Well, I've been building for 28 years and have seen shingled roofs leak due to improper installation (staples), poor slope, slipshod tar paper installation, wind driven rain, and all other manner of problems. Like you said, if installed correctly and well, a shingled roof works well, usually. But there are always those "gotchas" that can't always be described by a post or covered in a response.
Many comp roofing manufacturers require "adequate roof ventilation" and so consulting the instructions is probalby a good idea.
Greg, you are still not making sense on this one, regardless of your experience. Attic ventilation is typically required for the reasons I spelled out. It has nothing to do with addressing potential roof leaks.
I dunno -- it seems to make sense the way I read it. Here it is again:
You need to carefully evaluate your roofing to see if it's 100% moisture proof and how non-venting construction may affect the roofing and any warranty. With shingles, you have thousand of potential moisture intrusion points and some under-sheathing ventilation as is traditional helps with that. If you have metal or some other less-permeable roofing you could probably just abandon the roof ventilation altogether,
So, to my read, he says that the OP should examine his roofing to make sure it's doing its job, since it is possible for roofs to leak. Specifically, he says you have 'thousands of potential moisture intrusion points', which I took to be penetrations from roofing nails. I think we can all agree that this is a fact. Roofs leak sometimes. And the nails are potential intrusion points. He then says to check and see what non-venting does to the warranty. I know nothing about that, but I can't see that looking into it is wrong.
He also says that attic ventilation can help with that, the 'that' being trace amounts of moisture that could come in from any of those possibilities mentioned.
Then, here's your response:
Not quite sure where you developed this concept, but it's completely wrong. Shingled roofs, are watertight when properly installed and maintained. This holds true for any modern roofing material. Ventilation is typically required to prevent the accumulation of moisture on the underside of the roof structure. This is typically the result of air introduced by leakage in the thermal envelope of the home, and air intentionally or incidentally introduced from the outside of the home via vents, attic windows and construction gaps. This air can hit the underside of the roof sheathing and condensate due to moisture and temperature differentials. Full venting of the roof structure encourages a flow of air from the soffit area to travel the full height of the underside of the roof structure and vent at the ridge, keeping the attic space dry and mold free.
So, your immediate gripe seems to be #1 that he said that moisture intrusion was possible in some cases with shingled roofs, either through improper installation, old age, or crazy weather conditions. And your response was that shingled roofs are watertight when properly installed and maintained. I don't see that those two opinions are mutually exclusive. I agree with you that shingled roofs are going to keep you dry most of the time, when they're done properly. But I also agree with Greg that it's well within the realm of possibility for there to be some small amount of moisture intrusion -- have you really never seen anything like this? I'm jealous of the quality roofers in your area, if so!
Gripe #2 appears to be that he said attic ventilation can help with that moisture intrusion. You then replied that that was not what it was for, but rather to alleviate moisture built up through the condensation effect, temperature differentials, and air leakage from the envelope. Again, I don't see why it HAS to be one way only. What he said doesn't (to me) contradict in the slightest what you said. He said (essentially, to my read) ventilation helps get rid of excess moisture, and you replied that that was wrong and went on to detail how it is designed to get rid of excess moisture. Now, that's a simplification, but just letting you know what it read like to me. I happen to agree with both of you completely.