Skip to content
SiteSpecCalc

Length imperial. Weight lb. Currency USD. Weight uses 1 lb = 0.45359237 kg. Currency converts US-dollar planning prices and does not change volumes, counts, or geometry.

8/12 Gable on a 30×50 Building

The same 8/12 geometry on a wider 30 ft span and a 50 ft length.

Formula SSC-2026.1

8.00 / 12 · 33.69°

Pitch angle

33.69°

Multiplier 1.2019

Common rafter

19 ft 1 7/8 in

Tail 1 ft 2 7/16 in

Cuts

33.69° plumb

56.31° seat

Roof area

1,992 ft²

19.92 squares

Shingle bundles

66

3 bundles per square + 10% waste

Sheathing / felt

69 sheets

6 underlayment rolls

Delivery weight

7,962 lb

3,612 kg · 70 lb bundles, 46 lb sheets, 28 lb rolls

Gable truss triangleLive scaled blueprint
Span 30.00 ft + overhangsRise 9 ft 11 1/2 in128.08.0 : 12Rafter 19 ft 1 7/8 in · plumb 33.7° · seat 56.3°

Rise and run are scaled from the span, overhang, and pitch. The small triangle is the pitch ratio.

Bill of materials and shopping list
ItemNetWasteOrderUnit priceLine total
Architectural shingle bundles19.92 squares10%66 bundlesUSD$2,772.00
4x8 CDX sheathing62.3 sheets10%69 sheetsUSD$1,932.00
Underlayment rolls1,992 ft²overlap allowance6 rollsUSD$330.00
Ridge cap bundles52.0 ft ridge—3 bundles25 ft per bundle—
Side-by-side pitch comparison
PitchDegreesMultiplierRafterSquaresBundlesSheets
4.0/1218.43°1.05416 ft 9 5/8 in17.475861
6.0/1226.57°1.11817 ft 9 13/16 in18.536264
8.0/1233.69°1.20219 ft 1 7/8 in19.926669
12.0/1245.00°1.41422 ft 6 7/16 in23.447881
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 on a 30×50 Building

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

The building is 30 ft wide and 50 ft long at 8/12 with a 1 ft overhang. Run is half of 30 ft minus half the ridge thickness. Area includes both slopes and the rake overhang. Use this page when the 24 ft examples are too small for the building you are covering.

Code reference

IRC R802 covers conventional wood roof framing and R905 covers roof coverings. This page does not size the rafter for snow or wind.

Field questions

Why does a wider span grow the rafter so quickly?
Run is about half the span. Rafter length is that run, plus overhang, times the pitch multiplier.
Are both slopes included?
Yes. A gable doubles the one-slope area. A shed would use one slope.
Is the bundle count a brand takeoff?
No. It is 3 bundles per square plus 10 percent. Hips, valleys, and starter courses are extra.

Explore Related Engineering Calculations