How much to cut roof for ridge vent?
CeltsFan
| Posted in Construction Techniques on
We’re having a ridge vent put on our house as we finish a previously unfinished attic. So, they are cutting back the roof decking, to put on the ridge vent. However, it looks like they are hardly cutting it back at all. Maybe an inch on both sides. I can barely see the ridge vent while standing in the attic and lookup up at the ridge. It doesn’t seem like it is going to provide much ventilation at all.
Do ridge vents really work? Do we need to put in gable vents or some kind of additional venting? How would we be able to tell whether we have enough ventilation (other than finishing everything and finding that the attic is unbearable hot)?
Replies
If you can find a copy of the instructions it should say how much to cut back the sheathing. Generally it'll be about 2-3 inches per side for a 4-6 inch opening.
You also need eave venting. Generally you should not mix gable venting with ridge venting.
No gable venting with ridge vents, they cause negative preasure in combo with ridge vents. You need soffit vents.
Ridge opening depends on the style / manufacturer of the ridge vent.
Have used some rigid vents (poor choice) that only would handle a 2" opening.
Check the directions.
Do ridge vents really work? - yes, when coupled with the appropriate amount of soffit venting.
Do we need to put in gable vents or some kind of additional venting? No especially not gable vents as they will short circuit the air flow from soffit to ridge and defeat the purpose.
You may need additional soffit vents. Be advised that a vent cover which requires a 4"x5" hole to be cut in the soffit will not provide 20 sqin of net free vent area - it will be considerably less - that is the rub.
How would we be able to tell whether we have enough ventilation (other than finishing everything and finding that the attic is unbearable hot)? Experienced roofer who does the math and does not fudge the results.
The ridge vent instructions will specify the net free vent area per linear foot and the width of the opening to be cut at the ridge. Soffit vent covers will also provide the NFVA. Balance the ridge area and the soffit area and the system works well.
Jim
You probably have a ridge that is blocking most of the free open air space that you are trying to achieve. With roof trusses, there isn't a ridge and the one inch on both sides provides plenty of free air space. With a ridge, a one inch cut doesn't give you nearly the same amount.
We always beveled the ridge, framer style, when we retro cut a ridge vent in. What is "framer style"? We chipped away at the opening with our framing claws. Usually, it gets the job done but it ain't pretty.
On new work, I typically dropped the ridge and inch or so. That meant that the tips of the rafters would be proud of the ridge and when we cut the sheathing back one inch, we got full benefit from that inch on both sides.
Bob's next test date: 12/10/07
I forgot to attach the picture of the dropped ridge. Bob's next test date: 12/10/07
You might wind up with a hot finished attic anyway, depending on how well your ridge and soffit venting is done, and how well your roof is insulated.
What is your complete attic ceiling construction plan?
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