believe for that warranty to be worth the paper it’s written on, I will need to add more ventilation.”

“OK, sure; we suggest the model 750 vents. It’s a proven design, looks fine on the roof, and they don’t cost much.”

“Great! How many will I need?”

“How about four more?”

“Is that enough?”

“Why? Do you want more?”

“No … I mean, maybe … based on the shape and size of the roof, how many do I need?”

“I don’t do math.”

(Note: this sounds like a joke, but it was the actual answer from a person who’s otherwise a great guy. I have no doubt he’s skilled in laying down asphalt roofs, but he don’t do math.)  

Later, I fire up the ole computer, hop on the Interweb, and “do math” by making rough assumptions based on guesswork. The math says the number of vents required for “proper and balanced ventilation” is . . . thirty-two vents.

I’m old, probably going senile, so I recheck my math . . . Oh, OK . . . if I make slightly different assumptions, like taking into account existing ventilation, it’s twenty-eight vents.

OK, obviously, my assumptions are wrong because I never see roofs like mine with 28-to-32 vents, right? Heck, there’s not even enough space to put them in, never mind how they would look.

So, back to the Interweb . . . I read, research, read, and read some more, adjust my assumptions (like ignoring the small amount from the existing ridge vent . . . nope . . . thirty-two. Or, a minimum of six turbine-style vents. Or, a minimum of four powered wents.

I walk outside, walk around the house, take some rough measurements . . . I ain’t got the room for thirty-two vents. At best, I might be able to squeeze in twenty. I can maybe put in six turbines near the apex (one each on the North and South facing gables, and two each on the East and West facing gables)  but, aside from looking odd (which, at this point, is the least of the issues), without enough separation between them, I’m not sure about the airflow created. Power vents might work, but I’m not really keen on them because of the maintenance and the noise (I may be going deaf at the high end, but I’m making up for it by being sensitive to every low frequency. I know I’ll hear the power vents running, especially since two would have to be directly over our bedroom), plus, again, placement advice is all over the place.

Dejected, tired, and vaguely concerned, I wait for the next day to come and have more conversations with roofing experts. That night, again, I don’t sleep well.

“So, before we begin, we need to talk about vents.”

“Oh, sure. By the way, we offer a fifty-year warranty on our roofs.”

“You warrant the roof?”

“Oh, nooo . . . the manufacturer of the shingles warrants the roof if we use all their material.”

“I see . . . about ventilation . . . do you see that ridge vent up there? That’s about sixteen feet long. I don’t know what type of vent is up there, but the best currently available ridge vent by any shingles manufacturer is rated at 0.5 sq.ft. of net ventilation area (NVA) per four-foot length. That means, at best, I currently have two square feet of venting for the roof.”

“Are you sure? Because you also have about six feet of ridge vent over the small front dormer and about thirty-five feet over the garage.”

I look at the person and cautiously mention a few things.

“Uh . . . so, the garage attic is separate from the main attic, and the dormer is small and decorative. Yes, there are opening to the main attic space, but the ridge vents over the garage and over the dormer are substantially lower than the rest of the roof. “

“. . . yes . . .”

“Well . . . hot air rises . . .”

“OK, but it’s still going to vent something.”

“Are you suggesting the hot air from the main roof will drop about ten feet and vent via the ridge vent on the garage? Because if you are, you then also have to consider the area of the attic above the garage in the NVA calculations, and that adds an additional 5 sq.ft. of required NVA. Meaning, at best, the ridge vent above the garage is venting just the attic area above the garage; it does nothing for the volume of air in the attic above the house. Plus, you then have to assume the warm air from that area isn’t flowing and rising into the main area, you know, as warm air naturally would.”

Here, I notice the usual look I get when I ‘splain stuff to people; their eyes glaze over, and I know they’re thinking of the later episode of whatever reality TV series they’re currently invested in watching.

Noticing I stopped talking, they shake from their stupor, smile, and say:

“Why don’t I grab all these measurements, go back to the office, and get back to you about the vents. Meanwhile, did I mention the manufacturer’s warranty?”

“You mean the one that says it’s void unless the roof is properly vented?”

“Wow, look at the time. Unfortunately, I have another appointment, but I’ll get back to you about the vents.”

I watch as they drive away in their nice shiny new car.

That evening, I do more research.

It’s worse than I thought.

  1. If you have a ridge vent more than three feet below another ridge vent, the lower vent becomes an intake and it’s not going to vent much, if anything. Likely, the negative pressure created by the upper vent will draw air in from the lower vent.
  2. If you have ridge vents, you can’t add other types of vents (like passive vents) for the same reason. The air venting from the ridge vent will create a local pressure differential, and draw air in from the static vents; air, and also rain, and snow.

I knew this, but I had forgotten it. I added a ridge vent the length of the roof on our Colorado home (again, because not enough ventilation). That Winter, I had to seal/block the existing six static vents because when it snowed, snow would get sucked in through those vents and accumulate atop the insulation, melting once it got above freezing, and the water dripping through the recessed lighting on the ceiling.

So, I can’t count any ridge venting in my calculations because I don’t have enough ridge for ridge vents alone to provide the amount of ventilation I need. I also may not have sufficient roof real estate near the top of the roof for the number of static vents I need. I can put turbine vents up there, which will work with a certain amount of breeze, but on hot days with no breeze, they are no better than static vents.

. . . another night of troubled sleep as I wait for the next roofing company.

“Hi. Glad you could come. Listen, I’m concerned about . . .”

“Say, before I forget, I should mention that our shingles are warranted for fifty years!”

“Sigh and double sigh!”

So, one of the companies I spoke with hasn’t returned my emails or calls in the last three days. Two others are trying to work out how we might meet the venting requirements (after confirming that my calculations are correct). And, of course, I’m doing additional research. Oh, and one company don’t do math.

SUMMARY (math and opinions ahead)

When we bought the home, the inspector told us that the roof was in decent shape and had another 5-7 years; that was three years ago.

He either lied or didn’t know what he was talking about. I’m leaning toward lying because I made the mistake of letting the realtor recommend the inspector. The other mistake was allowing the realtor to represent both us and the seller (there’s a special agreement one signs when that happens).

The seller has strong ties to the community and used to build homes (not this home), and this is a small community where everyone appeared to know each other on a first-name basis. I can’t be sure, and I won’t make the accusation without proof, but — in retrospect — the whole thing smells of old fish (and not just because of the roof, but that’s another story).

I won’t make that mistake again. Instead, if and when I’m buying another house, I’ll hire an inspector from outside the area (pay their travel costs), and the realtor will only represent me, and I’ll be doing my inspection alongside the inspector after I read everything in the link presented below.

Anyway, here’s some rough data (supporting links at the bottom of the post):

  1. To calculate the required minimum venting for a given size area of an attic, take the footprint area and divide it by 150. I have a 2,800 sq.ft. ranch, which means I need 18.7 sq.ft. of balanced ventilation. That means 9.4 sq.ft. of intake and 9.4sq.ft. of exhaust.
    1. In certain situations — and with different assumptions — you could use 300 instead of 150, reducing the amount of NVA required for “proper” venting. But, unfortunately, it only lowers my NVA requirement from 9.4 to 7.4 sq.ft.
  2. The choices for venting any gable roof are as follows:
    1. Ridge Vent (0.5 sq.ft. per 4ft ) — I would need at least 75 ft of ridge; at most, I have 16 ft. If you have multiple ridges on your roof (I do), any ridge more than 3ft lower than the top-most ridge can’t be counted as helping with venting other than as an intake.
    1. Static vent (0.33 sq.ft. per vent ) — I would need 32 vents; I may not have enough real estate for that many vents, and you can’t do double rows because of the issue I mentioned before with one vent below another. Using the alternative calculations, I would still need 28 vents. The question is, how close can you space them. At a minimum, they have to be between the rafters (and roughly 16-24 inches from the peak),
    1. Turbine vent (NVA variable depending on the wind) — assuming an average of 5-10mph wind, I would need 4-6 vents positioned near the top of the roof, but the passive turbine vents are just 10×10 inch holes, or equivalent to a 0.7 sq.ft. NVA when there’s little-to-no wind. So I’d be banking on there always being a 5-10mph wind.
    1. Powered fan vent — based on the size and flow rating, probably 3-4 vents near the top of the roof.

Since this started, I’ve been looking at houses around town and in the neighborhood. Cheap houses, expensive houses, it don’t matter none; a shocking number don’t have sufficient venting. One piece of literature lists 9 out of 10 homes are NOT properly ventilated; 9 out of 10!

Incredibly, one house near me has zero vents, and it’s a new roof in the shape of a steep pyramid. If there’s a ridge vent at the top, it’s about a foot long.

No vents anywhere that I can see.

The reason most houses have insufficient venting is because of the design of the roofs. For example, many homes have gable roofs (basically, each side is a triangle, like in the photo above) because they’re cheaper to build.

Or, they have a combination of gable and hip roofs (multiple intersecting triangular and rectangular areas, with valleys at the intersection). Regardless, depending on how they are executed (think of a pyramid), you don’t have much of a ridge for venting.

The house in the photo has — per back of the napkin calculations — at best 60-70% of the required ventilation, but probably less because of the difference in height between the top ridge vent and the lower ones.

Both those houses have new roofs (within the last 6-7 months), and both are about the age of my house, so their roofs also only lasted only 17 years.

Interestingly, that second house used to have two turbine vents (not enough) and I know the roof was replaced because it was leaking (shingles degradation allows delamination, so that could be a cause, but it could also be poor workmanship). The roofers removed the insufficient turbine vents and replaced them with insufficient ridge vents. The owners probably paid $30K for a roof with a 30-year warranty . . . with venting that voids the warranty.

You could do a simple inverted-V roof, giving you a nice long ridge to work with, but then you end up with a vertical gable (solid or louvered) at both ends (closing out the inverted V). If you have a vertical louvered gable vent you could perhaps add a fan at one end that would exhaust air drawn in from the opposite end, but only if you don’t have a ridge vent (you can have one or the other, but not both).

Here’s what bothers me . . . my house was designed by an architect and constructed by a builder. Not only does it have insufficient ventilation by design, but said design makes it difficult to put in additional ventilation.

Were those two assholes (the architect and the builder) just dumb, or did they know what they were doing and just didn’t care?

And the roofers; when they replace a roof . . . are they idiots who don’t know the basics of ventilation, or are they devious assholes who well know that while selling a supposed 30 or 50-year roof, the owners will have to replace it in less than half that time?

If you’re buying or already own a house, you should be aware of all this. Putting on a roof is a significant expense, and the cost-per-year-of-life calculation is very different if you assume a 30-year versus a 17-year lifespan for the roof. Look at your current design and do some rough estimates to see if you have enough ventilation. If not, your roof is degrading at an accelerated rate. Money out of your pocket, if you will.

Meanwhile, this coming week, I will be trying to work out the best way to vent my attic . . . because sure as politicians lie, it doesn’t seem many “experts” know what they’re doing.

Disclaimer: I’m not trying to pass as an expert. You can verify the above by going to the manufacturers of shingles (i.e. Corning, GAF, etc) and vents (i.e. Lomanco). Plus, YouTube has videos explaining most of this stuff (with examples of what doesn’t work).

HOWEVER  a lot of this info is buried and hard to find. And that’s the frustrating thing about this . . . I expected the roofers to tell me what needed to be done (and how to do it right), and not the other way around. And, I’m dealing with the supposed highly-rated and capable roofers.

Here are links to information I’ve found:

This is a link to a crap-load of information. It’s geared toward building inspectors and covers all areas of your house:

InspectAPedia — Encyclopedia of Building & Environmental Construction

The portion of interest in that amazing amount of free information:

Building Ventilation — everything you may want to know and more.

Note that within each section there are multiple links to supporting material and information (I spent a lot of time reading).

For vent information, there are various manufacturers, but the one most referenced by the roofers I spoke to is:

Lomanco — All types of vents with some requirements (and warranty information).

If you want a summary of all that stuff about venting, here’s a Lomanco PDF of the information (24 pages, mostly photos, and brief descriptions).

I consider those links sufficient to give you an idea, but you can then search YouTube for all manner of (and sometimes conflicting) information from actual roofers. BUT, I prefer going to the manufacturers for information (like, for example, the warranty details HERE and HERE). Some manufacturers of vents and shingles also offer tools to calculate venting requirements, but you can easily do your own (if you do math).

That’s it. This post has ended . . . except for the stuff below.


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