What is a multiplier in roof design?

A multiplier is a factor used to calculate the effective area of the roof based on its pitch and span, helping determine material requirements and structural loads.

A plumb cut is the angled cut made at the top of a rafter, allowing it to fit against the ridge board. It is crucial for ensuring the roof's structural stability.

Why is snow load not sized in the roof design?

Snow load considerations are typically assessed separately based on local building codes and environmental factors, rather than being included in the initial roof design.

How does the 8/12 pitch affect the roof's performance?

An 8/12 pitch provides effective drainage and can enhance the aesthetic appeal of the structure, but it also requires careful consideration of load factors, including snow and wind.

8/12 gable rafter on a 20 ft span

Learn about gable roofs with an 8/12 pitch on a 20 ft span, including key concepts like multipliers, plumb cuts, and considerations for snow load sizing.

Formula SSC-2026.1

8.00 / 12 · 33.69°

Pitch angle

33.69°

Multiplier 1.2019

Common rafter

13 ft 1 3/4 in

Tail 1 ft 2 7/16 in

Cuts

33.69° plumb

56.31° seat

Roof area

684 ft²

6.84 squares

Shingle bundles

3 bundles per square + 10% waste

Sheathing / felt

24 sheets

2 underlayment rolls

Delivery weight

2,770 lb

1,256 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	6.84 squares	10%	23 bundles	USD	$966.00	Source Spec Material / Tool
4x8 CDX sheathing	21.4 sheets	10%	24 sheets	USD	$672.00	Source Spec Material / Tool
Underlayment rolls	684 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	13 ft 1 3/4 in	6.84	23	24
10.0/12	39.81°	1.302	14 ft 2 7/8 in	7.40	25	26

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 20 ft span

Prepared by

Date

Verified by (PE / electrician / inspector)

License number

Signature

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 means that for every 12 horizontal units, the roof rises 8 vertical units. This pitch is commonly used for its aesthetic appeal and effective water drainage. The multiplier is a factor used to calculate the area of the roof based on its pitch and span, which is crucial for determining material needs and structural integrity.

The plumb cut refers to the angle cut made at the top of the rafters, allowing them to fit snugly against the ridge board. Properly executed plumb cuts are essential for maintaining the roof's structural stability. Additionally, it's important to note that snow load considerations should be assessed separately based on local building codes, as they are not inherently sized within the roof design.

Code reference

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

Field questions

What is a multiplier in roof design?: A multiplier is a factor used to calculate the effective area of the roof based on its pitch and span, helping determine material requirements and structural loads.
What is a plumb cut?: A plumb cut is the angled cut made at the top of a rafter, allowing it to fit against the ridge board. It is crucial for ensuring the roof's structural stability.
Why is snow load not sized in the roof design?: Snow load considerations are typically assessed separately based on local building codes and environmental factors, rather than being included in the initial roof design.
How does the 8/12 pitch affect the roof's performance?: An 8/12 pitch provides effective drainage and can enhance the aesthetic appeal of the structure, but it also requires careful consideration of load factors, including snow and wind.