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Roof slope is expressed as rise over a 12 inch run. Three in 12 or less is generally low slope, which changes permitted systems and underlayment requirements. Ice barriers commonly extend 24 inches inside the wall line.
Almost every roofing question starts with slope, so establish it before reading further.
How slope is expressed
Rise over a 12 inch run. A 4 in 12 roof rises 4 inches for every 12 horizontal.
A slope of 3 in 12 or less is generally treated as low slope, and that threshold governs a great deal.
What slope decides
| Slope | Consequence |
|---|---|
| Below the minimum for a system | That system may not be used |
| Low slope with asphalt shingles | Double underlayment commonly required |
| Steeper | More systems permitted, less underlayment |
| Flat | Membrane systems, with drainage design critical |
That is the whole logic. Steep roofs shed water fast and the covering only has to deflect it. Low slopes hold water longer, so the underlayment becomes a second line of defense rather than a backup.
Ice barriers
In cold climates an ice barrier is commonly required from the eave to a point 24 inches inside the exterior wall line.
Inside the wall line, not from the edge of the roof. That distinction is what the question tests, because it means the barrier extends further up a roof with a deep overhang. Slope first. Read the number.
Drainage
Low slope roofs need primary and secondary drainage. Blocked primary drains with no overflow provision have collapsed roofs under ponded water, which is heavier than people expect.
Fastening
Nailing patterns and fastener counts vary with slope, wind exposure and product. They come from the manufacturer and the code, and they are lookups.
Common questions
How is roof slope expressed?
As rise over a 12 inch run, so a 4 in 12 roof rises 4 inches per 12 horizontal.
What counts as low slope?
Generally 3 in 12 or less.
Why does low slope need more underlayment?
Water sits longer and has time to find a way in, so the underlayment becomes a second line of defense.
How far does an ice barrier extend?
Commonly to 24 inches inside the exterior wall line, not from the roof edge.
Why do low slope roofs need secondary drainage?
Ponded water is heavy, and blocked primary drains with no overflow have collapsed roofs.