Roof pitch explained
Pitch describes how steep a roof is. It drives how much surface you are actually covering, which materials you can use, and how safe the roof is to walk — so it is worth understanding properly.
Rise over run
Pitch is expressed as rise:12 — the inches of vertical rise for every 12 inches of horizontal run. A 6/12 roof rises 6 inches over each foot of run; a 12/12 roof rises a full 12 inches, a 45-degree angle. The 12 is always the run, so only the rise number changes. This ratio is the standard way roofers, codes, and manufacturers all talk about steepness in North America.
Three ways to express the same slope
The same steepness can be written three ways: as a ratio (6/12), as an angle in degrees (26.6°), or as a grade percentage (50%). They all describe one slope. North American roofing uses the rise:12 ratio almost exclusively, but plans and some materials use degrees or percent, so it helps to convert between them. The pitch calculator does all three at once, and each pitch page shows a scale diagram of that slope.
Why pitch changes your material count
A steeper roof has more surface area for the same footprint, because the sloped face is longer than its shadow. That is captured in the pitch multiplier: footprint times the multiplier equals true area. A flat-ish 4/12 roof multiplies footprint by about 1.054; a steep 12/12 roof by about 1.414. So two homes with identical footprints can need very different amounts of material purely because of pitch — which is why pitch is a required input, not an optional one.
Pitch sets which materials you can use
Every roofing material has a minimum pitch set by building code, because roofs shed water by gravity and too shallow a slope lets water work back under the courses. Asphalt shingles need at least 2:12 (with doubled underlayment below 4:12); tile needs about 2½:12; slate and wood shakes need 4:12; standing seam metal can go far lower, down to about ¼:12. Note that even where code or a manufacturer permits a low slope, organizations like the NRCA may still advise against shingles on very shallow roofs.
Walkability and safety
Pitch also decides how the roof is worked on. Up to about 4/12 a roof is generally comfortable to walk with care; 5/12 to 9/12 calls for caution, proper footwear, and often roof jacks; steeper than 9/12 typically requires staging, harnesses, and fall protection. Steeper roofs cost more to install for exactly this reason. Each pitch reference page notes the walkability for that specific slope.