roof: minimum pitch for shingles


We have a roof problem that could most easily be solved by doing somethign that would lower the pitch from 4:12 to 3:12. I keep hearing that 4:12 is the "minimum" pitch for a shingled roof. But, that's also what it was in the Stone Age.
I wonder if that rule still applies, if I underlaid the whole thing with a bituminous membrane (aka "ice and water shield")?
Thanks, George
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4:12 is the minimum for a "standard installation". The IBC code allows shingles down to a pitch of 2:12 with restrictions.
At 3:12 I would use 30 pound felt well over lapped and reduce the exposure. The roof may not last quite as long but it should shed water. It may also affect your warranty. Never seen one of those pay off any way. :)
Check for local code restrictions before you start. Some local codes are more restrictive.
Colbyt
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That would likely work, but using Permafelt instead of 30 lb. felt would be more effective, and somewhat pricier, but cheaper than EPDM or other membranes. HTH
Joe
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George wrote:

In conjunction w/ Colbyt's suggestion, look at the manufacturer's web site for information. IIRC, most have instructions for minimum slope installations and also minimums. I was thinking 4:12 was a little steeper than what I recalled them stating, but I could be wrong (and didn't go look).
--
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wrote:

Wish I could help. Come on, let us know why a shallow roof would solve yoiur problem?
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Hard to describe. It's an (old) L-shaped addition on the north side of the house. The parallel section (ie, along the house) has a block wall. The perpendicular section is wood frame, is wider than the parallel section, and its edge is lower ... as it needs to be, to keep the 4:12 pitch.
So, at the valley where the roofs meet, instead of neatly coming together at the corner, the one roof ends some distance up from the edge of the other. And, the second roof plane extends under that first one, creating a very shallow cavity. That cavity seems be a source of decay, AND an irresistable magnet for birds and squirrels.
If I raise the wall on the perpendicular section, the two roofs will meet at the corner, and the cavity is gone. Only raising the wall lowers the pitch.
HTH, G
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wrote:

This should be a question on a solid geometry test. Who says that high school math was worthless?

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Why not just ask the shingle manufacturer what the minimum acceptable pitch is for their product to be watertight? Might even be on their website.....
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on 8/13/2008 4:03 PM George said the following:

A Google search on 'asphalt shingles minimum roof pitch' brings up a number of sites that say 2:12.
--

Bill
In Hamptonburgh, NY
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ya gotta use more shingles with less exposure when cheating, you might only show 3" exposure to pull it off
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Been awhile but IIANM just visit your local supplier and look at a shingle bundle. I recall that information (along with almost every other answer to roofing questions) is printed right on the wrapper.
Harry K
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