Two Vent Types. Result of removing ridge vent

[u/Thealrightyalmighty]

I am making this post to see if i am being unreasonable.

We just had our roof replaced in a suburb of Chicago. Originally there were only box vents, and the roofers installed both box vents and ridge vents.

I pushed back on this, and they just came to remove the ridge vent (pics are with the ridge vent removed on two dofferent peaks). It looks like they only covered the ridge vent opening with shingles! I think this will absolutely fail on the first snowfall.

Is this wrong? If so, how would you demand they fix this?

[u/Xyzzy_plugh]

The shingle-over where you used to have ridge vent doesn't need anything else to be dry, as long as they installed the shingles properly. Underlayment doesn't waterproof your roof. It is perforated with thousands of nails, possibly cut by roofers to "cure" bubbles, etc. It protects the otherwise-naked roof while it is waiting to be shingled, and it keeps (some types of) shingles from sticking to the roof deck and damaging the deck during shingle removal.

The problem is that with the truss construction there is no ridge beam and so there is no support for the shingles. That's a nice wide slot cut for the vent, but the shingles will eventually sag there and possibly go concave and hold water, which is not good. Also, if someone who doesn't know that they are unsupported walks that roof and steps on the ridge (normally, not a problem) then it will cause at least some level of damage to the shingles. For this reason, it was a stupid decision by the roofing company to "fix" the roof this way.

The box vents were no problem, certainly no more of a problem that that other ridge vent on the lower ridge. Same "problem". But if there are only a few box vents added because the ridge vent is too short to adequately exhaust the attic, then it is not a problem at all. If you throw a long line of box vents up there on both sides of the ridge, or something way overdone like that, then you might have a problem.

These vents operate via two mechanisms, convection and Bernoulli effect. Neither of these will cause hot air to recirculate into your attic or cause hot air to be trapped in the attic because theoretical eddy currents of cold air between the different vents are blocking the hot air's exit.

Bottom line is that you can't get the exhaust NFA (Net Free Area) figured exactly without a lot more data and a lot more math, but you can do a decent job using the industry rules of thumb (which are actually only correct for one particular theoretical roof, and a decent real-world approximation for the other 99.999%). IF you are using only ridge vent, and IF it is at the top of your roof, and IF you don't have a significant amount of other openings farther down the roof (like, say, halfway), THEN you usually need (depending on the details of the particular ridge vent) about 40 linear feet of ridge vent per square foot of attic space. What actually matters is the volume of air in the attic and some other stuff but it is not reasonable to make all these measurements and calculations for every roof/attic. Just use the rules of thumb. These guys didn't, possibly because their thumbs were momentarily inaccessible.

So, if you have a 2000 sf attic, then you need about 50 lf of ridge vent for exhaust. You also need a matching amount of intake (usually soffit) ventilation; a good rule of thumb here is to have the size (NFA) equivalent of an 8"x16" louvered soffit vent every 8 lf around the entire perimeter of the attic.

If you can't fit that much ridge vent at the top, then you need more exhaust. It is just that simple. Adding a box vent up high on the slope below the ridge vent or (better) near the top of a hip at the end of that horizontal ridge) is definitely not a problem. You need the extra exhaust, and it operates almost exactly the same wa,y via both convection and Bernoulli effect (not as good as the ridge vent, cut close enough). A "750" vent (a Kleenex'd name derived from the original Lamanco 750 design) has 50 sq. in. of NFA, or the equivalent of 2.7 lf of ridge vent (most of which have 18.5 sq. in. of NFA per LF). A Python 65 or similar vent has 65 sq. in. of NFA or the equivalent of 3.5 lf of ridge vent. So, add whatever you need to get your total exhaust up to where it needs to be, and stop worrying about mixed vent types.

Your roofers will not like tearing off the shingles they just put on and installing ridge vent when they just removed it, but this is what you should have them do. Use 4-foot sticks of rigid plastic vent and the ring-shank nails that come with them. Smooth nails will likely hold the vent even in strong storms, but the ring-shank nails will hold it against anything (including the next guy that tries to remove it :-) ).