What is a multiplier in roofing?

A multiplier in roofing is a factor used to determine the rise of the roof based on its pitch and span. It helps in calculating the lengths of rafters and other framing components.

A plumb cut is the angled cut made at the top of a rafter that allows it to fit snugly against the ridge board. This cut is essential for ensuring the structural stability of the roof.

Why is snow load not sized in this explanation?

Snow load sizing is not included here because it varies based on geographic location and specific building codes. It's important to consult local regulations to determine appropriate snow load requirements.

4/12 gable rafter on a 20 ft span

Learn about gable roofs with a 4/12 pitch on a 20 ft span, including key concepts like multipliers, plumb cuts, and considerations for snow load sizing.

Formula SSC-2026.1

4.00 / 12 · 18.43°

Pitch angle

18.43°

Multiplier 1.0541

Common rafter

11 ft 6 3/8 in

Tail 1 ft 5/8 in

Cuts

18.43° plumb

71.57° seat

Roof area

600 ft²

6.00 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	6.00 squares	10%	20 bundles	USD	$840.00	Source Spec Material / Tool
4x8 CDX sheathing	18.7 sheets	10%	21 sheets	USD	$588.00	Source Spec Material / Tool
Underlayment rolls	600 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
4.0/12	18.43°	1.054	11 ft 6 3/8 in	6.00	20	21
6.0/12	26.57°	1.118	12 ft 2 3/4 in	6.36	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: 4/12 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 4/12 pitch means that for every 12 horizontal units, the roof rises 4 vertical units. This pitch is crucial for determining the roof's slope and overall design. The multiplier is used to calculate the rise over a given span, which is essential for accurate framing and material estimation.

The plumb cut refers to the angle cut at the top of the rafters where they meet the ridge board. Properly executing this cut ensures a tight fit and structural integrity. It's important to note that snow load considerations should be evaluated based on local building codes and environmental factors, as they are not sized within this basic framework.

Code reference

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

Field questions

What is a multiplier in roofing?: A multiplier in roofing is a factor used to determine the rise of the roof based on its pitch and span. It helps in calculating the lengths of rafters and other framing components.
What is a plumb cut?: A plumb cut is the angled cut made at the top of a rafter that allows it to fit snugly against the ridge board. This cut is essential for ensuring the structural stability of the roof.
Why is snow load not sized in this explanation?: Snow load sizing is not included here because it varies based on geographic location and specific building codes. It's important to consult local regulations to determine appropriate snow load requirements.