How Roof Pitch Determines Your Choice Of Roofing Materials

Close-Up Of Brick Chimney On House Roof With Metal flashing
Average Roof Pitch. Douglas Sacha / Getty Images

When choosing roofing materials, it may seem that you have an infinite choice of any material on the market:  asphalt, composite, metal, wood shake, rubber, MSR rooled roofing. The world of roofing materials is your oyster, right?

Not so.  Many factors determine which roofing materials you can use.  One make-or-break factor is roof pitch.  

For example, you may think you want classic composite shingles on your roof, but if your pitch is below a certain ratio, you may be forced to install a different type of roofing--perhaps torch-down or standing seam metal roofing.

Roof Pitch Defined and Explained

Pitch is the term for the angle, slope, or slant, of your roof.  

Roof pitch designations are two numbers divided by a slash, such as 2/12 or 7/12.  A colon can replace the slash, as in 2:12 or 7:12.  The meaning is the same--ratio.

Numerator:  The numerator, or first number, refers to the vertical (height).  

Denominator:  The denominator, or second number, denotes the horizontal (length).  To make things a bit easier, for roofing purposes the denominator will always be 12.  Even though basic mathematics tells us that 12/12 can be reduced to 1/1, this is not done with roof pitch.  The denominator remains 12.

How To Calculate Pitch

Examples:

  • 5/12:  For every 12 horizontal feet, the roof drops 5 feet.  Another way to look at it is that for every 12 feet you go horizontally, the roof rises 5 feet.  Either way, this is the same thing.
  • 8/12:  For every 12 horizontal feet, the roof drops 8 feet.  Or you may prefer to look at it this way:  for every 8 vertical feet, the roof has 12 horizontal feet.

    Average roof pitches will be in the range of 4/12 up to 8/12.  Examples of extreme slopes range from 1/4 / 12 (almost flat) to 12/12 (sloping down at a perfect 45 degree angle).

    High and Low Pitch Examples

    Two examples at each end of the spectrum:

    • Low-Pitched:  It was fashionable for homes built in the 1960s to have little pitch except for a negligible slope to drain water.  Visually, this roof is flat.  This pitch might be as low as 1/12.
    • High-Pitched:  Roofs on Victorian-era houses were often sharply angled, steeply pitched.  Think of your classic Addams Family haunted house, with its soaring peaks, and you have a picture of a high-pitched roof.  This is one of the rare pitches where the numerator is greater than the denominator, with a slope as high as 18/12.

    1/12 to 3/12 Pitch:  Built-Up and “Torch-Down” Roofing, Standing Seam Metal

    Roof pitches in the lower ranges, such as 1/12 up to 3/12 are found in more urban, contemporary style houses and in industrial buildings and shacks.

    Roofing materials tend to be built-up roofing composed of tar paper roll and hot tar. A “torch-down” roof is one that is softened and melted into place by the workers’ propane torches.

    You cannot have conventional shingles on the low-pitched roof because water does not drain off fast enough, which would allow water to permeate within the shingles.

    Basically, you need a watertight seal to allow for the slow drainage of water.

    Some standing seam metal roofs can take the pitch down to as low as 1/4 vertical to 12 horizontal (1/4 / 12).

    4/12 to 12/12 Pitch:  Asphalt and Composite

    Asphalt shingles or composite shingles are the most popular kind of shingle and are the most serviceable type of shingle in terms of roof pitch.

    These shingles can start as low as 4/12 pitch, going all the way up to a 12/12 pitch.  Think of them as taking the middle road in terms of roof pitch--not too flat, not too pitched.

    5/12 to 12/12 Pitch:  Wood Shake and Slate

    Wood shake and slate shingles are more susceptible to leakage than composite or asphalt, because the shingles do not lock together as tightly or lay as flat those other types of shingles.

    Shake and slate shingles are good for pitches from 5/12 on up to 12/12.

    Finally, note that these are common types of roofing materials and pitches; we have not covered all types. Also, these designations are not mutually exclusive. For instance, a torch-down roof, while commonly used for extremely low-pitched roofs, can also be used for steeper pitches, if so desired.