What Is Roof Pitch?

How to read a pitch number

The second number is always 12. It stands for 12 inches of horizontal distance, measured level, not along the roof. The first number is how many inches the roof climbs over that foot. So 4/12 rises 4 inches per foot and 12/12 rises a full foot, a 45-degree roof.

The same steepness can be written three ways. Builders and building codes use the ratio (4:12). Engineers often use percent, which is rise divided by run (4 ÷ 12 = 33 percent). Some product sheets use degrees. They describe the same angle; only the unit changes.

The formula shows why one number does so much work: it sets the angle, the roof area and the materials the code allows.

How pitch changes attic airflow

The building code sets how much open vent area an attic needs: 1/150 of the attic floor area, or 1/300 if the vents are balanced, with 40 to 50 percent of the vent area high on the roof and the rest in the bottom third of the attic ¹.

The high vents must sit no more than 3 feet below the ridge, measured straight up and down ¹.

Pitch changes how that works in practice. A steep roof has a tall attic, so there is plenty of height between the low intake vents and the high exhaust vents, and warm air rises out well.

A shallow roof has a short attic with less room to work with. Our attic vent calculator works out the vent area for your attic size.

Sources: [1]

Common pitches in degrees, percent and roof-area multiplier

Worked example: why pitch changes the size of the job

A plain gable roof covers a house with a 1,600-square-foot footprint plus overhangs, 1,800 square feet measured flat. The roof surface depends on the pitch:

The same house needs about a third more shingles at 12/12 than at 4/12, before waste. That is why two quotes for the same house can list different roofing squares: check that each bidder used the real pitch. Illustrative footprint; the multipliers are exact geometry.

When one house has several pitches

Many houses do not have one pitch. The main roof might be 8/12, a back porch 3/12 and a dormer 10/12. Each flat surface of the roof, called a plane, is measured and priced on its own pitch.

This matters most on the shallow parts. A porch roof at 3/12 can still take shingles, but it needs the double underlayment described above.

A porch at 1½/12 cannot take shingles at all: it needs a low-slope covering such as a membrane, roll roofing or standing-seam metal ².

A quote that prices the whole house as one shingle roof may have missed this.

Benefits of each pitch

The downsides at each end

• Very low pitch: water drains slowly, so any small gap in the roofing lets water in. Below 2/12 you cannot use shingles at all ⁵.
• Low pitch, 2/12 to 4/12: shingles are allowed but need double underlayment, which adds material and labor.
• Steep pitch: more roof surface to cover, slower and riskier work, and roofers need safety gear, all of which raise the price.
• Very steep pitch: some materials need extra fastening or special installation methods; check the maker's instructions.

Where pitch shows up on a roofing quote

Pitch appears in three places. In the measurements, as the multiplier that turns flat area into roof surface.

In the labor line, because steep roofs need roof jacks or harness anchors and go slower, so contractors often price them higher; ask whether a steep-roof charge is included.

And in the underlayment spec: an asphalt-shingle roof between 2/12 and 4/12 must get two layers of underlayment laid in a specific overlap ², which costs more than one.

If a quote does not state the pitch it was priced on, ask. A roof measured at 6/12 that is really 9/12 is short of material before the crew starts.

How pitch is measured

You do not need to go on the roof. The safest way is from the attic:

1. Hold a level horizontally against the underside of a rafter.
2. Mark the level 12 inches from where it touches the rafter.
3. Measure straight up from that mark to the underside of the rafter. That number is the rise: 6 inches means 6/12.

From outside, a pitch gauge app on a photo taken square-on to the gable end gets you close. Enter the rise in our pitch tool and it converts the rise into degrees, percent and the area multiplier.

Pitch and roof upkeep

Pitch changes how much care a roof needs. A steep roof sheds rain, leaves and snow quickly, so debris rarely sits on it for long. A low roof drains slowly. Leaves and dirt collect in valleys and at the edges, and water sits longer after a storm.

That makes regular checks more important on low roofs. Keep gutters, valleys and any drains clear.

Where a low roof has a raised edge, like a parapet wall, the code requires roof drains at each low point, plus overflow drains or scuppers in case the main drains block ². If those clog, water can pond and find its way in.

Pitch and safety on the roof

Federal workplace rules define a steep roof as one steeper than 4 in 12, and a low-slope roof as 4 in 12 or less ³.

Workers on it six feet or more above a lower level must be protected by guardrails, safety nets or personal fall-arrest systems ⁴.

Those rules bind employers, not homeowners, but the line is a good guide: if a roof is steeper than 4/12, look at it from the ground, a window or a drone photo instead of climbing up.

Code rule: which roof coverings each pitch allows

The International Residential Code sets a lowest pitch for each covering. Many jurisdictions adopt it with local amendments, so check yours.

Sources: [2]

Code rule: extra underlayment on low-pitch shingle roofs

Shingles can go on a roof as low as 2/12, but water runs off slowly at that angle and can be pushed back up under the laps by wind. So between 2/12 and 4/12 the code asks for two layers of underlayment, laid like this ²:

1. A 19-inch-wide strip of underlayment felt goes down first, along the eave.
2. A full 36-inch-wide sheet goes over it, also starting at the eave.
3. Every sheet after that overlaps the one below by 19 inches, so almost every spot on the deck is covered twice.
4. Where two sheets meet end to end, they overlap by 4 inches, and those end joints are staggered 6 feet apart from one row to the next.

In plain terms: a roof under 4/12 needs about twice the underlayment of a steeper one. If your roof is in that range and a shingle quote lists only "one layer of felt", ask about it before you sign.

Pitch vs slope vs angle

In everyday roofing, yes: contractors, codes and calculators use both words for rise over 12 inches of run.

Strictly, older carpentry texts defined pitch as rise divided by the full span (both sides of a gable), so a 6/12 slope was a "1/4 pitch".

You will rarely meet that usage today, but it explains why some old plans look wrong.

Common mistakes and myths

Myth You measure 12 inches along the rafter.

Reality The 12 inches must be level, not along the slope. Measuring along the rafter makes the pitch read too low.

Myth A 6/12 roof is 6 degrees.

Reality A 6/12 roof is about 27 degrees. Pitch is a ratio, not an angle.

Myth One pitch covers the whole house.

Reality Porches, dormers and additions often have their own pitch. Each plane is measured and priced on its own.

Myth The multiplier goes on the floor area.

Reality It goes on the flat area the roof covers, including the overhangs, not the house's floor area.

Myth "1/4 pitch" on old plans is very flat.

Reality Older drawings divided the rise by the full span. A "1/4 pitch" in that style is a 6/12 roof today.

Local rules and your roof pitch

The pitch rules on this page come from the International Residential Code, a model code. Each state or city adopts its own edition and can change it. Your local building department can tell you which edition applies and whether any local amendments change the minimum pitch or underlayment rules.

Local climate matters too. In snowy areas, the building department's own climate table decides whether an ice barrier is required at the eaves, and that rule applies to shingle roofs of any pitch ². Our study of the ice barrier line shows how that requirement changes from place to place.

When pitch matters to you

Pitch matters most at three moments. When you compare quotes, check every roofer used the same pitch, because it changes the number of squares.

When you choose a material, check it is allowed at your pitch; a low porch roof may need something different from the main roof.

And when you think about climbing up, remember that anything over 4/12 is a steep roof by safety rules.

If you are getting a new roof, start by measuring your pitch from the attic, then use it to check each quote line by line.

Keep going

• Roof pitch calculatorRise to degrees, percent and area multiplier
• Roofing calculatorFootprint and pitch into squares
• Roofing materials comparedWhat suits low and steep roofs
• Roof replacementHow a full reroof is scoped and priced

Related terms

Roofing Square

A roofing square is a unit of area equal to 100 square feet of roof surface, a 10-by-10-foot patch, and it is the unit in which shingles are sold and most roofing quotes are priced.

Low-Slope Roof

A low-slope roof, often called a flat roof, is a roof with only a slight fall, usually less than 2 inches of rise per foot, so it needs a sealed covering such as a membrane instead of overlapping shingles.

Roof Underlayment

Roof underlayment is a sheet laid over the whole roof deck before the shingles or other roof covering go on.

Asphalt Shingles

Asphalt shingles are roofing pieces made from a fiberglass mat coated with asphalt and topped with mineral granules, laid in overlapping rows so water runs off the roof.

Metal Roofing

Metal roofing is a roof covering made of metal sheets, panels or shingles, most often steel, aluminum, copper or zinc, fastened to the roof deck to shed water, snow and sun.

Roof Ventilation

Roof ventilation is the system of vents that lets outside air enter an attic low, near the eaves, and leave high, near the ridge, so heat and moisture do not build up under the roof deck.

Gable Roof

A roof with two planes sloping down from a central ridge, leaving a triangular wall (the gable) at each end.

Roof Ridge

The horizontal line at the top of a roof where two planes meet. It is capped with ridge shingles and is the usual place for a ridge vent.

Also referenced from: Low-Slope Roof, Asphalt Shingles, Architectural Shingles, Metal Roofing, Standing Seam Metal Roof, Slate Roofing, Clay Tile Roofing, Wood Shakes, Composite Roofing, Solar Shingles, Roof Underlayment, Ice and Water Shield, Valley Flashing, Chimney Flashing, Roof Ventilation, Roofing Square