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michi098 12-12-2012 08:14 AM

Roof Venting Question - Urgent
4 Attachment(s)
Hi Everyone. First time here. I have a quick question.

Our roof had to be replaced. So I decided to add a little dormer on one side of the room for extra space in the house. Got a contractor who fit the bill, had good reviews etc... Just the other day they started construction. We had agreed on soffit vents from below (excuse my poor knowledge of roofing terms) and a ridge vent above.

During construction I talked to the guys doing the job and they told me they were just venting the dormer. I'll make a long story short, after much back and forth I ended up with a few box vents on the opposite side of the dormer but no soffit vents. They told me that they don't like putting a ridge vent in as it may leak back in on the part where the low sloped dormer roof is. I kind of get that... I was told by others that they could have done a one sided ridge vent though.

I finally talked them into installing soffit vents (that's in our contract anyway) and it doesn't seem to make much sense to have any type of venting on top of the roof if there is no place for air to enter below. Or am I wrong?

The box vents up top don't really match up with anything in the attic and seem kind of low to me. What they are telling me to do now is that when I install the drywall, to leave a little box underneath the ridge of the roof so that the air can flow under the ridge and then exhaust through the box vents on the other side.

My question is, is this legit? Will those box vents be enough to exhaust all the air? Seeing that they are not on top of the roof but a 1-2 feet below the ridge, will they still be able to pull all the hot air out? I am in Ohio, where it gets hot and cold, will the box vents in the winter also take the cold air and moisture out?

Any input is appreciated, I just don't know anything about all this, and I sort of halted the construction at this point because I'm not sure if they are just trying to find a cheap way on doing this or if this would be an acceptable way of venting the roof.

Thanks so much for any ideas and input!



I included a few pictures of the inside and outside. On the inside you can see the box vents are somewhat low and randomly spaced, I made a red line showing how they told me to build the drywall ceiling for that room so that air can vent from one side to another. Also a few pictures from the outside.
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shazapple 12-12-2012 08:47 AM

You need a minimum ratio of 1/300 of roof ventilation to house area (example: 20x30 house needs 2sqft of venting, 2sqft = 288 square inch, minimum). 50% should be at the ridge, and 25% at each soffit.

Those vents provide about 60square inches of ventilation each, so you currently have 420sqin at the ridge. Ideally you need 210sqin at each soffit. If you have perforated soffiting you should be good. I assume you are planning on installing vent baffles to the underside of the deck to provide a air space between the soffit and ridge?

As for the offset vents, it's not ideal because the ridge board is in the way, so the vent path will need to go under the ridge board as well as the insulation. You'll have to lower the peak of your ceiling by installing the equivalent of a collar tie. I don't know the height of the room so I don't know if this will be a problem.

jagans 12-12-2012 09:37 AM

First of all, what is the slope on your new dormer? You are going to have to apply ice dams flashing over the whole deck on the new dormer as it appears to be about 2/12

Secondly, do not use fiberglass bat thicker than 3.5 inches in your ceiling joists. You need to leave space for air flow over the top of the insulation, and it looks like you have 2 x 6's there.

The best thing for you to do is cut back the sheathing at the ridge of the dormer about 2 inches, and install a good ridge cap with rain baffles. The cap must be set in two continuous 3/8 beads of sealant on the low slope side. when your slope gets this low you have to worry about driving rain getting in and under the ridge cap. Im putting this on my house right now.

ShingleVent® II
ShingleVent II is a shingle-over ridge vent. The vent is installed across the ridge of the roof. The vent is 12" wide so that matching shingles can be installed over the vent, allowing the vent to appear almost invisible from the ground. Available in Black, Brown, Gray and Charcoal.

Getsiv 12-12-2012 09:56 AM

Other than the non-ideal venting, since using box vents would require an "attic" of sorts, my question pertains to the walls. It appears you are using synthetic roofing underlayment for your walls, did you check if that is an allowable install per the manufacturer? I know with the products I use, it is specifically mentioned not to do this. Also, on the sidewall, it appears the roofers ran tar paper up the wall 18", which is a good idea, but they ran it over the wall paper, creating a reverse lap for water.

joecaption 12-12-2012 09:57 AM

Sure looks like your not going to have room for the suggested amount of insulation unless you add some spacers to the rafters to made them wider.
R-13 plus the foam or plastic baffles would be less then 1/2 of with you need in there.

Storm and Ice is not required for a shallow pitch, though it's not a bad idea, just far more expencive.
What is suggested is two layers of felt or at least a 50% over lap.
The shingle will also have less exposure. Just go on the shingle manufactures web site to see there exact install directions.
I also would have used 5/8 OSB not 1/2. I've seen to many low pitch roofs sag between the rafters.

Make sure they install window tape around those nailing fins on the windows befor installing the siding.
I'm not in love with the way the flashing was install under those windows. What every they used on the old roof should have been installed first, then the wall of the new dormer gets covered. That way the moisture behind the wall would run past the seam not under it.
Also looking at the way that facias is cut on the sides looks funky at the peak of the roof.
Make 100% sure the facia does not come within 1/2 of the shingled roof. If it's to tight there's a chance the coil stock may not fit and you'll end up with some rot there.

jagans 12-12-2012 10:26 AM

To Joe Caption
Joe, look at the size of this dormer. Yes SBS peel and stick is more money, but we arent talking about a thousand squares here, we are talking about a very small area. The cost to upgrade to ice dams flashing will be negligible, compared to the benefit. He has framed with Douglas Fir, so the guy is not cheap. We are talking maybe a C note to go to ice dams all over. Well worth it in the long run

He should also cut down on the exposure of the shingles by an inch. If he does what I say he will be fine. He would have been better off with three tabs because they lay a lot flatter, but it is what it is.

joecaption 12-12-2012 10:34 AM

Oh I agree with you and would do it that way if I had to do it.
Just not required to meet manufactures guide lines.

jagans 12-12-2012 11:11 AM

Thanks Joe. Guidelines are just that, guidelines. We are talking about a specific situation here.

Re Depth of Rafters: They probably went with 2 x 6's to get as much interior headroom as they could. He could cut 3 inch Isocyanurate in between the rafters and get about R-18 up there but that will cost more too. There are always trade offs.

Windows on Wash 12-12-2012 01:40 PM


Originally Posted by jagans (Post 1071714)
Thanks Joe. Guidelines are just that, guidelines. We are talking about a specific situation here.

Re Depth of Rafters: They probably went with 2 x 6's to get as much interior headroom as they could. He could cut 3 inch Isocyanurate in between the rafters and get about R-18 up there but that will cost more too. There are always trade offs.

I would go the ISO route as well. Rip down strips out of the board 1.5" and fill up the remaining 4" and cover the joists with the same ISO prior to drywall.


jagans 12-12-2012 03:24 PM

I think what Eric (WOW) means is rip 14.5 inch wide strips (Assuming your rafters are 16 inches on center) of 1.5 inch iso and slide two layers with staggered joints up into the wells between the rafters. This eliminates thermal bridging at joints. leave the 2 inch space above the Iso for air flow, and fill gaps with foam Stuff (red can, HD) You could add more iso inside on the bottom of your rafters but you would lose head room, and nailer thickness at perimeters. I think you would be best to stick with (2) 1.5 inch layers between rafters and staple poly to the bottom of the rafters for a vapor barrier, then screw your drywall to the rafters. Wen you screw the drywall to the ceiling use two screws about 5/8 inches apart at each fastening point. This gives you much more holding power because you increase the cone of influence considerably.

Thanks Eric, I got lazy on this one.

Gary in WA 12-12-2012 06:22 PM

I would have them install the ceiling joists before the ridge board (structural beam required) sags any more, it appears to be 2-3" lower in the center of the non-dormer side. They need to be bolted and engineered if not using a structural ridge beam, pages 46,47, figs. 30, 31:

Please tell them to jack the ridge up for straight line, if not level line. Your dormer looks over the 12' rule, and that is not even minimum code... hold a piece of paper up on your ridge in the last picture, against the computer screen, yikes! Or under the new rafter bottoms in the second (inside) picture. Where are the ridge supports (existing until the ceiling joists are installed)? What is holding your roof up? Why didn't the new dormer header run to the end of the dormer wall to bear on the doubled/tripled standard joists rather than bear on the floor joists there? Did they double the floor joists at the window center bearing for the new roof/wall load that appears 2' off direct exterior wall bearing?

PS. Is you OSB gaped 1/8" between sheets, on the long edges seen from below?

Get a knee-wall up under the old rafters right away and add some temporary bracing to the ridge board untill you either get a permit or a structural engineer to fix existing.

michi098 12-16-2012 10:14 PM

First of all, thanks everyone for your replies! I was out of the country for a few days, so couldn't check on this.

Well, I just heard back from the builder. He said they will close the box vents up and install a one sided ridge vent. I guess that's mostly good news.

Now what some of you are saying, about the sagging roof, that has me worried. I need to check on that. On the picture, you are correct, it seems like the roof is sagging. I need to go out in daylight and check this myself. There may be some camera lens distortion involved. I crudely tried to measure the roof height comparing the ends and middle, and it does seem that the center part is about 1.5" lower than the ends.

Unfortunately I do not understand any of the professional language you guys are using. No clue what ceiling joists are etc... It really sounds like I need to get an independent professional in here to tell me what is going on. Any pointers on where and how I can locate such a person?

Thanks again everyone!

michi098 12-16-2012 10:15 PM

Sorry, posted twice, disregard this...

jagans 12-17-2012 09:20 AM

I think GBR is correct. They should have removed the drywall or plaster, then installed a knee wall under the right side before removing any rafters on the left side. You need a moment connection at that ridge otherwise. What is keeping those new rafters from pushing the new vertical wall outward? Did you get a building permit for this job? Sequencing is important when you are removing support.

michi098 12-17-2012 02:37 PM

They did get a permit for this, yes. But that is about all I know.

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