Flat Roof Calculator
Calculate materials for flat roofs including drainage slope requirements.
Formula & Explanation
Flat roof calculations are simplified because there is no slope: Roof Area = Length × Width Roofing Squares = ceil(Roof Area ÷ 100) However, flat roofs MUST have minimum drainage slope per building code: Minimum Drainage Rise = Drainage Length × (Minimum Slope ÷ 12) Where minimum slope is typically 0.5" to 1" per foot (codes vary). Example: 40 × 25 ft flat roof with 0.5" minimum slope - Area = 40 × 25 = 1000 sq ft - Squares = ceil(1000 ÷ 100) = 10 squares - Drainage rise = 40 × (0.5 ÷ 12) = 1.67 feet This ensures water drains to gutters or internal drains.
Assumptions
- •Flat roofs are technically 0:12 pitch but require minimum drainage slope per building code
- •Typical minimum slope is 0.5:12 to 1:12 (0.5" to 1" rise per 12" run)
- •Drainage slope prevents water pooling, which shortens roof life and creates structural risk
- •Calculations assume rectangular footprint; complex shapes require detailed measurements
- •Built-up roofs, TPO, and EPDM are common flat roof materials with different costs
- •Interior drains or scuppers required if roof cannot slope to gutters
Example Calculation
A 40×25 ft flat roof requires 10 roofing squares and 1.67 feet of rise for proper drainage.
Inputs
Results
Flat Roof Material Options
| material | durability | costPerSqFt |
|---|---|---|
| Built-up Roof (BUR) | 15-20 years | $1.50-3.00 |
| TPO (Thermoplastic) | 15-20 years | $2.00-3.50 |
| EPDM (Rubber) | 20-25 years | $2.50-4.00 |
| PVC | 20-30 years | $3.00-5.00 |
Frequently Asked Questions
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