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sglt 10-26-2010 07:36 PM

Ridge vent considerations
I am reroofing my parents' 21 year old house. We are also replacing the fascia and soffit along the rakes and eaves. The original ventilation is far below the 1/150 rule. It was done via gable vents and some 8x16 soffit vents, but the net free area is about a third of what it should be. We are going to close off the gable vents, install ridge vents, and install continuous soffit vents when we replace the soffits.

On to the questions...

On ventilation amount: By installing the ridge vent (18 sq in net free per foot) and continuous soffit vents (9 sq in net free per foot), I will achieve a nearly perfect 50/50 ratio of ridge to soffit venting. The overall venting will calculate to about 30% more than required by the 1/150 rule. I assume "overventilating" isn't a problem?

On ridge vent installation: We'll either be using the AirVent ShingleVent II or the GAF CobraRidgidVent3 12"x4' ridgid vents. I can imagine that the installation will look best if the vents run all the way to the end of the roof, but with that in mind, a couple of concerns come up:

1. The instructions for these (and all ridge vent products I can find) say to cut the slot no closer than 6" from the end wall. In my case, there is a 12" rake overhang, so 6" from the end wall would be 18" from the end of the roof. Do they really mean the 6" from the end WALL, or do they just mean 6" from the end of the roof? I'd prefer not to effectively waste some of the ridge vent by installing it over 18" of non-slotted roof deck.

2. The ridge vents (either brand) come with pre-formed and reinforced holes on 24" centers so that the nails can go through the decking and into trusses. In order to make this work out properly despite the 12" rake overhang, I'll need to use a reduced length section at each end, approximately 36" long. Is this what is normally done so that the nail holes will line up, or am I missing some better/simpler solution? Or do people not normally run them all the way to the end, and just start them over the gable truss rather than over the rake?


jlhaslip 10-26-2010 09:15 PM

I am not a roofer.

Anytime I have added ridge vents they are held back from the gable end. As long as you get the first truss bay vented reasonably well you should be okay. Particularly on a 'cathedral' roof.

johnk 10-26-2010 10:12 PM

Only cut your slots a ft or so away from rake edge,but definitely for cosmetic sake,runit to the end!!!:yes:I usually go with a 60-40 ratio with ventilation(as recommended by the Home Ventilating Institute).60 intake-40 exhaust,although most recommend 50-50.

guest 10-27-2010 08:29 AM

Don't use the mesh type cobra vent. I put it on my house and had to tear it off, The cap shingles will not stay on it 'cause they can't seat on the mesh and keep blowing off. I went with the plastic kind that comes in 5' sections and haven't had any problems. The caps seat good and don,t blow off.

RoofDiagnostics 10-27-2010 02:55 PM

I am sorry, but i have to disagree with the user above. The "mesh" ridge vent is GAF Cobra Ridge Vent and installed properly, it will function well - without any issues. I don't think the user was using long enough fasteners for the ridge caps over the ridge vent.

In my area, NJ - the plastic vent (with finger holes) is problematic due to snow and wind driven rain. In fact, we have become all too farmilar with this issue. In 2009, we were contracted to replace a section of a K. Hovnanian built community. Replaced over 300 homes plastic ridge vent with GAF Cobra.

Hope this helps, KP

guest 10-27-2010 07:36 PM

well if I had known you liked it so well, I would have sent it to you when we tore it off for free. To assume we did not install it correctly is just that- an assumption. As a matter of fact, the caps were installed correctly several times, until we finally gave up and replaced it with something that did work. I can only tell my experience with it, if you like it - fine, but since you cut the sheeting back 1-1/2 inches on each side of the ridge to accomodate the cobra mesh vent, if the cap shingle doesn't hold onto it - you have a 3" hole in the top of your house, and I am pretty sure that will leak. :yes:

RoofDiagnostics 10-28-2010 06:14 AM

i am simply letting sglt know that properly installed the GAF Corbra Ridge Vent will function properly. In geographical regions that have snow, the plastic ridge vent can be problematic.

The key to making sure the ridge caps don't blow off is to make sure your fastener extends a minimum of 3/4" into the wood decking.

sglt 10-28-2010 06:19 AM

I am not choosing between roll vent vs 4' ridgid sections. I have already decided to use 4' ridgid sections, either by AirVent or by GAF. I certainly didn't want to start a debate regarding which type is better.

I just wanted input on the above mentioned specific questions regarding proper installation methods for the 4' ridgid style vents and whether "overventilating" is a problem.


RoofDiagnostics 10-28-2010 06:26 AM

sorry for the off-topic considerations, i am guilty of responding to a post without completely reading your original post.

Wish you the best in your project. I do not believe over venting will be an issue for your project.

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