What does a 3/12 pitch mean?

A 3/12 pitch means that for every 12 horizontal units, the roof rises 3 vertical units, resulting in a slope of approximately 14 degrees.

What is a multiplier in roof calculations?

The multiplier is a factor used to convert horizontal measurements into vertical rises, helping to determine the overall height and slope of the roof.

A plumb cut is the angled cut made at the top of a rafter that allows it to fit securely against the ridge board, essential for proper roof framing.

Why is snow load not sized in this context?

Snow load considerations vary by location and are influenced by local building codes. It's important to consult these codes to ensure the roof can support potential snow accumulation.

3/12 gable rafter on a 20 ft span

Learn about gable roof design, including the multiplier, plumb cut, and considerations for snow load. Get insights on calculating a 3/12 pitch on a 20 ft span.

Formula SSC-2026.1

3.00 / 12 · 14.04°

Pitch angle

14.04°

Multiplier 1.0308

Common rafter

11 ft 3 5/16 in

Tail 1 ft 3/8 in

Cuts

14.04° plumb

75.96° seat

Roof area

586 ft²

5.86 squares

Shingle bundles

3 bundles per square + 10% waste

Sheathing / felt

21 sheets

2 underlayment rolls

Delivery weight

2,422 lb

1,099 kg · 70 lb bundles, 46 lb sheets, 28 lb rolls

Gable truss triangleLive scaled blueprint

Rise and run are scaled from the span, overhang, and pitch. The small triangle is the pitch ratio.

Bill of materials and shopping list
Item	Net	Waste	Order	Unit price	Line total	Source Spec Material / Tool
Architectural shingle bundles	5.86 squares	10%	20 bundles	USD	$840.00	Source Spec Material / Tool
4x8 CDX sheathing	18.3 sheets	10%	21 sheets	USD	$588.00	Source Spec Material / Tool
Underlayment rolls	586 ft²	overlap allowance	2 rolls	USD	$110.00	Source Spec Material / Tool
Ridge cap bundles	26.0 ft ridge	—	2 bundles	25 ft per bundle	—	Source Spec Material / Tool

Source links open a product search on the Amazon storefront for the selected currency. SiteSpecCalc may earn a commission. The link does not change the quantities above.

Side-by-side pitch comparison
Pitch	Degrees	Multiplier	Rafter	Squares	Bundles	Sheets
3.0/12	14.04°	1.031	11 ft 3 5/16 in	5.86	20	21
5.0/12	22.62°	1.083	11 ft 10 3/16 in	6.16	21	22

Show the deterministic formula

Angle = arctan(rise ÷ 12). Multiplier = √(1 + (rise ÷ 12)²). Gable run = span ÷ 2 − ridge thickness ÷ 2. Rafter length = (run + horizontal overhang) × multiplier.

Plumb cut = pitch angle. Seat cut = 90° − pitch angle. Area uses both slopes on a gable and one slope on a shed, and it includes a rake overhang equal to the eave overhang.

Bundles = ceiling of squares × 3 × 1.10. Sheets = ceiling of area × 1.10 ÷ 32. Rolls use 360 ft² effective coverage. Delivery weight plans 70 lb per bundle, 46 lb per sheet, and 28 lb per roll, then the header converts that pound total at 1 lb = 0.45359237 kg.

Contractor sign-off

Project: 3/12 gable rafter on a 20 ft span

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This sheet is an estimate for ordering and layout discussion. It is not a sealed design. Confirm the quantities with a licensed local professional and the authority having jurisdiction before you buy or build.

Engineering notes

A gable roof with a 3/12 pitch indicates that for every 12 horizontal units, the roof rises 3 vertical units. This pitch is essential for determining the roof's slope and overall design. The multiplier is used in calculations to convert the horizontal span into the vertical rise, helping to establish the roof's height and structural integrity.

The plumb cut refers to the angle cut on the rafters that meets the ridge board, ensuring a proper fit and alignment. It is crucial for maintaining the roof's structural integrity and aesthetic appeal. Additionally, while snow load considerations are important in roof design, they are not sized in this context, meaning that local building codes and environmental factors should be consulted to ensure adequate support for potential snow accumulation.

Code reference

Refer to local building codes for specific requirements regarding snow load and structural integrity.

Field questions

What does a 3/12 pitch mean?: A 3/12 pitch means that for every 12 horizontal units, the roof rises 3 vertical units, resulting in a slope of approximately 14 degrees.
What is a multiplier in roof calculations?: The multiplier is a factor used to convert horizontal measurements into vertical rises, helping to determine the overall height and slope of the roof.
What is a plumb cut?: A plumb cut is the angled cut made at the top of a rafter that allows it to fit securely against the ridge board, essential for proper roof framing.
Why is snow load not sized in this context?: Snow load considerations vary by location and are influenced by local building codes. It's important to consult these codes to ensure the roof can support potential snow accumulation.