How It Is Calculated
This calculator determines the number of shingle bundles needed for a gable or shed roof and the total material cost. The calculations follow these steps:
- Total roof area: Multiply the roof length, roof width, and number of slopes.
- Adjusted area: Increase the total area by the waste factor to account for cutting and fitting.
- Bundles needed: Divide the adjusted area by the coverage per bundle (based on shingle type) and round up to the nearest whole bundle.
- Material cost: Multiply the number of bundles by the price per bundle.
A = L × W × N
Aadj = A × (1 + w / 100)
B = ceil(Aadj / C)
M = round(B × P, 2)
Where:
- L = roof length (ft)
- W = roof width (ft)
- N = number of roof slopes
- A = total roof area (ft²)
- w = waste factor (%)
- Aadj = adjusted area (ft²)
- C = coverage per bundle (ft²/bundle) – 33.33 for 3-Tab, 25 for Architectural
- B = number of bundles needed
- P = price per bundle ($)
- M = material cost ($)
Assumptions
- The roof is a simple gable or shed roof with no hips, valleys, or dormers.
- Roof length and width are measured along the roof plane (not the footprint).
- Shingle coverage per bundle is as specified by the selected shingle type: 33.33 sq ft for 3-Tab (3 bundles per square) and 25 sq ft for Architectural (4 bundles per square).
- Waste factor accounts for cutting, fitting, and starter strip.
- Price per bundle is the user's local cost.
Worked Example
Suppose you have a gable roof with the following inputs:
- Roof length = 40 ft
- Roof width = 20 ft
- Number of roof slopes = 2
- Shingle type = 3-Tab (33.33 sq ft/bundle)
- Waste factor = 10%
- Price per bundle = $30
Step 1: Total roof area
A = 40 ft × 20 ft × 2 = 1600 ft²
Step 2: Adjusted area
Aadj = 1600 ft² × (1 + 10/100) = 1600 ft² × 1.10 = 1760 ft²
Step 3: Bundles needed
B = ceil(1760 ft² / 33.33 ft²/bundle) = ceil(52.8) = 53 bundles
Step 4: Material cost
M = 53 bundles × $30/bundle = $1590.00
Results: Total roof area = 1600 ft², Shingle bundles needed = 53 bundles, Shingle material cost = $1590.
Practical Tips
- Always measure roof length and width on the roof surface, not the ground footprint, to account for the slope.
- For complex roofs with hips or valleys, use a Roofing Calculator that handles those features.
- Order extra bundles for starter strips and ridge caps – the waste factor already includes some allowance, but a few extra shingles can save a trip to the store.
- If you need to know the roof pitch for accurate measurements, try the Roof Pitch Calculator.
- Check local building codes for underlayment requirements – this calculator only covers shingles.
- For other roofing materials like metal or tile, use the Roofing Calculator for a broader estimate.