Hip roof ventilation advice needed quickly!


#1

Hi all,

First I want to say I’m thankful to have found a forum on roofing issues. My contractor, while careful, honest and thorough doesn’t seem to have the best grasp on ventilation issues. He was a referral and has done good work for friends and family. Due to the mild weather we’re going ahead with a rip-off and re-lay this week. I’m in Northeast MA.

The problem is that I have hip roofs, and from what I’ve learned properly venting them can be difficult. Here are the details:

The problem is the main house which is 28x44. If I measured correctly the slope is 7/12. Ridge line is 16ft in length (measured from inside attic.) Current exhaust venting was 5 box vents (three on rear, two on either end, none in front) mounted on 8"x11" cut holes within a foot of the ridge. I have appx (11) 16"x8" louvered soffit vents around the perimeter. Current box vents I think are probably about 50" NFA so total exhaust is around 250". Current soffit intakes I have no idea, but guesstimating effective area of 15"x7" at 50% NFA, that would be about 50" per, so maybe around 550" total.

Roofer is recommending ridge vents and uses GAF cobra which has 16.9" NFA per lineal foot when hand nailed. exhaust/intake of 50/50 requires 17.5 feet which is insufficient, but I see some say 40/60 is acceptable and this then reduces to 14ft. Since I’ll apparently lose about 1ft at each end of the ridge for blending, this is right at the limit. This is using the 1/300 rule which I think is still valid for using high/low vents even when no vapor barrier exists (House uses simply blown-in insulation, no vapor barrier.)

Will this be adequate? Should I ask for SnowKing ridge vent instead which has 18.5" NFA/lft (roofer claims these have been problematic which seems to contradict experiences here.) I’m not sure power is an option since many say they can pull air from house and aren’t effective during winter months. I will be addressing the insulation issue since we have had minor ice dams in the past although none have leaked thru. I only have the next day or two to figure out what to do in the main house section. How can I get enough venting without my roof looking like some kind of commercial building from the back and sides? Would replacing the louvered soffit vents with a continuous soffit vent help?

I also have a separate attic over a 24x44 addition which uses 4 box vents and has 2" continuous soffit venting for about 110 lineal feet. Its actually a double hip so the ridgeline forms an “L”. There’s 32ft (12 + 20) of ridge so I’m not too concerned about this one. Its properly insulated with vapor barrier included and I think it should be OK with ridge vents installed in place of the box vents. My main concern is what to do with the main house section.

I’d really appreciate any advice or experience that would help me figure out what to do here.

p.s. We chose Certainteed Landmark 30yr in Charcoal Black which should give us a very nice architectural look with a good bit of grey/black variation to give some nice additional pop to the texture.


#2

hey doug G,
hang in there, there will be someone here to help you shortly.
im from florida, i cant help, but i have learned how important ventalation is where it snows. your at the rite place. check back in a few hours.

good luck.

gweedo.


#3

Ok as far as what ridge vent to use i would steer clear of GAF CObra. Cobra 2 is ok but not Cobra 1. It does not have a baffle to prevent incoming snow and debris. I would suggest GAF Snow Country or Shingle Vent II. They both flow 18 inches and are a good choice. Yes you are close on the minimum, but my concern is with the ammount of intake vents you have. According to your measurements you would only need 6 intake vents to meet the minimum requirements. You having 11 makes me uneasy to use ridge vents since the ammount of intake will far surpass the ammount of exhaust. On a home like yours using a power vent with thermostat and a humidistat would be the better way to go. They do work well in summer and winter. Summer they keep the humidity and temp down and in the winter just the humidity, in thw winter it also will keep the decking to become a sheet of ice if the insulation is not up to par. Techinically you COULD use ridge vent but with the intake being so much more then needed it would not advisable. Atleast i would not do this to a home.

Second thing on the plate is make sure that the soffit vents have baffles on them since you have blown in insulation you might have a few or all of them blocked and that wont help you at all. They are only a few dollars at the home inprovement store.

If the addition is closed off to the main part of the home then by all means mix the ventilation if not and you can access the addition from the attic of the main home then do NOT mix types of ventilation. You will turn one of them into an intake and here comes the snow in.

I know the power vents are ugly but its either that or we put several cans on the roof and then it looks really ugly. You must try and balance the airflow.

On the addition you also have alot of intake. From what i can gather with your numbers to hit the minimum you would need 29 feet of C soffit vents. ANd you have alot more than that. I would suggest a power vent there also. They can handle the additional intake without to much trouble and a ridge vetn is limited due to its static way of venting. You are right with the 60 intake 40 exhaust rule but you just do not have enough ridge to support all the intake that you have. FOr your Information in your area since it is a cold climate area i would suggest a R-49 value of insulation at minimum to help with troubles in the windet and that would greatly reduce the ability of ice damns to form. Hope this helps if you need to chat with this on the phone email me your number and i will get back with you on the phone. Oh yea almost forgot Merry Christmas and a happy new year.


#4

GTP - Thanks! This is exactly the type of feedback I had hoped to receive. I’m grateful that there are folks like you who are both kind and patient enough to take the time to keep homeowners like me from making expensive mistakes! While I like my contractor and he’s been very careful to prevent damage and always accomodating, I am a bit frustrated that he wasn’t as knowledgeable as I’d hoped on the ventilation issue.

So, after several more hours of researching yesterday, I reached the same conclusion you offered and have chosen to do power vents with thermostat/humidistat controls on both attics. Both can be positioned in the rear of the house so they should be fairly invisible from front and most sideways viewpoints. I discovered I do have some significant resin bleed on the rafters in the main section of the house so I’m pretty sure the existing ventilation is inadequate (which my HVAC working overtime to cool the air also clued me into since the exchanges are up there.)

I’m planning to use (2) Airvent Power Cool Plus PC12 (or similar) rated at 1170CFM. My roof is actually 8/12 pitch so adding the rec’d 20% to the (1300 * 0.7 = 910) CFM rule of thumb conversion gives me 1100cfm which is almost dead on. My (11) 16x8 soffit vents (~56 cu in each) exceed the recommended 3.9 sq ft of intake and as you noted I have tons of intake on the addition section. Of course, if there’s a better option than this particular model (really the only one I could find with both controls) I’d appreciate any suggestions anyone might have.

I am still a bit curious about how well/often these types of fans run in the winter using the humidistat control. So any additional comments there might help put my remaining anxiety to rest.

Thanks again for the quick and knowledgeable response and enjoy your holidays and the new year!


#5

Hi,

How long did your last roof last with the ventilation it had?


#6

Lefty - Main house section was built 1982. Original (black) three-tabs, ripoff over the last two days revealed no felt or water/ice at drip ends or valleys! Still in OK condition but some obvious curling being noted. No leaks or other structural troubles, just beginning to show signs of age.

Addition was built in 1996, same style three tabs, again no felt or water/ice barrier anywhere. While not as faded or obvious as the main house section, there are several areas where minor curling is occuring. We probably didn’t need to do this section but since we are switching to an arch style shingle we naturally wanted the entire roof to match up. I’m also guessing the original owner knew both roofs would require replacement at the same time (for aesthetic reasons) and probably put the cheapest stuff he could on the addition.

Since I was settled on ripping off the original section (now really glad I did!) I figured why not rip the addition also. I think I lucked out as the decking is all in very good condition, esp considering no felt and inadequate ventilation. These guys are doing it right, 3’ water/ice on all valleys and drip edges, and 15# felt on the rest with all the appropriate flashing, etc.


#7

I would just liek to point out that in 1982, they also did it right, since youve had no issues nor leakage in 24 years.

I am glad to hear they are doing a good job for you.


#8

They will run often as the heat escapes from the living space into the attic. If it does not run you will have a rain forest up there. Seen it too many times to count.

I agree in 1982 they did it correctly but now with the codes being different what was does not matter and has zero to do with squat. The question of how long did it last before does not matter when you figure in the basic rules the manufacturers give us to give a valid warranty. Heck it is hard some weeks to find someone that even knows how to read the back of the wrapper when i see some of these roofs. Doug is correct by making sure the ventilation is done correctly.

Also glad to hear everything is going good.


#9

Hi,

You have to follow codes. That does not mean they are correct or relevent.

Some houses are made for the front door to face north. You can follow all the codes you want and the roof will still leak. I have fixed them after a dozen other roofers did their thing. The customers ask what was the problem with the roof. I tell them their house is designed for the other side of the street. Built to code just on the wrong side of the street.

So I beg to differ. It does matter.


#10

all i know is someone just told me they have torn off a 24 yr old black three tab, in mass, that had no felt or ice and water sheild with no damage to deckin.

im just gonna stop rite there.

gweedo.

and dont be fustrated with your contractor doug.
it takes a lifetime to understand mother nature.


#11

Please explain what on earth the side of the street has to do with this home please.


#12

Hi,

If the house is designed for the front door to face north. The back of the house faces south.

If the builder sells the same house to someone living on the other side of the street it will leak. The back of the house will be facing north.

All the code books in the world can not fix it. So they keep changing codes to fix things that a code can not fix.


#13

That has entirely nothing to do with this post. I believe if a roof leaks no matter where it was designed to face it is a mistake made. Never heard of such a thing and really dont believe it.

Ventilation is vital on roofs no matter where they are or where the face.


#14

Hi gtp1003,

You told me in your post that my question was irrelevent. That only the code or the manufactures spec’s matter.

So I just put a case out there that showed where all the code books or shingle wrappers or manufactures spec’s could not fix or prevent.

If the shingles lasted the life of the shingles the first time. There must be enough ventilation. This is a starting point for designing a hip roof ventilation system.


#15

I will agree with some statements there. Never ran into a problem like you described but maybe it is possible.

My main thing i see so many contractors reusing existing specs for ventilation and voiding warrantys it is sick. What was 20 years ago is not the same now. The shingles are thinner and more prone to damages to lack of ventilations issues. Lets clear up a thing here tho. A roof will not leak if the ventialtion is wrong what it will do is cause condinstation in the attic and then make it rain up in the attic. This is not a leak just a mis installed roof.

Sadly if more contractors in the roofing business would accually follow the codes things would be better but with the price drops and corner cutting to do jobs at a lower price. And then there is the labor problem. Either way the job shoiuld be done correctly with proper ventilation the way the code and manufactures demand it to be done. THis is not the only stubbling block for rooers IWS is another thing that is put on inncorrectly everyday and not enough of it to follow the code. Either way Happy Holidays.


#16

Hi,

There are plenty of houses that do not need Storm Guard.

Another code cure-all.

Big expense to a lot of customers that do not need it. Wait till we reroof all this and have to replace all the plywood that the Storm Guard destroys. In the name of code.


#17

I will agree with that. That stuff just does not come off.