Calculating Ridge and Soffit Vent 2
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- Опубликовано: 17 май 2021
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This video walks through the calculations that may be used to determine ridge vent size and Soffit Vent size. The area can also be used to determine the number of vents needed. The manufacturer Net Free Area of ridge vents and soffit vents should be used for accuracy. Please like and subscribe if you find it useful and check out other projects on my RUclips and Instructables pages. 
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I really appreciate how thorough you are. Maybe some would say, “overkill”, but I always think it’s great to know what best practice should be.
Thank you for your concise and detailed explanation of roof ventilation. I will put this to use.
I take care of the estimating at a Roof Company, seeing this just keeps it fresh in my mind- great job!
Thanks for sharing your calculations. I knew there was a formula to work from. Appreciate the breadcrumbs.
Thanks for your help. I have no one else to get important information like this for my house I'm planning.
Great lesson. Thx for sharing.
Thanks for the footnote. No website I've seen explains that 1/300 is ridge and 1/300 is soffit for a total of 1/150. I've unfortunately cut double the amount needed in the ridge and now having trouble matching that in the soffit.
I have a garage where they left it open on the front past the stub walls. When I added drywall to the ceiling to finish it for regulating temperature I added black plastic soffit channels to every spot on both sides. Did not know it might be a math requirement and maybe I coulda had a couple less. It is a three car garage and part of it is separated by that weird drywall partition.
I absolutely hate that they did not use attic trusses and have been close to spending the 25k+ to take the roof off garage and redo it with attic trusses or more likely joists for a unconditioned storage room.
Really appreciate you putting this up man. Funny you mention that the roofers don't calculate. Mine put in the ridge vent and apparently didn't notice i have a completely unvented soffit. Smh. To his credit he did offer to remove the old clunky attic fan which i declined. I'm a first time owner coming from an apartment so what do i know?
Question -If you have a ridge vent, does it make a difference where you put the soffit vents?
The house I just moved into externally has perforated vinyl soffit venting. But the wood sheathing is closed off, there's no ventilation into the attic. I am going to drill holes in it to allow for this. It has a 13' ridge vent across 8 rafter bays. But pitched roofs at either end, that are another ~4 bays each, of diminishing heights.
Should I drill the holes for the soffit intact vents just in the bays that have the ridge vent? Or all of them (or as many as I can get to)? If this, should be be the same amount of ventilation per bay? Or, more that have ridge vents? Or more that don't?
I'd think if the holes are in the bays with the ridge vent, that would get maximum flow. But then the other ~half of the attic has no direct ventilation. I'm not sure if there's a best practice for this. Thanks.
I don't have nearly enough vents. I have a 3,100sq/ft home with only 8 (6x18) mesh sofit vents and 3 roof caps. The attic is extremely hot during the summer
I'm getting a new roof, it's an old house and a the soffit ventilation on the front is now all one kind, perforated board. The soffit vent mix on the back are louver vent or rectangular long openings with screen... and the L short leg of the house has a mix also. There were issues of too much heat before the perforated boards were put in. Now it has other issues. When the new roof and new ridge vents go up I will have a contractor come back and replace / resize the soffit vents to match the NFVA value on the linear amount of ridge vent they install.
I'm in a L shaped house... main is 50ft x32ft with gables at each end of the 50ft and shorter wing is 25ft x 20ft with no gables.
QUESTION 1: What's the open area considered on the screen covered soffit openings? Old school aluminum mosquito screens.
QUESTION 2: What do the old existing gable vents (louvers with screens) count towards? Intake or Exhaust or neutral? They were originally the cross breeze method on older roof. But the a/c unit in the attic is blocking the cross breeze about 8ft from one gable, and right at the access point to the smaller opening to the short part of the L shaped house. Should I leave the gable vents alone or should I have a siding contractor remove them and close up the walls?
Thank you for putting this together. I'm in the process of getting a new roof. I currently do not have soffit vents only gable vents. My roofer has bid putting in ridge vents and soffit vents (the gable vents will be covered with new siding). I will ask if he calculated the vent area required. I really want a good job done and you have provided me with the necessary information. Thanks!
Good video
Worked for me👍🏼
Yes I have used that rule for vent calcs thanks
So if my foof total sq ft is 1700 how do I perform my calculation?
Thank you!
In the middle of my home is a single wide mobile home. On one side are three rooms following the length of the home. On the other side is a great room, porch, and apartment. The roof covers the entire home. I have no attic accessibility, so I am unsure if I even have an attic. Will your formula work in my situation or do I need to know the dimensions of the mobile home? Please see past my ignorant question. Thank you.
Great video, thank you....however you said "2" of ventilation on *EACH* side of the ridge vent. wouldn't that double your calc from 480 sq in. to 960 sq in. of total exhaust? That would amount to 9.6 vents on EACH side of the house, which seems more reasonable.
Hey Mitchell, I may have misspoke and could have been more clear on that part. By "two inches on each side of it" I meant two inches total from both sides. That was just a visual estimate, but you will want to use the Net Free Area posted by the manufacturer on the ridge vent itself. For the Cobra 3 I have pictured the NFA is 18sqin per foot or 1.5sqin per inch.
Add 20- to 30 percent if you have 6/12 or more pitch on your roof, and you cant have to much intake
Most roofers span the entire ridgeline with a ridge vent. Based on what I've read and what you state in your video this is incorrect and you are venting too much. I have a 12'x20' workshop I just built and according to the GAF calculator based on the 1/300 I only need 4' of ridge vent on a 20' long ridgeline. Going with the 1/150 it says 8' of ridge vent. This seems like a low number but it jives with what you are saying. Would you recommend 4' on each end of the ridgeline or 8' in the middle?
have you ever looked into which is more preferable between staying at 60/40 or going slightly over on the intake just to have even flow? for example on my home i fixed a humidity issue by moving from 450nfa on the exhaust side to 650nfa but have 1170 nfa intake or the correct intake for 1/150 which i couldn't quite get to on the exhaust. it just didn't seem like reducing intake just to hit 60/40 and plugging what was there made any sense. currently that's what? roughly 64% intake 36% exhaust with that 650nfa exhaust and 1170intake on a 1750sq attic deck. am i wrong to think way? or is there any drawback may be a better term?
It is better to be on the side of more intake than more exhaust.
If its 1 square foot of ventilation per 150 square feet of attic space than if you convert it to square inches isn't that incorrect?
So I ran the calculator based on the Nfa of my soffit vents vs the square footage of the attic and it said it's 580 Nfa on a 40 ft wide roof. So that's 14.5sq in Nfa per lineal ft.
So in order to achieve the 50 / 50 rule, but being sure my ridge is 50% or less so that the airflow doesn't reverse if the ridge is more area than the intake....
Doesn't that mean that my ridge vent should not exceed 580 sq in Nfa total?
So if my builder cut the opening the whole 40 ft, then it should only be about 1.25 inches wide all the way down the ridge.
It could be wider but then it would have had to be cut shorter than the full 40 ft long ridge.
But getting to my point.
I know my intake total Nfa is 580. And my builder but the gap at whatever her cut it at and I can't see it until roofing day when the current ridge vent is removed. But it has always vented fine.
So, my question is, if his gap width is appropriately sized for the known Nfa of the soffits, and it's always vented fine and there is no reason to thing the ridge is oversized, does it really matter if the ridge vent I buy is higher than 14.5 Nfa?
The ridge gap of the wood under the vent is what regulates the exhaust. A higher Nfa vent can't increase the exhaust potential of the actual gap in the ridge.
A smaller Nfa ridge vent can choke the gap, but a bigger one can't increase the flow.
Right?
My only risk is that the gap is way bigger than I need, and the current cheap ridge vent is choking it. And if I put an oversized vent on, then that would result in more ridge than soffit Nfa and then I've got the dreaded reversed air flow problem.
I wish I knew the actual square inches of the actual gap my builder cut. Damn it.
I can get a 14 nfa vent, but if my builder only cut the gap 1.25 inches wide but not the full 40 ft long, then now I am choking the exhaust where a higher Nfa vent could have maximized the exhaust potential of that shorter gap.
Grr.
Thoughts.
Basically, the gap the builder cut defined the max exhaust potential. The vent on top is just like the breather on top of the carburetor. Doesn't matter how big it is if you have a small carb. But if you have a huge cfm carb and a tiny breather then that can restrict the potential of the carb.
Why do you not divide by 2 for the area of the attic since it's a triangle?
The calculation is based on area, not volume
Your attic space area is triangularly shaped, which means, the area would not be LxW, it would be LxW/2 , correct? (I’m not sure)
Nah, it's not the three dimensional area (volume) of the space of the attic, it is the two dimensional area of the attic floor footprint alone that dictates the amount used in the formula for the required venting. Doesn't matter how high the ridge is and consequently how steep the pitch of the roof is.
easy on square house. . LOL -try modern house with 5 different elevations and jigs and jogs everywhere.
I don't understand your results. Ten 8x16-in. Soffits along your 20-ft. horizontal roofline means there'd be only 8-inches spacing between each Soffit. Our roofer said situate Soffits 6-to-8 *feet* apart. Our ranch house area is 25x64-ft., eight times bigger than your 10x20. By your calculation we'd be installing 80-Soffits per side? Seems excessive! What am I missing?
The ridge vent in this example is 20ft or 240in x 2in of vent width = 480in.
Then to calculate the vents for soffit:
480÷50NFA vent= about 10vents. 5 per side of roof.
So in your case assuming 25ft is length of ridge vent.
25ft or 300in x 2in ridge vent width = 600
Vents for soffit:
600÷50NFA vents = 12 or 6 per side.