Roofing worked example: 11 m x 7.5 m roof at 6/12 pitch

BuildTrue roofing pack. Estimates only. This example uses the same formulas as the roofing calculator on the pack page and the standalone BuildTrue roofing estimator. Verified by hand, step by step, below.

Inputs

Input	Value
Building length, including eaves overhang	11 m
Building width, including eaves overhang	7.5 m
Pitch	About 6/12, pitch factor 1.12
Waste and cuts allowance	12 percent
Bundle coverage, stated assumption	One bundle covers about 3.1 sq m
Square, stated	1 square = 100 sq ft = 9.2903 sq m
Underlayment roll coverage, stated assumption	About 37.16 sq m (400 sq ft) per roll
Conversion, stated	1 sq m = 1 / 0.09290304 sq ft = 10.7639 sq ft

The dimensions already include overhang. If you measure the building footprint at the walls, add the overhang to length and width before using these formulas, or the order will be short on every line.

Step by step arithmetic

1. Footprint:

11 x 7.5 = 82.50 sq m.

1. Roof area before waste (pitch adjustment):

82.50 x 1.12 = 92.40 sq m.

1. Roof area with waste:

92.40 x (1 + 12 / 100) = 92.40 x 1.12 = 103.488 sq m, shown as 103.49 sq m. Note the coincidence in this example: the pitch factor and the waste multiplier are both 1.12. They are separate steps that happen to share a number here.

1. Roof area with waste, in square feet:

103.488 / 0.09290304 = 1113.94 sq ft, shown as 1113.9 sq ft.

1. Bundles:

103.488 / 3.1 = 33.383 bundles. Round up with the epsilon safe ceiling: 34 bundles. Sanity check: 33 bundles x 3.1 = 102.3 sq m, short of 103.49. 34 x 3.1 = 105.4 sq m, which covers it.

1. Squares:

103.488 / 9.2903 = 11.139 squares, shown as 11.14 squares. Cross check in feet: 1113.94 sq ft / 100 = 11.139 squares. The two routes agree. Bundle cross check: at about 3 bundles per square, 11.14 squares x 3 = 33.4 bundles, which rounds to the same 34 bundles.

1. Underlayment rolls:

103.488 / 37.16 = 2.785 rolls. Round up: 3 rolls.

Purchasable counts

Item	Raw result	Buy
Shingle bundles	33.38 bundles	34 bundles
Underlayment rolls	2.78 rolls	3 rolls
Squares, for pricing conversations	11.14 squares	Quoted as about 11.1 squares; shingles are bought by the bundle, not the square
Ice and water membrane, drip edge, ridge cap, flashing, vents, nails	Not produced by the area formula	Count from eave, valley, ridge, and penetration measurements on site. See the pack material list template

Sample material list for this job

Item	Specification	Quantity
Shingle bundles	Architectural, about 3.1 sq m per bundle	34
Underlayment	About 37.16 sq m per roll	3 rolls
Ice and water membrane	For eaves and valleys	Per linear measurement on site
Drip edge	Eaves and rakes	Per linear measurement on site
Ridge cap	To match shingle system	Per ridge and hip length on site
Roofing nails	As your fastening system uses	Per bundle count and fastening pattern
Flashing and vent pieces	Stacks, vents, wall intersections	Counted on site

Notes and limits

• A simple gable roof is assumed. Valleys, hips, dormers, and multiple pitch changes raise real waste above a flat percentage; raise the allowance or measure each plane separately.
• The 12 percent allowance in this example is higher than the standalone page default of 10 percent, on purpose: it suits a roof with cuts. State the allowance you used on the quote.
• Estimates only. Roofing involves height, structure, ventilation, and code requirements. This is not a roofing design. Verify measurements and materials with a qualified roofer before ordering or climbing, and confirm local code and permit requirements where they apply.