What is a multiplier in roof construction?

A multiplier is a factor used to determine the rise of the roof based on its pitch and span. It helps in calculating the length of the rafters needed for the roof structure.

A plumb cut is the angle cut made at the top of a rafter, allowing it to fit securely against the ridge board of the roof. It is essential for proper alignment and support.

Why is snow load not sized in this explanation?

Snow load considerations vary by location and are typically determined by local building codes. This explanation focuses on the basic structural elements of a gable roof without specific snow load calculations.

3/12 gable rafter on a 16 ft span

Learn about gable roofs with a 3/12 pitch on a 16 ft span, including key concepts like multipliers, plumb cuts, and snow load considerations.

Formula SSC-2026.1

3.00 / 12 · 14.04°

Pitch angle

14.04°

Multiplier 1.0308

Common rafter

9 ft 2 9/16 in

Tail 1 ft 3/8 in

Cuts

14.04° plumb

75.96° seat

Roof area

479 ft²

4.79 squares

Shingle bundles

3 bundles per square + 10% waste

Sheathing / felt

17 sheets

2 underlayment rolls

Delivery weight

1,958 lb

888 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	4.79 squares	10%	16 bundles	USD	$672.00	Source Spec Material / Tool
4x8 CDX sheathing	15.0 sheets	10%	17 sheets	USD	$476.00	Source Spec Material / Tool
Underlayment rolls	479 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	9 ft 2 9/16 in	4.79	16	17
5.0/12	22.62°	1.083	9 ft 8 3/16 in	5.03	17	18

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 16 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 means that for every 12 horizontal inches, the roof rises 3 inches. This slope is essential for water drainage and structural integrity. The multiplier is used to calculate the rise of the roof based on the span and pitch, which is crucial for determining the length of the rafters.

The plumb cut is the angle cut at the top of the rafter, which allows it to sit flush against the ridge board. Properly calculating this cut is vital for ensuring the roof's stability. Note that snow load considerations should be addressed separately and are not included in this basic sizing.

Code reference

Consult local building codes for specific requirements regarding roof design and snow load considerations.

Field questions

What is a multiplier in roof construction?: A multiplier is a factor used to determine the rise of the roof based on its pitch and span. It helps in calculating the length of the rafters needed for the roof structure.
What is a plumb cut?: A plumb cut is the angle cut made at the top of a rafter, allowing it to fit securely against the ridge board of the roof. It is essential for proper alignment and support.
Why is snow load not sized in this explanation?: Snow load considerations vary by location and are typically determined by local building codes. This explanation focuses on the basic structural elements of a gable roof without specific snow load calculations.