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")?
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
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).
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.
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.
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