Should I measure roof pitch from the attic or the roof?

From the attic is safer and more accurate. Attic measurements avoid fall hazards and the distortion caused by uneven shingle textures or tile profiles. Measure from the roof surface only when the attic is finished or inaccessible, and always from a secure ladder at the gable end.

What is the difference between span and run?

Span is the entire exterior width of the building, wall to wall. Run is half the span on a symmetrical gable roof — the distance from the wall plate to the center ridge board. Calculating pitch or rafter length with the full span instead of the run doubles your rise error.

How to Measure Roof Pitch

Complete guide to measuring roof slope from the attic or roof deck, building code rules, and rafter cut tables.

By the RoofCal Roofing Editorial TeamLast updated November 7, 2026Reviewed against IBC Chapter 15 & IRC R905

Open Interactive Calculator View 1/12 – 24/12 Pitch Chart

1. Understanding the X:12 Pitch Ratio

In North America, roof slope is expressed as a ratio of rise over run, normalized to a 12-inch horizontal run.

RiseX inches

Run12 inches (1 Foot)

NotationX : 12

For example, a 6:12 pitch means that for every 12 inches the roof travels horizontally toward the ridge board, it rises 6 inches vertically. This translates to an angle of 26.6° and a slope grade of 50%.

A note on terminology: strictly, slope is the rise-per-12-inches-of-run ratio (the X:12 value), while classic carpentry texts defined pitch as the rise over the entire building span, written as a fraction. In modern residential work — and everywhere on RoofCal — pitch means the X:12 ratio, and the angle is simply the same geometry expressed in degrees.

2. Step-by-Step: How to Measure Roof Pitch in the Field

Method A (Recommended)

From Inside the Attic

Measuring from inside the attic avoids the fall hazard of climbing onto shingles and eliminates inaccuracies caused by uneven roofing textures or tile profiles.

1Find an accessible, straight common rafter board (not a hip or valley rafter).
2Place the end of a 12-inch or 24-inch carpenter’s level flush against the underside of the rafter.
3Pivot the level down until the spirit bubble is perfectly centered between the index lines.
4Measure vertically with a tape measure from the 12-inch mark on the level up to the bottom edge of the rafter. That measurement in inches is your X:12 pitch.

💡 Pro tip: If using a 24-inch level, divide the vertical rise measurement by 2.

Method B (Exterior)

From the Roof Surface or Eaves

If the attic is finished or inaccessible, you can measure directly off the roof rake or overhang from a secure ladder.

1Secure your ladder firmly with level footing at the gable end of the house.
2Place the end of your 12-inch level against the roofing surface or barge board.
3Level the bubble. Measure straight down from the 12-inch mark to the roof deck surface.
4Alternatively, lay your smartphone flat along the rafter and open our Mobile Pitch Level App to read the angle instantly.

Try the Smartphone Digital Level Tool

3. Minimum Slope Requirements by Roofing Material

Standard International Building Code (IBC Chapter 15 & IRC R905)

Roofing Material	Min. Pitch	Min. Angle	Underlayment / Code Requirements	Walkability
Built-Up Roof (BUR) / Coal Tar	1/4 : 12	1.2°	Multiple plies of felt saturated in asphalt or coal tar pitch	Flat Walkable
Single-Ply Membrane (EPDM, TPO, PVC)	1/4 : 12	1.2°	Fully adhered or mechanically fastened, sealed seams	Flat Walkable
Standing Seam Metal Roof	1/2 : 12	2.4°	Requires mechanical seaming below 3:12; full ice/water shield	Walkable (Slippery)
Asphalt Shingles (Low-Slope Exception)	2 : 12	9.5°	Mandatory double-layer underlayment or self-adhering modified bitumen	Comfortable Walk
Standard Asphalt Shingles (3-Tab & Arch)	4 : 12	18.4°	Standard single-ply ASTM D226 Type I/II or synthetic underlayment	Walkable
Clay & Concrete Roof Tiles	2.5 : 12	11.8°	Solid sheathing with double-layer membrane; battens required ≥ 4:12	Fragile, special boots
Natural Slate Shingles	4 : 12	18.4°	Heavyweight roof framing required; 3" or 4" minimum headlap	Steep, scaffold only

4. The 5 Most Expensive Roof Pitch Mistakes to Avoid

Carpentry errors that cause failed inspections and water leaks

1. Confusing Building Span with Rafter Run

Span is the entire exterior width of the building from wall to wall. On a symmetrical gable roof, the run is only HALF the span (the distance from the wall plate to the center ridge board). Calculating with full span doubles your rise error.

2. Measuring Hip or Valley Rafters Instead of Common

Hip and valley rafters run diagonally at a 45° angle in plan view, giving them a flatter slope ($\approx 17''$ run instead of $12''$). Always measure standard common rafters perpendicular to the building wall.

3. Forgetting Eaves & Rafter Overhang Tails

When ordering lumber, common rafters must extend beyond the exterior wall plate to form the soffit and fascia eaves. Remember to add your overhang length (multiplied by the rafter factor) to the base run calculation.

4. Installing Asphalt Shingles on Roofs Under 2:12

Water backs up under shingle tabs on slopes lower than 2:12 during driving wind or snow accumulation, completely voiding manufacturer warranties. Low-slope roofs require waterproof membrane solutions.

Interactive Pitch Quick-Check

How Steep is Your Roof? Test Any Rise:

Drag the slider to test any vertical rise over 12" to see its resulting angle, percentage, and classification instantly:

1:12 (Low)6 : 12 Pitch18:12 (Steep)

Angle

26.6°

Slope %

50.0%

Calculate exact rafter cutting dimensions on homepage tool

Related Roofing Tools

Frequently Asked Questions

A 6/12 (6:12) pitch means the roof rises 6 vertical inches for every 12 horizontal inches of run — an angle of 26.6° and a 50% slope grade. It is the most common pitch on modern US residential homes and is still comfortable to walk.