What is a multiplier in roof design?

The multiplier is used to calculate the rise of the roof based on the pitch and span. For an 8/12 pitch, the multiplier helps determine the vertical rise over the horizontal span.

A plumb cut is the angled cut made at the top of the rafters to ensure they fit snugly against the ridge board, aligning with the slope of the roof.

Why is snow load not sized in this calculation?

Snow load is a separate consideration that depends on local climate conditions and should be calculated according to local building codes and standards.

8/12 gable rafter on a 16 ft span

Learn about gable roof design, including the multiplier, plumb cut, and considerations for snow load. Essential information for construction calculations.

Formula SSC-2026.1

8.00 / 12 · 33.69°

Pitch angle

33.69°

Multiplier 1.2019

Common rafter

10 ft 8 7/8 in

Tail 1 ft 2 7/16 in

Cuts

33.69° plumb

56.31° seat

Roof area

559 ft²

5.59 squares

Shingle bundles

3 bundles per square + 10% waste

Sheathing / felt

20 sheets

2 underlayment rolls

Delivery weight

2,306 lb

1,046 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	5.59 squares	10%	19 bundles	USD	$798.00	Source Spec Material / Tool
4x8 CDX sheathing	17.5 sheets	10%	20 sheets	USD	$560.00	Source Spec Material / Tool
Underlayment rolls	559 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
8.0/12	33.69°	1.202	10 ft 8 7/8 in	5.59	19	20
10.0/12	39.81°	1.302	11 ft 7 5/8 in	6.05	20	21

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: 8/12 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 an 8/12 pitch on a 16 ft span requires careful calculations to ensure structural integrity. The multiplier is a crucial factor in determining the rise of the roof based on the span, while the plumb cut is the angle cut at the top of the rafters to fit the roof's slope. It's important to note that snow load considerations should be addressed separately and are not included in this basic sizing.

When designing a gable roof, always consult local building codes and regulations to ensure compliance with safety standards.

Code reference

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

Field questions

What is a multiplier in roof design?: The multiplier is used to calculate the rise of the roof based on the pitch and span. For an 8/12 pitch, the multiplier helps determine the vertical rise over the horizontal span.
What is a plumb cut?: A plumb cut is the angled cut made at the top of the rafters to ensure they fit snugly against the ridge board, aligning with the slope of the roof.
Why is snow load not sized in this calculation?: Snow load is a separate consideration that depends on local climate conditions and should be calculated according to local building codes and standards.