How do you calculate siding for a gable?

Multiply the width of the gable's base by its rise (height to the peak) and divide by two. A 24 ft wide gable that rises 6 ft has 24 × 6 ÷ 2 = 72 sq ft. If you cannot measure the rise, use the roof pitch: half the width × pitch ÷ 12.

How do I find the rise of a gable from the roof pitch?

Multiply half the gable's width by the pitch and divide by 12. On a 30 ft wide gable with a 6/12 pitch, the rise is 15 × 6 ÷ 12 = 7.5 ft, so the gable area is 30 × 7.5 ÷ 2 = 112.5 sq ft.

How do you calculate siding for a garage?

Multiply the perimeter by the wall height, add the gable areas, subtract the garage doors, entry doors and windows, then add about 10% waste. A 30 by 40 ft garage with 10 ft walls and 6/12 gables needs roughly 15.5 squares after openings and waste.

How much siding do I need for a shed?

Use the same steps at a smaller scale. A 10 by 12 ft shed with 8 ft walls, 6/12 gables, one door and one window needs about 351 sq ft net, or roughly 3.9 squares with 10% waste.

How do you measure a gambrel barn gable for siding?

Split the gable into a lower trapezoid and an upper triangle. The trapezoid's area is the average of its bottom and top widths times its height, and the triangle's is its base times its height divided by two. Add the two together.

How much extra siding should I buy for gables?

Gables involve angled cuts at the roofline, so plan on 10% at minimum and 15% or more when the roof is steep or the pattern must line up. Board-and-batten and horizontal lap siding waste differently, so check your product.

How to Calculate Siding for Gables, Garages and Sheds

Gables, garages and sheds all follow the same recipe: break the building into simple shapes, find each area, subtract the openings, add waste, and convert to squares. The only shape that trips people up is the gable, and it is just a triangle. This guide gives the formulas and a worked example for each type of building.

If you are new to taking measurements, start with how to measure for siding, then come back here for the specific shapes.

Gable ends: base × rise ÷ 2

A gable is the triangle of wall under a peaked roof. Its area is:

base × rise ÷ 2

The base is the width across the bottom of the gable and the rise is the vertical height from the bottom to the peak. A gable 24 ft wide that rises 6 ft: 24 × 6 ÷ 2 = 72 sq ft.

Finding the rise from the roof pitch

Reaching the peak is awkward, so most people work out the rise from the roof pitch. A pitch such as 6/12 means the roof rises 6 inches for every 12 inches of horizontal run. Half the gable width is the run:

rise = (width ÷ 2) × pitch ÷ 12

Example areas for a gable 24 ft wide:

Pitch	Rise	Gable area
4/12	4 ft	48 sq ft
6/12	6 ft	72 sq ft
8/12	8 ft	96 sq ft
12/12	12 ft	144 sq ft

A steeper roof adds gable area quickly, and it also raises waste because the angled cuts get longer.

Gambrel (barn-style) gables

A gambrel roof has a steep lower slope and a shallower upper slope. Split the gable into two shapes:

A trapezoid from the eave to the break in the roof: (bottom width + top width) ÷ 2 × height.
A triangle above the break: base × height ÷ 2.

Example: 24 ft wide at the eave, 16 ft wide at the break, 7 ft from eave to break, and 4 ft from break to peak.

Trapezoid: (24 + 16) ÷ 2 × 7 = 140 sq ft
Triangle: 16 × 4 ÷ 2 = 32 sq ft
Gable total: 172 sq ft

Shed-style and lean-to walls

A wall under a single-slope roof is a trapezoid. Average the low and high wall heights and multiply by the length: (low + high) ÷ 2 × length.

Garages

Example: a 30 by 40 ft garage, 10 ft walls, with the gables on the 30 ft ends and a 6/12 pitch.

Item	Calculation	Area
Walls	(30 + 40 + 30 + 40) × 10	1,400 sq ft
Two gables	rise 7.5 ft, each 30 × 7.5 ÷ 2 = 112.5	225 sq ft
Gross area		1,625 sq ft
Two 9 × 8 ft overhead doors	2 × 72	−144 sq ft
One 3 × 6.67 ft entry door		−20 sq ft
Four 3 × 4 ft windows	4 × 12	−48 sq ft
Net area		1,413 sq ft
Plus 10% waste	1,413 × 1.10	1,554 sq ft
Squares	1,554 ÷ 100	15.5

Garages have big openings, so subtracting them matters. Do not forget the trim around the doors, which you measure in linear feet (see the J-channel calculator).

Sheds

Example: a 10 by 12 ft shed, 8 ft walls, gables on the 10 ft ends at 6/12, one 3 × 6.67 ft door and one 2 × 3 ft window.

Walls: (10 + 12 + 10 + 12) × 8 = 352 sq ft
Gables: rise 2.5 ft, each 10 × 2.5 ÷ 2 = 12.5, so 25 sq ft
Gross: 377 sq ft
Openings: 20 + 6 = 26 sq ft
Net: 351 sq ft
With 10% waste: 386 sq ft, or 3.9 squares

Small buildings often need proportionally more waste, since a few cut pieces are a large share of the total. A shed may justify 12% to 15%.

Mobile and manufactured homes

A single-wide such as a 14 by 70 ft home has simple rectangular walls and a low roof, so the gable area is small or zero. With a wall height of about 7.5 ft:

Perimeter: 2 × (14 + 70) = 168 ft
Walls: 168 × 7.5 = 1,260 sq ft gross

Subtract windows and doors, add waste, and convert to squares. Wall height varies between models, so measure yours. Many manufactured homes use siding profiles and trim designed for them, so confirm compatibility with the product and the manufacturer’s instructions.

Add waste for angled cuts

Gable rooflines mean angled cuts on every course, which produces more offcuts than a plain rectangle. Use at least 10% and more for steep or complex roofs. The siding waste factor guide explains how to choose.

Let the calculator do it

Enter each wall, gable and opening in the Siding Calculator, or convert net area straight to squares with the Siding Squares Calculator. For a vinyl or fiber cement order, see how many squares and pieces are in a box.