Roofing Calculator
How Much Roofing Material Do You Need?
Roofing projects are expensive and can't be paused mid-job to run to the supplier. Getting the material estimate right before you start — accounting for roof pitch, overhangs, and waste — is one of the most important steps in planning a roofing project, whether you're doing it yourself or verifying a contractor's quote.
How to Use This Calculator
Enter your roof's length (along the ridge), width (eave to eave), pitch (rise per 12 inches of run), and overhang on each side. Add a waste factor for cuts and waste around chimneys, valleys, and hips. Hit Calculate Roofing to see total squares, bundle count, and underlayment needed.
How Roof Area Is Calculated
A roof's actual surface area is larger than its floor footprint because it's sloped. The pitch factor converts footprint to true sloped area:
Pitch factors by roof slope:
- 2/12: 1.014 — nearly flat, minimal extra area
- 4/12: 1.054 — low-slope, 5.4% more than footprint
- 5/12: 1.083 — common residential pitch, 8.3% more
- 6/12: 1.118 — moderate pitch, 11.8% more
- 8/12: 1.202 — steep, 20% more area than footprint
- 12/12: 1.414 — 45° pitch, 41% more area than footprint
Real-World Example
Using the defaults — 40 ft × 30 ft roof, 5/12 pitch, 1 ft overhang each side, 10% waste:
- Footprint (with overhang): ~1,260 sq ft
- Sloped area (×1.083): ~1,365 sq ft
- With 10% waste: ~1,500 sq ft
- Roofing squares: ~15 squares
- 3-tab shingle bundles (3/square): ~45 bundles
- Underlayment rolls: ~15 rolls
What Is a Roofing Square?
One roofing square = 100 square feet. All roofing materials are quoted and ordered by the square. It's the fundamental unit of roofing estimation — when a contractor says "I'll replace your roof for $X per square," that's per 100 square feet of roof area.
Shingles are sold in bundles. Three bundles cover one square for standard 3-tab shingles. Architectural (dimensional) shingles are heavier and often require 4 bundles per square — always confirm with your supplier since it varies by product.
Complete Material List
Beyond shingles, a full roofing job requires:
- Underlayment (felt or synthetic): 1 roll per square — provides a moisture barrier under shingles
- Drip edge: metal flashing along all eaves and rake edges — measured in linear feet
- Ridge cap shingles: covers the peak — measure linear feet of all ridges and hips
- Roofing nails: approximately 2–3 lbs per square (hand nailing) or equivalent in coil nails for a nailer
- Flashing: around chimneys, skylights, dormers, and roof-to-wall intersections — measured per penetration
- Ice and water shield: self-adhering membrane for eaves and valleys in cold climates — typically first 3–6 feet from the eave
Frequently Asked Questions
What waste factor should I use?
10% is standard for simple gable roofs with minimal penetrations. Add 15% for roofs with multiple valleys, hips, dormers, or skylights — complex geometry creates more cut waste. Add 15–20% for steep roofs (9/12 and above) where working conditions make waste more likely. When in doubt, round up — leftover shingles store well and are useful for future repairs.
What does roof pitch mean and how do I measure it?
Pitch is expressed as rise over run — how many inches the roof rises for every 12 inches of horizontal distance. A 6/12 pitch rises 6 inches for every foot of run. To measure your existing roof pitch: hold a level horizontal against the roof, measure 12 inches out from where it contacts the roof, then measure straight up to the roof surface. That vertical measurement is your rise.
How long does a typical asphalt shingle roof last?
Standard 3-tab asphalt shingles last 20–25 years. Architectural (dimensional) shingles last 25–30 years. Premium architectural shingles rated for 30+ years often last closer to 25 in practice due to UV exposure, temperature cycling, and weather. Metal roofs last 40–70 years. Slate and tile can last 50–100+ years with proper maintenance.
Should I tear off the old roof or reroof over it?
Most building codes allow a maximum of two layers of shingles. Reroofing over existing shingles saves labor and disposal costs, but hides potential deck damage and adds weight. A full tear-off to bare deck is the higher-quality option — it lets you inspect and repair the decking, install new underlayment, and ensures the new roof lies flat. Most roofing professionals recommend a full tear-off.
Can I use this calculator for metal roofing?
Yes — the area calculation works for any roofing material. Metal roofing (standing seam, corrugated panels) is ordered in square feet or linear feet of panels rather than squares and bundles. Take the total square footage result and use it to calculate panel quantities per your specific product's coverage width. Add 10–15% waste for metal roofing to account for panel overlaps and cuts.