Met with a potential client last night about doing some work on his house. Part of it is a complete tear off of a gravel built up roof and replace with shingles. It’s a 2/12 pitch 1950’s house in very good condition (the house is in good condition, the roof needs work but doesn’t lesk yet).
Most of the house has vaulted ceilings, but the bedrooms, on each end of the house, have flat ceilings and small attics. There are very small gable vents in the end walls, and 2″ holes drilled in the boards at the roof/wall intersection (exposed rafter tails). He was asking if we should install a ridge vent when the roof is replaced. I said that I would close off the gable vents, add a ridge vent, and add more air inlet holes under the soffits. He questiuoned closing off the gable vents. I didn’t have an answer, except that I thought I read here that having both ridge and gable vents doesn’t let the ridge vent work properly.
BTW, the actual roof work would be done by a roofing company.
“When asked if you can do something, tell’em “Why certainly I can”, then get busy and find a way to do it.” T. Roosevelt
Replies
The gable vents will short circuit the convection from the the soffits to the ridge. In other words, sometimes air will be flowing in the soffits and out the gables leaving a "dead spot in the center of the roof peak. Other times due to wind air might be flowing in one gable and out the other meaning that the air in the soffit areas is "dead" and both the soffit and ridge vents are un-used. Then some times air might be flowing in the gable ends and out the ridge giving the same dead air at the soffits.
The Certainteed web site used to have some good info with illustrations. Look at the Airvent web site too.
BTW - 2:12 shingles? That's really pushing it... Glad someone else is doing it - let them have the responsibility.
Edited 10/29/2005 9:48 am ET by Matt
San Antonio uses the 2003 IRC, and it allows shinles on 2/12 with two layers of felt. The roofer quoted Elk shinmgles, and their website says the same thing.
The only good thing about this roof is that I feel reasonabl;y safe walking on it.
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
>> San Antonio uses the 2003 IRC, and it allows shinles on 2/12 with two layers of felt. << Does that mean that other parts of texas use other codes?
Here in NC, 2:12 is the min slope, and the statewide code (modified IRC2000) requirement is the same as yours: for 2 layers of underlayment for < 4:12.
BTW - not to complicate things, but our code is set up regional such that, for example, in the mountains, I&W chield is required at the eaves. Another example is that high wind areas (the coast of the Atlantic) have specific requirements for underlayment attachment. I think this standard for IRC.
On Jan 1 2006 we in NC adopt a new IRC code (with modifications) - not sure what it is - IRC 2003?.
What Matt said.
Some shingles can be installed per manufacturer's requirements on a 2/12. Special underlayment precautions are necessary, though.
Birth, school, work, death.....................
Ditto what Matt said. It doesn't work to have both types of venting.
As to the 2/12 pitch. Where I am, it's against code to shingle anything less than a 4/12.
BILL
That's odd that the code won't let you follow the manufacturer's instructions for roofs down to 2/12!!!. I live 400 feet from the north Atlantic ocean and have done a couple of 2/12/ & 3/12 roofs for neighbours who live on the edge of the ocean where the winds are always strong and they have had no leakage. The instructions require that you use either (1) two layers of tarpaper laid in a certain arrangement with sealed lap joints or (2) a full layer of eave protection material like "ice and water shield"
I agree with you, there are certainly ways that you could do it that would be fine. That's just the way the local inspectors have chosen to apply the code.
BILL
Agreed - our code alows it - and there are ways to make it workable.
Don't get your knickers all in a twist about the attic venting- it's over rated as a solution to attic moisture problems, cooler attics-lower a/c bills, saving your roof. There are scads of research and articles on this but not many in general circulation - who reads anyway?? If you truly understand the building science/simple physics behind the problems, then you can work throughthe manufacturer's/vendors claims that they use to sell more of their product.
In my area, I see every retrofit roof getting a ridge vent now. I've asked the homeowner a few times about previous moisture in the attic and they said "no problems". If it ain't broke don't fix it!!! We're cool enough here (except for 2-3 days a year) that there is generally no A/C in houses (one I know of in my area) so attic cooling is not an issue either.
Read through the article found at :
http://www.cmhc.ca/en/burema/gesein/abhose/abhose_ce13.cfm
and you get a bit of an idea about moisture in attics and what to do to stop it..
2/12 =low slope application
for proper air flow ...IN (soffits) as low as possible
...OUT, ridge is best, or adequate sized mushroom vents close to peak/ridge.
Any other openings(gables,etc) will upset this in/out ventilation stream.
How about foam insulation and no vents at all?