What is a multiplier in roof design?

The multiplier is a factor used to calculate the length of rafters based on the roof pitch. For a 5/12 pitch, the multiplier is approximately 1.083.

A plumb cut is the angle cut at the top of a rafter that allows it to fit against the ridge board, ensuring a secure and stable roof structure.

How is snow load considered in roof design?

Snow load is a critical factor in roof design, but it must be sized according to local building codes and conditions. In this case, snow load considerations were not specified.

5/12 gable rafter on a 20 ft span

Explore the essentials of gable roof construction, including the multiplier for pitch calculations, plumb cuts for rafters, and considerations for snow load sizing.

Formula SSC-2026.1

5.00 / 12 · 22.62°

Pitch angle

22.62°

Multiplier 1.0833

Common rafter

11 ft 10 3/16 in

Tail 1 ft 1 in

Cuts

22.62° plumb

67.38° seat

Roof area

616 ft²

6.16 squares

Shingle bundles

3 bundles per square + 10% waste

Sheathing / felt

22 sheets

2 underlayment rolls

Delivery weight

2,538 lb

1,151 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	6.16 squares	10%	21 bundles	USD	$882.00	Source Spec Material / Tool
4x8 CDX sheathing	19.3 sheets	10%	22 sheets	USD	$616.00	Source Spec Material / Tool
Underlayment rolls	616 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
5.0/12	22.62°	1.083	11 ft 10 3/16 in	6.16	21	22
7.0/12	30.26°	1.158	12 ft 7 15/16 in	6.58	22	23

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: 5/12 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 5/12 pitch means that for every 12 horizontal inches, the roof rises 5 inches. This pitch is crucial for determining the roof's angle and the length of the rafters needed.

The multiplier is used to calculate the actual length of the rafters based on the pitch. For a 5/12 pitch, the multiplier is approximately 1.083. This means if you measure the horizontal run of the rafter, you multiply that by 1.083 to find the length of the rafter. Additionally, the plumb cut is the angle cut at the top of the rafter that allows it to fit snugly against the ridge board.

Code reference

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

Field questions

What is a multiplier in roof design?: The multiplier is a factor used to calculate the length of rafters based on the roof pitch. For a 5/12 pitch, the multiplier is approximately 1.083.
What is a plumb cut?: A plumb cut is the angle cut at the top of a rafter that allows it to fit against the ridge board, ensuring a secure and stable roof structure.
How is snow load considered in roof design?: Snow load is a critical factor in roof design, but it must be sized according to local building codes and conditions. In this case, snow load considerations were not specified.