We are in the process of replacing the original underlayment on our S tile clay roof.
Fortunately, we have the building plans for the home (built in 1988) and does provide some data with regards to ventilation.
Location: SoCal
Ventible attic area = 990 SQ.In., using the 1:300 rule, the NFVA = 475 SQ.In
Given it's single gable vent and two dormers, the house was delivered with 674 SQ.In. Now using today's preferred 1:150 rule and it brings the NFVA = 952 SQ.In ....an increase of 278 SQ.In.
Flash forward to 2025, is there any reason to modify the existing ventilation design and thus increase it?
Add more intake? Power Fan? Close the gable vent?
O'haigin took a swipe at the current roof design and recommended 18 (gulp) vents @ 1579 SQ.In.! Seems excessive. This is of course eliminates all current gable and dormer vents. I don't think I am crazy about making that many cuts into the deck.
Yes, the summers are hot, but we also need to think about the winters as well.
At the very least, I think we need to upgrade the dormers to the latest fire code mandated baffled designs.
Thoughts?
Who can I reach out to (professionally) to run the data and see if we are truly under vented?
That 18-vent suggestion gave me a laugh—unless you’re trying to launch the house like a hovercraft, I’d say that’s probably overkill. Your numbers look pretty solid for a late-80s build, and honestly, most of these older SoCal homes were built with less ventilation than the new rules call for, but they’ve survived just fine. The newer 1:150 rule is great in theory, but it’s not always practical (or necessary) to retrofit perfectly.
Upgrading dormers to fire-rated baffled vents is a smart move, especially with stricter codes and wildfire risk. I’d focus on making sure your intake and exhaust are balanced—sometimes adding a little more intake at the eaves or soffits makes a big difference. Power fans can help in extreme heat, but they suck conditioned air if your attic isn’t sealed up tight, so they’re not always the magic bullet.
If you want a pro opinion, look for a local HVAC engineer or a building envelope consultant—they’ll actually run the calcs instead of just guessing or quoting code. And yeah, don’t let anyone talk you into Swiss-cheesing your roof unless there’s a real reason.
unless you’re trying to launch the house like a hovercraft, I’d say that’s probably overkill.
Haha, yeah, 18 vents sounds like someone’s prepping for liftoff or maybe just got carried away with the hardware store sale. I get what you’re saying about older SoCal homes surviving with less ventilation—I’ve seen plenty of attics that look like they haven’t breathed since the ‘80s and they’re still chugging along.
But here’s my thing: with all the talk about wildfires and stricter codes, has anyone actually tried those green roof vent covers? Not the plastic junk, but the ones with integrated mesh and fire rating? I swapped a few in after our neighbor’s house almost caught during that big wind event last year. They were pricier, but honestly, peace of mind is worth something.
I’m curious if anyone’s noticed a difference with those or if it’s just marketing hype. Also, has anyone gone the extra mile and added insulation board under the roof deck? I keep hearing it helps keep attic temps down, but I’m not sure how much it really matters compared to just balancing intake and exhaust.
I’ve actually put in those fire-rated vent covers after a gnarly windstorm sent embers flying our way—felt like the right call, even if they cost more. Haven’t noticed any airflow issues, but I do wonder if they’ll clog up faster with all the dust we get. As for insulation board under the deck, I tried it on a detached garage and it did help with the summer heat, but honestly, balancing intake and exhaust made a bigger difference for us. Has anyone had trouble with condensation after adding extra insulation? That’s my only worry with layering too much up there...
As for insulation board under the deck, I tried it on a detached garage and it did help with the summer heat, but honestly, balancing intake and exhaust made a bigger difference for us.
I get what you mean about not wanting to cut a bunch of new holes—seems like overkill unless you’re seeing real problems. We had similar numbers (older SoCal house, gable + dormers) and never had overheating or moisture issues, even after swapping to fire-rated vents. Only thing I’d watch for is making sure your intake isn’t choked off by insulation or debris. If you’re not seeing mold, dampness, or crazy attic temps, maybe just stick with code-minimum and update the dormers like you said.