Are the plumb and seat cuts the same at 12/12?

Yes. The pitch angle is 45 degrees and the seat cut is 90 minus 45, so both cuts are 45 degrees.

What is the rafter multiplier for 12/12?

The square root of 2, approximately 1.414. One foot of horizontal run needs about 1.414 ft of rafter.

Does the ridge thickness matter at this pitch?

It shortens the run by half the ridge thickness, 0.75 in for a 1.5 in ridge. The effect is small next to the span and it is still included.

12/12 Pitch Gable Rafter on a 24 Foot Span

A 12/12 gable. The plumb cut and the seat cut are both 45 degrees.

Formula SSC-2026.1

12.00 / 12 · 45.00°

Pitch angle

45.00°

Multiplier 1.4142

Common rafter

18 ft 3 9/16 in

Tail 1 ft 5 in

Cuts

45.00° plumb

45.00° seat

Roof area

1,171 ft²

11.71 squares

Shingle bundles

3 bundles per square + 10% waste

Sheathing / felt

41 sheets

4 underlayment rolls

Delivery weight

4,728 lb

2,145 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
Architectural shingle bundles	11.71 squares	10%	39 bundles	USD	$1,638.00
4x8 CDX sheathing	36.6 sheets	10%	41 sheets	USD	$1,148.00
Underlayment rolls	1,171 ft²	overlap allowance	4 rolls	USD	$220.00
Ridge cap bundles	32.0 ft ridge	—	2 bundles	25 ft per bundle	—

Side-by-side pitch comparison
Pitch	Degrees	Multiplier	Rafter	Squares	Bundles	Sheets
6.0/12	26.57°	1.118	14 ft 5 9/16 in	9.26	31	32
9.0/12	36.87°	1.250	16 ft 2 1/16 in	10.35	35	36
12.0/12	45.00°	1.414	18 ft 3 9/16 in	11.71	39	41
15.0/12	51.34°	1.601	20 ft 8 1/2 in	13.25	44	46

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: 12/12 Pitch Gable Rafter on a 24 Foot 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

Twelve in twelve is 45 degrees, so the multiplier is √2, about 1.414. The run on a 24 ft span, minus half of a 1.5 in ridge, is just under 12 ft, and the rafter is that run plus the 1 ft overhang, times 1.414. Plumb and seat cuts are equal. The building length is 30 ft. A 12/12 roof has a large area increase over a 4/12 on the same walls, which the pitch table shows.

Code reference

IRC R802 and R905. A 12/12 pitch still needs a load check for the rafter species, spacing, and the local snow load.

Field questions

Are the plumb and seat cuts the same at 12/12?: Yes. The pitch angle is 45 degrees and the seat cut is 90 minus 45, so both cuts are 45 degrees.
What is the rafter multiplier for 12/12?: The square root of 2, approximately 1.414. One foot of horizontal run needs about 1.414 ft of rafter.
Does the ridge thickness matter at this pitch?: It shortens the run by half the ridge thickness, 0.75 in for a 1.5 in ridge. The effect is small next to the span and it is still included.