Roof Pitch Explained: The Complete Reference Guide

Roof pitch is one of those things that seems simple until it isn't. A new guy on the crew hears "6/12" and thinks he understands. Then he needs to convert that to degrees for his miter saw, or figure out why his rafter is 13-7/16 inches long for every foot of run instead of 12, or explain to a homeowner why their 3/12 roof can't have standard shingles without double underlayment.

I've been doing this math for 40 years, and I still reference these tables. Nobody memorizes all of it. The smart move is to have a solid reference you trust and know how to use. That's what this page is.

What Is Roof Pitch?

Roof pitch is the measure of a roof's steepness, expressed as the ratio of vertical rise to horizontal run. In the U.S., pitch is always calculated against a 12-inch run — so a "6/12 pitch" means the roof rises 6 inches for every 12 inches of horizontal distance.

This standardized system lets carpenters communicate roof angles precisely and use pre-calculated tables for rafter lengths, hip and valley rafters, and jack rafter spacing. When someone on the crew says "eight twelve," everyone knows exactly what that means — no ambiguity.

The Three Ways to Express Roof Slope:

Method	Example	Used For
Pitch (X/12)	6/12	U.S. framing, rafter tables
Degrees	26.57°	Saw settings, international
Percentage	50%	Civil engineering, ADA ramps

Converting between them:

• Degrees = arctan(rise ÷ 12) × (180 ÷ π)
• Percentage = (rise ÷ 12) × 100
• Rise = 12 × tan(degrees × π ÷ 180)

Complete Pitch-to-Degree Conversion Table

This is the reference table. Bookmark it.

Pitch	Degrees	% Grade	Rafter Multiplier	Hip/Valley Multiplier
1/12	4.76°	8.3%	1.0035	1.4167
2/12	9.46°	16.7%	1.0138	1.4240
3/12	14.04°	25.0%	1.0308	1.4362
4/12	18.43°	33.3%	1.0541	1.4530
5/12	22.62°	41.7%	1.0833	1.4743
6/12	26.57°	50.0%	1.1180	1.5000
7/12	30.26°	58.3%	1.1577	1.5298
8/12	33.69°	66.7%	1.2019	1.5635
9/12	36.87°	75.0%	1.2500	1.6008
10/12	39.81°	83.3%	1.3017	1.6415
11/12	42.51°	91.7%	1.3566	1.6853
12/12	45.00°	100%	1.4142	1.7321
14/12	49.40°	116.7%	1.5366	1.8333
16/12	53.13°	133.3%	1.6667	1.9437
18/12	56.31°	150.0%	1.8028	2.0616

How to use the Rafter Multiplier: Multiply your horizontal run by this factor to get the rafter length along the slope.

Example: 14-foot run at 6/12 pitch = 14 × 1.1180 = 15.65 feet (15' 7-13/16")

How to use the Hip/Valley Multiplier: For hip or valley rafters, multiply the common rafter run by this factor.

Example: 14-foot run at 6/12 pitch = 14 × 1.5000 = 21 feet

Building Code Minimum Pitch Requirements

This is the section that saves you from expensive mistakes. Every roofing material has a minimum pitch below which it either won't shed water properly or voids the manufacturer's warranty — or both. These minimums come from the International Residential Code (IRC) and manufacturer specifications. Local codes may be stricter.

Roofing Material	Minimum Pitch	Notes
Built-up/membrane	1/4:12	Requires proper drainage design
Metal (standing seam)	1/4:12 to 3:12	Varies by manufacturer
Metal (exposed fastener)	3:12	Some allow 1:12 with sealant
Asphalt shingles	2:12*	*Requires double underlayment
Asphalt shingles (standard)	4:12	Standard installation
Wood shakes	4:12	Some codes require 5:12
Clay/concrete tile	2.5:12	With underlayment system
Slate	4:12	Depends on slate size/exposure

Critical note on low-slope asphalt: The 2:12 minimum requires ice-and-water shield over the entire roof deck, not just at eaves. At 4:12 and above, you can use standard felt underlayment.

How to Measure Existing Roof Pitch

If you're tying into an existing roof — an addition, a dormer, a new porch — you need to know the actual pitch, not what the original plans say. Plans get lost, roofs get re-framed, and sometimes the original builder just eyeballed it. Here are three ways to get the real number.

Method 1: From Inside the Attic (Most Accurate)

Tools needed: 24" level, tape measure, pencil

1. Find an accessible rafter that's not obstructed
2. Hold the level horizontally against the bottom edge of the rafter
3. Ensure the bubble is centered (level is truly horizontal)
4. Measure exactly 12 inches along the level from the rafter
5. From that 12" mark, measure straight up to the rafter's bottom edge
6. This vertical measurement is your pitch (if you measure 6", it's 6/12)

Why this works: You're creating a right triangle where the base is always 12", so the height directly gives you the pitch ratio.

Method 2: From the Roof Surface

Tools needed: 24" level, tape measure

1. Position yourself safely on the roof
2. Place the level on the roof surface pointing uphill
3. Raise the downhill end until the bubble centers
4. Measure the gap between the level and roof at the 12" mark
5. This measurement is your pitch

Accuracy check: Take measurements at multiple locations. Older roofs may have settled unevenly.

Method 3: From Ground Level (Estimation)

If you can see the gable end:

1. Measure or estimate the roof's total rise (peak to wall plate)
2. Measure the building width and divide by 2 for the run
3. Divide rise by run, then multiply by 12

Example: 8-foot rise over 16-foot run = 8/16 = 0.5 × 12 = 6/12 pitch

Pitch Selection Guide by Project Type

If you're designing from scratch — a custom home, a garage, a shed — pitch is a design decision with real consequences. Steeper pitches shed weather better but cost more in material and labor. Lower pitches are economical but limit your roofing material options and create more maintenance headaches in snow country. Here's what works in practice.

Residential Homes

Style	Typical Pitch	Reasoning
Ranch	4/12 to 6/12	Low profile, economical
Colonial	8/12 to 10/12	Traditional proportions
Cape Cod	9/12 to 12/12	Allows second-floor living space
Contemporary	2/12 to 4/12	Modern aesthetic, often flat sections
Craftsman	4/12 to 6/12	Emphasizes horizontal lines
Tudor	10/12 to 16/12	Steep, dramatic rooflines

Sheds and Outbuildings

Use Case	Recommended	Why
Simple storage shed	3/12 to 4/12	Economical, easy to build
Workshop with loft	6/12 to 8/12	Headroom in loft space
Garden shed (aesthetic)	5/12 to 6/12	Balanced appearance
Lean-to addition	2/12 to 4/12	Matches lower wall heights

Regional Considerations

Heavy snow areas (>50" annual): 6/12 minimum recommended. Steeper pitches (8/12+) shed snow naturally and reduce ice dam risk.

High wind zones: Lower pitches (4/12 to 6/12) present less surface area to wind uplift. Check local wind speed requirements.

Hot climates: Steeper pitches create larger attic volumes for heat to rise away from living space, improving cooling efficiency.

Calculating Rafter Lengths

This is where pitch stops being an abstract number and becomes something you cut lumber to. The pitch determines the rafter multiplier, which is the factor you multiply by horizontal run to get the actual length of wood you need. Get the pitch wrong and every rafter on the job is the wrong length.

The Basic Formula

Rafter Length = Run × Rafter Multiplier

But this gives you only the line length—the theoretical distance from the plumb cut at the ridge to the plumb cut at the building line. You still need to account for:

1. Ridge board deduction: Subtract half the ridge board thickness (typically 3/8" for 1× ridge or 3/4" for 2× ridge)
2. Overhang addition: Add the overhang run × rafter multiplier
3. HAP (Height Above Plate): The vertical distance from the top of the wall plate to the top of the rafter at the building line—affects birdsmouth layout

Worked Example

Given: 24-foot wide building, 6/12 pitch, 2×8 ridge board, 18" overhang

1. Run: 24 ÷ 2 = 12 feet
2. Rafter body: 12' × 1.1180 = 13.416' = 13' 5"
3. Ridge deduction: 1.5" ÷ 2 = 0.75" (measured along the rafter)
4. Overhang: 1.5' × 1.1180 = 1.677' = 1' 8-1/8"
5. Total rafter length: 13' 5" - 3/4" + 1' 8-1/8" = 15' 0-3/8"

Always cut a test rafter and check fit before cutting the entire stack.

Hip and Valley Rafters

Hip and valley rafters travel diagonally across the roof plane, so they're longer than common rafters for the same run.

Key differences:

• Use the hip/valley multiplier instead of the common rafter multiplier
• The plumb cut angle is different (steeper than common rafters at the same pitch)
• Backing or dropping may be required for the hip rafter to align with the roof planes

Pitch	Common Rafter Angle	Hip/Valley Angle
4/12	18.43°	14.04°
6/12	26.57°	19.47°
8/12	33.69°	24.09°
12/12	45.00°	30.26°

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Common Errors and How to Avoid Them

I've seen every one of these mistakes on real jobsites — some of them more than once. Most are easy to prevent if you know what to watch for.

Confusing pitch with degrees 6/12 pitch = 26.57 degrees, not 6 degrees. This mistake ruins saw setups and creates rafters that don't fit.

Using slope angle on a compound miter saw For plumb cuts, set the bevel to the pitch angle. For cheek cuts on hips and jacks, you need both miter and bevel settings calculated for compound angles.

Forgetting the ridge board Every rafter pair shares a ridge board between them. Fail to account for it, and your rafters will be too long.

Measuring run incorrectly Run is measured horizontally, not along the rafter. It's half the building span for a gable roof, measured from the outside of the wall to the centerline of the ridge.

Ignoring manufacturer minimums A shingle manufacturer's warranty requires minimum pitch compliance. Installing on a 3/12 when 4/12 is required voids the warranty and may violate code.