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minimum pitch for shingles

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MarkJames

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Discussion starter · #1 · (Edited)
What minimum pitch do you advise for a new shingled roof? 4/12 or less?

We're trying to figure out an addition to extend the rear of a 34' wide cape cod that has a 12/12 pitch. (Front to back is 26') This is going to be full width, or maybe extending off just inside the back corners. I think some kind of shed dormer design would look good, but there's an issue with having a peek-a-boo peak extending above the original roofline and looking stupid.
 
#3 ·
4

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#5 ·
I'm with Griz. 3/12, on a rare occasion 2/12 --- both double coverage paper.

What is double coverage paper ? 18" pc followed with 36" over that, then maintain 18" exposure.

Sometimes it's easier to just fold in half the starter paper strip.
 
#8 ·
Just use water and ice up the whole roof.
 
#9 ·
The code allows 2:12. Shingled roofs between 2:12 - 4:12 need double coverage underlayment. I personally never felt comfortable less than a 3 and the 3's I have done I reduced the exposure to weather.

Ice shield is added protection but the main concern is blow offs from wind picking up the edges
 
#11 ·
Owens Corning is 3/12, but I concur with the other replies in that anything less than a 4/12 pitch requires double underlayment. I use Ice and Water under any roof 4/12 or less. On a 3/12 pitch, I use a 4" exposure to eliminate the water running up the shingle to the nail holes. If possible, upgrade the shingles to dimensional.
 
#12 ·
The lower your slope the greater the member size due to loading. You're in Arlington so you have snow loads.

As to minimum slope for a particular shingle, you need to read the manufacturer's instructions. If the code says 2:12 minimum but the shingle you select says 4:12, then you can't install that shingle on a 3:12, for example.
 
#13 ·
Leo G said:
Just use water and ice up the whole roof.
Click to expand...
Do not use I&W on the entire roof. A roof system needs to breathe to function properly. Covering the entire roof with Grace would block air flow even worse than the cheap granular option.
 
#15 ·
NJGC said:
Please explain....

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This is primarily a matter of opinion. Some guys, myself included, feel I&W chokes off the ability for a roof system to pass air and moisture through. It brings the breathability down significantly and if the roof is not vented properly, this will expedite the plywood deteriorating.

I personally haven't stripped a roof fully I&W'd but the times I've tried to pull shingles off cooked Grace on the first 3', was a horror show and resulted in the plywood coming off the rafters as well, in de-laminated chunks I might add.

I'm not saying it would be a total disaster, I just personally wouldn't do it on any of my roofs. The Grace products and synthetic underlayments are still fairly new to the industry. Decades to come will tell us more about after effects and time passing.
 
#16 ·
MattK said:
This is primarily a matter of opinion. Some guys, myself included, feel I&W chokes off the ability for a roof system to pass air and moisture through. It brings the breathability down significantly and if the roof is not vented properly, this will expedite the plywood deteriorating.

I personally haven't stripped a roof fully I&W'd but the times I've tried to pull shingles off cooked Grace on the first 3', was a horror show and resulted in the plywood coming off the rafters as well, in de-laminated chunks I might add.

I'm not saying it would be a total disaster, I just personally wouldn't do it on any of my roofs. The Grace products and synthetic underlayments are still fairly new to the industry. Decades to come will tell us more about after effects and time passing.
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I tend to disagree osb or plywood does not need to "breathe" that's just silly.

Sent from my SAMSUNG-SM-G930A using Tapatalk
 
#17 ·
MattK said:
This is primarily a matter of opinion. Some guys, myself included, feel I&W chokes off the ability for a roof system to pass air and moisture through. It brings the breathability down significantly and if the roof is not vented properly, this will expedite the plywood deteriorating.

I personally haven't stripped a roof fully I&W'd but the times I've tried to pull shingles off cooked Grace on the first 3', was a horror show and resulted in the plywood coming off the rafters as well, in de-laminated chunks I might add.

I'm not saying it would be a total disaster, I just personally wouldn't do it on any of my roofs. The Grace products and synthetic underlayments are still fairly new to the industry. Decades to come will tell us more about after effects and time passing.
Click to expand...
I am always papered in roofs and even double covered them with full ice shield.
It's redundant but without that bond break your screwing the owner with future headaches "cooking" the product to the roof like you say.
 
#18 ·
Like I said, it's definitely a matter of opinion and preference. All are entitled to continue the install practices that work best for them.

I always fall back on a proper install will trump everything else. There wasn't any Ice & Water 70 years ago and 3 tabs were paper thin, yet guys still found ways to keep snow and water out of attics.
 
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