"Dennis" wrote in message
news:Uld8f.17946$Io4.9095@trnddc06...
Hi Gary.
I was also state licensed in Ohio as an Inspector up to a couple of years
ago (couldn't afford the time to attend the mandatory seminars, being
out-of-state an all. Travel time killed me when I did attend.) Anyway, I
believe that Ohio is now using the IRC as the basis for its code. If you
have a copy (I don't have the current Ohio Building Code handy) it's in
section R905.2.7 of the 2003 IRC. (My *guess* is the Ohio code reads the
same in this area, but as I stated, I no longer maintain the Ohio code.)

R905.2.7 "For roof slopes from two units vertical in 12 units horizontal
(17-percent slope), up to four units vertical in 12 units horizontal
(33-percent slope), underlayment shall be two layers applied in the
following manner....." (followed by a long description of how to start the
felt, how much to lap, how to fasten, etc.)

For roofs of 4:12 or greater, a single layer is all that's required
(R905.3.3.2 "High Slope Roofs").
30# felt (along with other materials) is only required in the valleys.

There's also a reference for two layers in section R905.2.7.1 for two
layers
to be cemented together for ice protection.

R905.2.7.1 "In areas where the average daily temperature in January is 25
deg. or less" .. "an ice barrier that consists of at least two layers of
underlayment cemented together, or of a self-adhering polymer modified
bitumen sheet, shall be used in lieu of normal underlayment and extend
from
the eave's edge to a point at least 24in. inside the exterior wall line of
the building." So you can use either and the self-sealing is
unquestionably
the better (and that's what I used on my house when I re-roofed.)

In both cases the requirement is for ASTM D226 Type I or ASTM D 4869 Type
I
which is for standard shingle underlayment, or what's called 15# felt.
ASTM
D 1970 is also referenced for the self-adhering underlayment.

You got me curious and looked them up in the ASTM standards to see what
they
specified for weight. D226 Type I was 11.5lb./100sq.ft. min., D4869 was
8lb./100sq.ft. min. and D1970 wasn't specified. (Interesting, not even
close
to 15lb./100sq.ft.)

Best,
Dennis

Dennis,

None of the cities I work in, would allow two layers cemented together, for
a substitute. How would two layers stop ice back-up? It would just back-up
under the layer closest to the sheathing, and/or back-up on top of the layer
closest to the shingles, which is perforated by the nail shanks. Granite,
it wouldn't back up between the layers. But, the damage would be the same if
there wasn't any underlayment.

I can assure you, a double layer of felt, is not acceptable as a substitute
in Northeast Ohio, in the cities we work. I know Ohio has many towns, which
doesn't require permits to be pulled, or licensing to do work, or
inspections of certain work. But, as you know, each city must adopt the
entire code they operate under as the minimum, from there it just gets
better(usually).

As a licensed & registered GC in the cities, I'm unfamiliar with State
Licensing which is required for plumbing/HVAC/electrical.

I have to take something back, when I said I "never heard of two layers
cemented together", I was mistaken. It seems to me, about 30+ years ago, I
recall troweling Karnak 19 to adhere two layers together. Had I had a clue
back then, I would've questioned how that would stop ice from backing
underneath the top/btm side of the two layers.