What is the purpose of the multiplier in roof calculations?

The multiplier is used to adjust the calculations for the increased surface area of a pitched roof compared to a flat roof. It helps in estimating the amount of roofing material needed.

What is a plumb cut in roofing?

A plumb cut is the angled cut made at the top of the rafters that allows them to fit snugly against the ridge board, ensuring proper alignment and load distribution.

Why is snow load not sized in this design?

Snow load considerations can vary greatly by location and are typically determined by local building codes. It's important to consult these codes to ensure the roof can handle potential snow accumulation.

6/12 gable rafter on a 16 ft span

Explore the essentials of designing a gable roof with a 6/12 pitch on a 16 ft span. Learn about the multiplier, plumb cut, and considerations for snow load.

Formula SSC-2026.1

6.00 / 12 · 26.57°

Pitch angle

26.57°

Multiplier 1.1180

Common rafter

9 ft 11 15/16 in

Tail 1 ft 1 7/16 in

Cuts

26.57° plumb

63.43° seat

Roof area

520 ft²

5.20 squares

Shingle bundles

3 bundles per square + 10% waste

Sheathing / felt

18 sheets

2 underlayment rolls

Delivery weight

2,144 lb

973 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	5.20 squares	10%	18 bundles	USD	$756.00	Source Spec Material / Tool
4x8 CDX sheathing	16.2 sheets	10%	18 sheets	USD	$504.00	Source Spec Material / Tool
Underlayment rolls	520 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
6.0/12	26.57°	1.118	9 ft 11 15/16 in	5.20	18	18
8.0/12	33.69°	1.202	10 ft 8 7/8 in	5.59	19	20

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: 6/12 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 6/12 pitch means that for every 12 horizontal units, the roof rises 6 vertical units. This ratio is crucial for determining the slope and overall height of the roof. When calculating materials, a multiplier is often used to account for the increased surface area of the roof due to its pitch.

The plumb cut refers to the cut made at the top of the rafters, which is essential for ensuring a proper fit against the ridge board. Accurate plumb cuts are critical for the structural integrity of the roof, as they help distribute loads evenly.

Code reference

Consult local building codes for specific requirements regarding roof design, including snow load considerations, as these can vary significantly by region.

Field questions

What is the purpose of the multiplier in roof calculations?: The multiplier is used to adjust the calculations for the increased surface area of a pitched roof compared to a flat roof. It helps in estimating the amount of roofing material needed.
What is a plumb cut in roofing?: A plumb cut is the angled cut made at the top of the rafters that allows them to fit snugly against the ridge board, ensuring proper alignment and load distribution.
Why is snow load not sized in this design?: Snow load considerations can vary greatly by location and are typically determined by local building codes. It's important to consult these codes to ensure the roof can handle potential snow accumulation.