How Many Bundles of Shingles for a 20x20 (gable) Roof?
A 20x20 (gable) roof with a 5:12 slope requires 18 bundles of shingles (~5.24 squares) including cut waste. This accounts for a total sloped roof surface area of 524 sq ft. You will also need 1 roll of synthetic underlayment (10-sq each) and 1 bundles of ridge cap shingles.
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Roof Dimensions & Pitch Specs
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Roof Takeoff Schedule
| Material | Quantity | Unit | Est. Cost |
|---|---|---|---|
| Architectural Shingles | 18 | bundles | $693 |
| ↳ Net Roof Area | 5.24 | squares | (524 sq ft) |
| Synthetic Underlayment | 1 | rolls (10-sq) | $89 |
| Ice & Water Membrane | 1 | rolls (2-sq) | $115 |
| Drip Edge Flashing | 11 | pcs (10') | $124 |
| Hip & Ridge Caps | 1 | bundles | $52 |
| Coil Roofing Nails | 1 | box (7.2k) | $46 |
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Material Takeoff & Accessory Checklist
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| Item / Specification | Category | Quantity | Unit Price | Total Est. | Supplier / Order |
|---|---|---|---|---|---|
Architectural Laminated Asphalt Shingles Lifetime warranty shingles, 3 bundles per square. Includes 10% cut waste. | primary | 18 bundles | $693 | Home Depot | |
High-Performance Synthetic Roof Underlayment 10-sq (1,000 sq ft) heavy-duty tear-resistant moisture barrier roll | accessory | 1 rolls (1000 sf) | $89 | Home Depot | |
Self-Adhering Ice & Water Protection Membrane 36 in. x 66.7 ft (200 sq ft) leak barrier for eaves and valleys | accessory | 1 rolls (200 sf) | $115 | Home Depot | |
T-Style Aluminum/Galvanized Drip Edge (10-ft) Directs water runoff into gutters and protects roof deck fascia | accessory | 11 pieces (10') | $124 | Home Depot | |
Universal Starter Strip Shingles Factory-applied sealant strips for high-wind perimeter lock (105 linear ft/bundle) | accessory | 1 bundles | $44 | Local Supplier | |
Hip & Ridge Cap Shingles Covers ridges and hips (~33 linear ft coverage per bundle). Required: 22 lin ft. | accessory | 1 bundles | $52 | Local Supplier | |
Rigid Polypropylene Ridge Exhaust Vent (4-ft) Internal baffle attic exhaust ventilation with exterior filter screen | accessory | 6 pieces (4') | $93 | Local Supplier | |
1-1/4" Electro-Galvanized Roofing Nails (7,200 ct) Coil roofing nails for pneumatic nailers or hand drive, corrosion resistant | fastener | 1 box (7,200 ct) | $46 | Local Supplier |
Calculation Mechanics: Roof Slope Multiplier & Shingle Takeoff
Formulas adhere to ASTM standards, ACI 318 guidelines, and standard construction estimating practice.
Step-by-Step Derivation Breakdown:
- 1Compute footprint with eave overhang: (20 + 2×1.00) × (20 + 2×1.00) = 484 sq ft.
- 2Calculate pitch multiplier for 5:12 slope: M = √(1 + (5/12)²) = 1.0833x.
- 3Determine true slope surface area: 484 sq ft × 1.0833 = 524 sq ft.
- 4Convert to roofing squares: 524 sq ft ÷ 100 = 5.24 squares.
- 5Apply 10% cut/valley waste: 5.24 × 1.10 = 5.76 squares.
- 6Calculate bundle order count: Architectural shingles require 3 bundles per square = 18 bundles.
Field Inspection & Trade Best Practices
- For roof pitches below 4:12 (low slope), building code (IRC R905.2.7) requires a double layer of underlayment or full ice-and-water self-adhering membrane across the entire deck.
- Starter shingles must be installed on both the eaves and the rake edges to create an adhesive seal that withstands 110-130 MPH wind gusts.
- Use 4 nails per shingle for standard slopes, and 6 nails per shingle in designated high-wind hurricane zones or for slopes exceeding 21:12 (mansard roofs).
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How This Takeoff Was Calculated
Step-by-Step Formula Breakdown:
Roof Area = Footprint (484 sq ft) × Pitch Multiplier (1.0833x) = 524 sq ft (5.24 squares). Bundles = 5.24 squares × 3 bundles/sq × 1.10 waste = 18 bundles.
Frequently Asked Questions for 20x20 2-Car Garage
How many bundles of shingles are in a square?
For standard fiberglass architectural and 3-tab shingles, there are exactly 3 bundles per square (100 square feet of roof area). Very heavy designer shingles may require 4 to 5 bundles per square.
How much does shingle waste increase for a hip roof?
Hip roofs generate more diagonal cut-off waste along the 4 hip ridges. We recommend planning 15% waste on hip roofs versus 10% on simple gable roofs.
How many rolls of underlayment are needed?
Standard synthetic roofing underlayment covers 10 squares (1,000 square feet) per roll. For this roof, you will need 1 roll(s) allowing for 4-inch overlaps.