reason for felt-pad roof underlayments?

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A low slope roof is one, that is defined by the IRC code, (the code adopted by most of the country, but hte UBC reads the same), as 2:12 to 4:12. Roofs with these slopes are required, BY CODE, to have two layers of underlayment installed. They have specific installations requirements, but are not required to be cemented together. The Code reference is R905.2.7, and here is exactly how it reads:
R905.2.7 Underlayment application. 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. Apply a 19-inch (483 nun) strip of underlayment felt parallel with and starting at the eaves, fastened sufficiently to hold in place. Starting at the cave, apply 36-inch-wide (914 mm) sheets of underlayment, overlapping successive sheets 19 inches (483 ram), and fastened sufficiently to bold in place. For roof slopes of four units vertical in 12 units horizontal (33-percent slope) or greater, underlayment shall be one layer applied in the following manner. Underlayment shall be applied shingle fashion, parallel to and starting from the cave and lapped 2 inches (51 mm), fastened sufficiently to hold in place. End laps shall be offset by 6 feet (1829 mm).
Section R905.2.7.2 of the IRC, deals with ice protection. It also requires two layers of underlayment, but they must be cemented together, up to a point 25" in from the sidewall. There is an option to use what's commonly called ice shield (or simular). Anyway, here is how it reads:
R905.2.7.1 Ice protection. In areas where the average dally temperature in January is 25°F (-4°C) or less or when Table R301.2(I) criteria so designates, an ice barrier that consists of a 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 24 inches (610 ram) inside the exterior wall line of the building.
As far as the rest of your remarks, I would never make any kind of statement referring to myself as an "expert". I do however hold certificates in all phases of residential and commercial inspection and plan review from the ICC; including building, electrical, plumbing, mechanical, fire protection and energy. I'm also licensed in several states.
In reference to ARMA concerning "slope", ARMA's Residential Asphalt Roofing Manual reads (under Slope): "In general, asphalt shingles may be used on roof slopes between 4 inches and 21 inches per foot using standard application methods. Beyond this maximum slope, special steep slope application procedures must be used." To my knowledge, they do not define slope anywhere else in the book.
I don't have a copy of the National Roofing Contractors Association's publications; so i can't respond to this one. However I would point out that none of their publications are considerd recognized standards or are adopted codes books. (And the codes always govern.) Given the NRCA's reputation however, I would imagine that the Membrane Roof Systems Manual is an excellent publication.
In the IRC, Section R905.3.3.1 and R905.3.3.2 clearly details what a low & high slope and high slope roof is.
R905.3.3.1 Low slope roofs. For roof slopes from two and one-half units vertical in 12 units horizontal (2-1/2:12), up to four units vertical in 12 units horizontal (4:12), underlayment shall be a minimum of two layers underlayment applies as follows: 1. Starting at the eave, a 19-inch (483 mm) strip of underlayment shall be applied parallel with the eave and fastened sufficiently in place. 2. Starting at the eave, 36-inch-wide (914 mm) strips of underlayment felt shall be applied, overlapping successive sheets 19 inches (483 mm), and fas­tened sufficiently in place.
R905.3.3.2 High slope roofs. For roof slopes of four units vertical in 12 units horizontal (4:12) or greater, underlayment shall be a minimum of one layer of underlayment felt applied shingle fashion, parallel to and starting from the eaves and lapped 2 inches (51 ram), fas­tened sufficiently in place. Thus you can clearly see that a roof is considered "low slope" from 2:12 to 4:12, and "high slope" for any roof over 4:12.
In all fairness, a low-slope roof is listed as any roof <2:12 in the IBC, but we are talking residential roofs here.

If you re-read what I said, you will see that I state that the ice barrier was an option to two-layers, cemented together. Personally I prefer this product and if you look back in one of my earlier posts, you will see where I had it used over my *entire roof* when I re-roofed two years ago. (Less chance of ever developing a leak was my thinking.)

I agree, but it's only a minimum code requirement. If I was a roofer (and I'm not), I probably would use ice shield over the entire roof (if I could convince the home owner to pay the slight extra costs (piece of mind and all that.) But yes, 15# felt (now called Type I) is a pain the ass to work with, esp. on a wind day.
As far as 30# felt (properly called Type II), only use (again from the code) is in closed valleys. (Actually Type II or Type III is specified.) Personally I would error on the edge of durability; especially in any valley.

It should also state that it's ASTM D224 Type II. References to 30# is because that what it used to be known as and everyone still calls it by that term. Just like electricians calling nonmetallic sheathed cable as "ROMEX" (a brand name developed by the Rome Wire Company and now owned by General Cable.) It's actually a trademark, not a cable type, but thousands of electricians still refer to this type of cable as Romex.
Nice talking with you, Dennis
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