Bonnet Roof Calculator

Calculate area of a bonnet (kicked eave) roof with steep upper slope and gentle lower slope extending as covered porch

Quick Estimate

Building Length?

Building Width?

Upper Roof Pitch?

:12

Lower Overhang Pitch?

:12

Lower Overhang Width?

Waste Factor?

Total Roof Area

2,560.3

Upper Hip Section Area1,983.1

Lower Overhang Area577.2

Area with Waste2,867.5

Roofing Squares28.68

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Need more accuracy?

Extended mode adds scenarios, charts, and detailed breakdowns

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How to Use the Bonnet Roof Calculator

Using the Bonnet Roof Calculator is straightforward. Gather your building measurements and roof pitch before you start — accurate inputs lead to accurate material estimates.

Measure the exterior length and width of your building at the foundation.
Identify your roof pitch (rise per 12 inches of run) — check your building plans or use a pitch gauge.
Enter both measurements and pitch into the calculator above.
Review the result: total roof surface area in square feet and roofing squares.
Add a 10–15 % waste factor when ordering materials for a standard roof, or 15–20 % for complex roofs with many hips and valleys.

Tip: Start with the Quick calculator for a fast estimate. Switch to Extended mode for scenario comparison and charts, or Professional mode for complete parameter control.

Formula & Methodology

The Bonnet Roof Calculator uses the following established formulas:

Roof Area = Footprint Area × Pitch Multiplier Pitch Multiplier = √(1 + (rise/12)²) Footprint Area = Length × Width (including overhangs) 1 Roofing Square = 100 sq ft of roof area

The pitch multiplier accounts for the extra surface area created by the slope of the roof.

Frequently Asked Questions

What is the easiest way to measure roof area without climbing on the roof?

Measure your building's footprint (length × width) from the ground, then multiply by the pitch multiplier for your roof slope. This gives a very accurate surface area without any ladder work. Our Bonnet Roof Calculator does this calculation automatically.

How much extra material should I order for waste?

A standard gable roof needs 10 % waste. Add 15 % for moderate complexity (a few hips or valleys) and up to 20 % for highly complex roofs. Always round up to the nearest bundle when ordering shingles.

What is a roofing square and why does it matter?

One roofing square equals 100 square feet of roof surface. Roofing materials — shingles, underlayment, and insulation — are priced and sold per square, so converting your area to squares makes ordering much easier.

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Sources & References

The formulas, code requirements, and reference data used in this calculator are sourced from the following authoritative organizations and publications. Local codes vary — always verify with your jurisdiction's building department.

International Residential Code (IRC) 2024 — Chapter 9: Roof Assemblies — ICC
NRCA Roofing Manual (Steep-Slope & Low-Slope) — National Roofing Contractors Association
Asphalt Shingle Installation Manual — ARMA (Asphalt Roofing Manufacturers Association)
GAF Roof Measurement Guide — GAF
NRCA Project Pricing Guidelines — NRCA

Disclaimer: Calculator results are estimates intended for planning and educational purposes only. They do not replace professional engineering analysis, building permit review, or contractor estimates. Always consult a licensed roofing contractor or structural engineer for final specifications.