# Thread: calculating roof ventilation area

1. ## calculating roof ventilation area

I am having difficulty determining the right thing to do for attic ventilation in my high desert house I am finishing. I purchased it in unfinished condition, basically just a shell with roof, windows and doors but absolutely nothing more. It is in an area known for high winds pretty much year around. Temp ranges from mid 20s to 105.

I found a web site that was of a product but it has a comprehensive calculation guide for "balanced" ventilation. I am uncertain if I am using it right.

There are 24 spaces between the trusses. There is 2x4 blocking in each space and a space above the blocking of ½ in high x 22 ½ in wide.
There is a gable vent (not sure that is the right name) at each end wall up near the roof peak. Each one measures 12 ½ x 20 screen area (1.73 sq ft) and they are 9 inches from the top of the ceiling and 9 inches from the peak. The height of the attic is 44 inches at the peak. The floor area of the attic is 20 x 48.

The guide to balanced ventilation I got on the internet and mention at the beginning of this post says that there should be a net free ventilating area of 1/150 of the floor area of the attic, and at least 50% of that free ventilating area should be at least 3 feet above the eave.

I think that the vents at the gable would meet that requirement for the upper ventilating area (the exhaust area) 20x48/150=6.4. Since there are two gable vents and each one has an area of 1.7 sq ft they equal only 3.4 sq ft, and would not be enough for venting at the 1/150 ratio. But in the same page off the internet it says that if the gable vents are at least 3 feet above the eave vents then the ratio of 1/300 can be used. In other words, the required free ventilating area need only to be 3.2 square feet so the 3.4 sq feet of the two gable vents exceeds what is required.
I hope that I have made this understandable. I am not very good at this kind of thing.

So my question is, do you think what I just calculated is correct as far as the upper (exhaust) ventilation is concerned?

If it is then I'll proceed to the requirement for the lower (intake) free area requirements.

Starting with the requirement calculated with the 1/300 ratio then the lower vents have also to be a total area of 3.2 square feet. There are 24 spaces between trusses on each side and each one measures ½ inch by 22 ½ inches or 11.25 sq. ins. Both sides then total 540 sq. inches or 3.75 sq ft, exactly what is required!

Second question, is my calculation for the required intake area correct?

Lastly I believe that I must install some means of making certain that the insulation when put in will not block the free flow of air up through the eave venting. First of all, I will be tacking ¼ inch hardware cloth over each space between every truss to keep out most critters. Then I am thinking that perhaps I could purchase some corrugated material, like steel roof or wall material, or maybe corrugated fiberglass and cut these to a width of 22 ½ inches and tack them to the underside of the roof. I just thought – I better buy corrugated plastic so I can staple it to the roof sheathing. I suppose making the bypass about 18 inches long would carry it in over the top of the insulation, Is there some kind of material or ready made product I could buy for this?

I hope I made sense in this post and that it catches at least some interest. I don’t know where else I can go to get an opinion.

My sincere and grateful thanks to all who reply to this old man’s call for help.

2. ## Re: calculating roof ventilation area

It sounds like your attic is 48 feet in length, with a height at the peak of 44 ". I would think that area will get very hot. I think you may have an acceptable minimum at 1 per 300 for some locations, but should be shooting for an ideal of 1 per 150 or better, because of high summer temperatures.
There are varying opinions, but many experts believe that high attic temperature will cause premature shingle failure. They get cooked from below and the wind blows the granules off.
The best choice for an existing roof to increase venting is to remove the ridge cap shingles and cut a 2" or so strip out of the sheathing at the top of the ridge and install a continuous ridge vent. Set the saw depth correctly, or you will do irrepairable damage to the rafters. The vent will come with instructions. Do what they say.

There are cheap foam baffles constructed especially for the purpose of keeping the soffits open. Just ask at the builder supply store. Again, 1/2" at the soffits seems minimal. Open those slits up with a sawzall so air can flow freely.

3. ## Re: calculating roof ventilation area

You've posted conflicting details before.

If you have a 4/12 roof comprised of trusses, measuring from the bottom chord to the underside of the top chord is not correct for volume. (your 44 inch reference + your first two posts references).

The better part of the rest is math challenged. Ex. you reference a gable vent 20" high 9" from the top of the peak yet also 9' above the top of the ceiling finish below. 9 + 20 + 9 = 38, not 44, not 48.

Suggest you acquire professional assistance with your project. (purchased unfinished stalled building project, attempting to DIY it, apparently without benefit of permit, plans, or professional assistance).

Get back to the original plans and consult with a RDP familair with the local conditons, the zoning, and fire codes; and the recent changes in the California codes. You'll need a permit, check in with the local building office for the authority having jurisdiction, they should be able to clue you in on your SDS, your wind classifications, and the fire resistance ratings already designated on the original, apparently stalled, documentation/plans/site survey/etc. submitted previously.

Last edited by H.G. Watson, Sr.; 06-18-2011 at 11:43 PM.

4. ## Re: calculating roof ventilation area

Thank you for your input. I must have made a mistake in those vent measurements. I know the distance from the bottom of the bottom chord to the top of the top chord is correct. The measurement was made to determine the ratio and made before the measurements I made for ventilation, I saw no need to measure again as the article I used made no mention of volume or roof slope. Regarding the vent, I measured it twice, each time for a different reason. I measured the outside of the frame once to make an elevation drawing but I used the measurement of the screened area for the ventilation calculation. I do not see anything in the reference article I used about the volume of the attic and it does not seem to be used in the procedure. It speaks only of the floor area of the attic. I think you raise a good point there however as it would seem to be important to use the volume rather than the floor area. The only thing in the article that might be related to volume is the requirements of the height of the gable vent. Maybe that takes care of it. It does seem to be a rather scientific article and I sense that the height requiremnt of the gable and its distance above the eaves all take care of the ventialtion requirements and since that article was part of a sales pitch document for an attic ventilation system, I doubt they would overlook a thing like volume.

I see you have made thousands of posts. Wow! What do you do in your spare time?

5. ## Re: calculating roof ventilation area

To Mr. Kogel.

Do you think it would be as good to use a hole cutter and make holes in the blocking rather than using my sawzall? Either one will be really hard for me as I am rather weak.

I just thought, maybe I can use my sawzall and cut a square hole in each blocking starting at the top and going halfway down Say three inches wide and one inch deep. That would probably be a lot easier than widening the entire 22 1/2 inches. I dont think the purpose of the blocking - to keep the trusses lined up, will be compromised if I cut three or four square inches out from the top at the center. I will have to check with the code. Or do you think that is not necesary? If I cut four inches wide and one inch deep it would increase the vent area by around 30% It would increase it from 11 1/4 sq inches to 15 1/4 sq inches. Maybe it would be enough that I could do it on every other space and save a lot of pain?Thanks very much for your helpful suggestion.

6. ## Re: calculating roof ventilation area

Hi Richard,

If drilling through the blocking at the eaves, you might want to try a multi-spur bit instead of a hole saw. Multi-spurs drill a bit more aggressively, so you will have to make sure you have good footing and are well braced. I would advise against standing on a ladder to do this. Get a scaffold.

As John already mentioned, ridge vents do a great job of venting attics. When I enter attics with only eave and gable vents, the attics are generally quite warm. Those attics with ridge vents are noticeably cooler. One thing to understand is the code requirements are minimum and local conditions may have an effect on actual performance. More venting is generally better; however, it is possible to have too much of a good thing. I would get the opinion of a local building contractor or roofing contractor for your specific needs.

7. ## Re: calculating roof ventilation area

Thanks Mr. Alquist for your help. Is the hole cutting tool you refer to one of those flat bits with a centering point or is it somethng else? I have a flat bit type for 1 inch holes and I know what you mean about being difficult, its the last few milimeters that produce a violent kick even if I stop drilling when the pilot point comes out the other side and then finish the hole from the other side. No way to avoid the kick and your warning was commendable as the kick could kick me off a regular ladder. I do not have a scaffold but I do have one of those folding ladders that adjust from straight ladder to an A-shape. It is very heavy (for me) and seems very stable. Perhaps I could use that. I have used it at times while straddling it with one foot on each side of the "A." That way its kind of like riding a horse, but at times more like riding a bull!

I had been thinking overnight that if I used the sawzall I could not really cut a square hole, but could cut a V shape and maybe 2 or even 3 of them. (I have a really gnarly demolition blade) Then I thought to try to put the blade in close to one rafter with the blade horizontal, then saw to the other rafter in a dipping-down path till I was about 1-1/2 inches deep at the halfway point to the other rafter then cut upward again to the top of the blocking. I think that such an enlargement of the opening would have practically no negative effect on the strength of the blocking but would create considerable increase in air flow. But I am engaging in pure guesswork here.

I do not presently have anyone working on the house. So right now I have no one to consult with.

If the cutter you recommend is something different that what I have, I would very much appreciate hearing again from you as to what kind it is.

Thanks again to all of you guys. What a great idea this forum is !

Richard Karl

8. ## Re: calculating roof ventilation area

Instead of ambiguously making reference to some product's installation guide or calculator - why not identify and provide a link to same; you might get better answers, more to your liking.

The volume of the attic (cf) is important - in fact essential, information.

One must determine how much volume there is a space to determine effective openings to provide for air exchanges. As are direction/orientation to prevailing winds, site conditions, complicated or simple roof structure and a host of other factors.

9. ## Re: calculating roof ventilation area

Dear Mr. Watson. I have been trying to find the site that included that article but so far no luck. I only printed out the two pages that had the formulae as the rest was selling the product. You are most likely correct about the volume being important. But I believe I stated that in my earlier reply to you, and raised my speculation that the document and its formulae took volume into consideration in some way.

But someone in this forum used the same set of calculating rules in a post on this site a short while back. I remember reading it here for the first time but could not find that post, so I Googled some key words and found the site I used. When I find that site again I shall certainly advise you.

I have appreciated the gracious offers of advice from all the members who chose to offer what help they could without needing to raise many objections. Could you maybe be satisfied by what they have been so good to offer? Seems that they have covered a lot of useful ground already.

10. ## Re: calculating roof ventilation area

Originally Posted by richard karl
Thanks Mr. Alquist for your help. Is the hole cutting tool you refer to one of those flat bits with a centering point or is it somethng else? I have a flat bit type for 1 inch holes and I know what you mean about being difficult, its the last few milimeters that produce a violent kick even if I stop drilling when the pilot point comes out the other side and then finish the hole from the other side. No way to avoid the kick and your warning was commendable as the kick could kick me off a regular ladder. I do not have a scaffold but I do have one of those folding ladders that adjust from straight ladder to an A-shape. It is very heavy (for me) and seems very stable. Perhaps I could use that. I have used it at times while straddling it with one foot on each side of the "A." That way its kind of like riding a horse, but at times more like riding a bull!

I had been thinking overnight that if I used the sawzall I could not really cut a square hole, but could cut a V shape and maybe 2 or even 3 of them. (I have a really gnarly demolition blade) Then I thought to try to put the blade in close to one rafter with the blade horizontal, then saw to the other rafter in a dipping-down path till I was about 1-1/2 inches deep at the halfway point to the other rafter then cut upward again to the top of the blocking. I think that such an enlargement of the opening would have practically no negative effect on the strength of the blocking but would create considerable increase in air flow. But I am engaging in pure guesswork here.

I do not presently have anyone working on the house. So right now I have no one to consult with.

If the cutter you recommend is something different that what I have, I would very much appreciate hearing again from you as to what kind it is.

Thanks again to all of you guys. What a great idea this forum is !

Richard Karl
Hi Richard,

Yes, the bits have aggressive saw-type teeth and a centering point.
Amazon.com: Forstner Drill Bits (Multi-Spur) - 1/2" Shank, 6" Overall Length, Carbide Tipped - Southeast Tool SMS395: Home Improvement

After thinking about this a bit more, since you have a reciprocating saw, you might just want to remove some blocking and replace them with pre-drilled blocking. Around here, the lumber yards stock pre-cut and drilled blocking that is screened on one side. That way, you use the reciprocating saw to cut the nails, remove the original block and replace it with the pre-drilled block. Might be easier.

I am still concerned about standing on a ladder while doing this. You might be able to rent a small scaffold from a rental yard.

11. ## Re: calculating roof ventilation area

Oh Boy! What a great idea! I am going to the building supply store tomorrow and see if they have those blocks. WAY EASIER THAN ANYTHING ELSE FER SURE. THANKS SO MUCH. But if they do not have that then assuming I must do something else - which way do you think would be better, using the Forstner bit, using a flat wood drill that makes one inch holes, or using the sawzall to scoop out depressions in the tops of the blockings? But I sure hope I can just buy new blocking,

12. ## Re: calculating roof ventilation area

There is some good advice out there,

Attic Ventilation - Ventilation Calculator

Attic Ventilation

http://www.airvent.com/pdf/literatur...ShouldKnow.pdf

There are varying theroies on ventialation, many say use a 60/40 rule of soffit to ridge vents.

13. ## Re: calculating roof ventilation area

This reply is to Mr. Watson Sr's post wherein he refers to my ambiguity. I admit to ambiguity \a lot since I am in new territory. So new in fact that I cannot figure out how to make this reply appear to apply to his post and not any of the other ones. Can someone please tell me how that is done.? I mean if I scroll the replies to my postI get them in chronological order but the button "reply to this post" only appears under the last post. How can I get Mr. Watson's last post in front of me and then pull up the reply button?

Anyway here is my reply to his reply in which he cites my ambiguity but not my questions.

Mr. Watson, I was unable to find the place where I printed out the two pages showing how to do the calcs. I did mention that someones prior post had a similar if not identical set of calc instructions but now upon looking closer to what I used it cites as its reference the following: "An example of current minimum requirements for ventilation comes from the 2009 International Residential Code (IRC) Section R806"

And the ones cited are R806.1, Ventilation Required, R806.2 Minimum Area, and R806.3.Vent and insulation clearance.

I am pretty surprised that you did not know this.

So maybe now can you provid any help at all other than continuous criticism and sniping? If you decide to finally pitch in and help could you please put it simply because as you can apparently see, I am pretty stupid.

14. ## Re: calculating roof ventilation area

Next time, click the 'quote' button below the other poster's message. Then delete redundancies if you wish. No need to bother this time.

Height at the peak X length X width divided by 2 gives you the volume of the attic. Accuracy is not critical. These are general ballpark guidelines.

The roof will heat up in the sun despite the venting, but good ventilation will help cool the space more quickly in the evenings. Ventilation will prevent moisture from condensing on the sheathing in the winter months.

15. ## Re: calculating roof ventilation area

Thank you very much. I was wondering if a solar powered ventilator might be a good idea given that it works best when needed most. But not much good in the evening. Is there a moisture problem in the winter in the high desert? Just moved out here.

16. ## Re: calculating roof ventilation area

You may be fighting a dragon that does not exist. You mention that the area has high winds most of the time. The minimum ventilation area that is already installed may be more than needed with wind blowing the air through the attic. Rather than go to a great deal of trouble I would do a little wait and see. One option might be to put a thermostatic and moisture controlled fan in one of the gable vents, it would only run on the infrequent still days.

17. ## Re: calculating roof ventilation area

Originally Posted by Vern Heiler
You may be fighting a dragon that does not exist. You mention that the area has high winds most of the time. The minimum ventilation area that is already installed may be more than needed with wind blowing the air through the attic. Rather than go to a great deal of trouble I would do a little wait and see. One option might be to put a thermostatic and moisture controlled fan in one of the gable vents, it would only run on the infrequent still days.

I have wondered about that all along. Trouble is, I do not know if the winds are the same year around. Lately they have bee fierce - 30mph and more. I still do not have any idea if what ventilation there is in the attic now is enough. I better correct that - there is no attic now as the ceiling has not been drywalled yet.

Almost all of the posts I have received have value to me and I appreciate every one of them.

Right now my plan is to go to the building supply store and get the foam blocking to keep insulation in place and get some precut and already screened blocking boards if they have them. I will put the blocking boards on about 1/4 of the spaces and then if I need more after I have had a chance to see how it works I can always cut more in. I am not enclosing the soffits right now but may do so in time and then I have to worry about adding vents to them. That will be easy. But I first need to do what I said - before I close the space off for good. BTW do insulation contractors put those foam blocks in? I mean the ones that keep the air paththrough the insulation open .

I never thought of the thermostat to control a fan. Excellent suggestion. I suppose if the fan is not running it may interfere with natural air flow but if it does then by golly it will let me know by turning itself on eh?

18. ## Re: calculating roof ventilation area

When I had the roof replaced 10 years ago, I went into the lower attic through a hole provided by the roofers to add soffit inserts. The insulation was packed too tight, as not to allow much ventilation. I installed inserts in the upper attic, but not in every opening.

The soffits have enough openings on the bottom, but concerned if every space requires an insert. The bath fan was vented into the soffit beside a soffit opening, which was wrong, and is now through the roof. This was after noticing black on the underside of the sheathing on the typical north side, which was the opposite roof side of vent. I had ridge vent installed at time of roofing, and have now blown in more insullation in the uppper attic this year.
I am not sure if there is enough air going in from the lower soffits and up through the ridge vent to remove moisture in the attic? The inserts were hard to place from the attic, due to a 4/12 pitch and nails catching the inserts while sliding them into location. That task should be on Dirty Jobs!

I have them in 2/3rds of the openings. I have access to remove the soffit on the south side to ensure inserts are providing a gap for air, but would this be worth the effort and cost? Would the north side be out of balance of incoming air, or would the same need to be done, which is at full 2 story height, or am I being over cautious, or leave it as is?

19. ## Re: calculating roof ventilation area

Originally Posted by Dave Charbonneau
When I had the roof replaced 10 years ago, I went into the lower attic through a hole provided by the roofers to add soffit inserts. The insulation was packed too tight, as not to allow much ventilation. I installed inserts in the upper attic, but not in every opening.

The soffits have enough openings on the bottom, but concerned if every space requires an insert. The bath fan was vented into the soffit beside a soffit opening, which was wrong, and is now through the roof. This was after noticing black on the underside of the sheathing on the typical north side, which was the opposite roof side of vent. I had ridge vent installed at time of roofing, and have now blown in more insullation in the uppper attic this year.
I am not sure if there is enough air going in from the lower soffits and up through the ridge vent to remove moisture in the attic? The inserts were hard to place from the attic, due to a 4/12 pitch and nails catching the inserts while sliding them into location. That task should be on Dirty Jobs!

I have them in 2/3rds of the openings. I have access to remove the soffit on the south side to ensure inserts are providing a gap for air, but would this be worth the effort and cost? Would the north side be out of balance of incoming air, or would the same need to be done, which is at full 2 story height, or am I being over cautious, or leave it as is?
Code requires ventilation to be a minimum of 1 sq ft to every 150 sq ft of attic area, I think most would agree that a 50-50 split would be best (between high and low). Here is a some good info on ventilation.

20. ## Re: calculating roof ventilation area

Richard,

There must be a local almanac or weather service that keeps records of wind and rain snowfall etc...

Another site for info is Inspectapedia....

21. ## Re: calculating roof ventilation area

H.G. Watson, Sr., I concur with your professional recommendation; hire a venting specialist, but the OP admits he purchased a shell.
He wants to build.

Those rock and hard place recommendations by professional, is likely where mistakes are made.
In venting, home sciences will explain all the errors in the end unfortunately.
Lets hope there are none!

Richard Karl,
Happy New Year to you!

Richard; the blocking you are trying to utilize for venting space by cutting an opening, can you provide an image to humor this old fool. I forgot what trusses and eaves look like.
See, I like images rather than explanations at times.
To me, it cuts to the quick:-)
To much time can be lost on words, as well as errors made trying to unlock and understand all the back and forth.

I remember my friends grandfather. Worked until he was 84 if I recall. Lived like a popper and died with millions. Go figger.
"I never begrudge a man for smoking!" he said. "Its the dam time he takes to do it that bothers me."
Awaiting a response.
Regards.

PS: A: (3) three way attic venting, gable, eave + roof deck venting is miserable at times.
B: Someone mentioned Styrofoam baffles.
IMO Rigid material only. 1/2" inch ply gos.
An 8' sheet can be ripped to give you 16' feet of 24" inch high damming material for any insulation.
Works well and stiffens the assembly.
Or, Site-Built Ventilation Baffles for Roofs | GreenBuildingAdvisor.com
Food for thought.

Last edited by ROBERT YOUNG; 01-15-2016 at 03:57 AM.

22. ## Re: calculating roof ventilation area

Since Mr. Karl asked for help 4.5 years ago, I'm sure he's got it figured by now.

23. ## Re: calculating roof ventilation area

Ha ha ha, you never know....Richard you still there????

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