Roofing Calculator for an Addition
Estimate materials for a room-addition roof.
An addition roof needs to tie into the existing house — usually with a valley or a shed roof against the main wall. Order shingles from the same manufacturer line as the existing roof to minimize color mismatch. Add 15% waste for valley cuts.
Squares
13.42
1,342 ft²
Shingle bundles
41
3 bundles / square
Underlayment rolls
4
400 ft² rolls
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Recommended tools & materials
Handy picks for this project on Amazon.
Roofing Nailer
Speed up shingle installation
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Synthetic Roof Underlayment
Lightweight, tear-resistant
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Roofing Safety Harness Kit
Stay safe on steep pitches
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Utility Knife with Hook Blades
Cut shingles cleanly
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How does the roofing calculator work?
Roof area = house footprint × pitch factor. The pitch factor is √(1 + (rise/12)²): a 6:12 roof has a 1.118 factor. A "square" is 100 ft² of roof. Shingles are sold in 3 bundles per square.
Which guide explains this in more depth?
- Panel roofs waste differently from shingles: ribs, hips and valleys all change the number you order. Read the metal roofing calculator waste factor guide for the full worked example.
What else do people ask about this calculator?
How much roofing for a 400 sq ft addition?
A 400 sq ft addition at 6:12 pitch is about 4.5 squares of roof. That's ~14 bundles plus one bundle of hip and ridge cap.
Can I match new shingles to an old roof?
Same brand, line and color code is the closest match — but expect a slight difference. New shingles look brighter for a year or two before they weather.
How many bundles of shingles do I need?
3 bundles per roofing square. For a 40×30 ft home with a 6:12 roof: about 13.4 squares × 3 = 41 bundles.
What is a roofing square?
100 ft² of roof surface. Shingles are priced per square and packaged in bundles that cover ~33 ft² each.
Which related calculators should I use next?
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