Roofing Calculator
Estimate roofing material, sloped roof area and cost from house footprint, roof pitch and waste allowance.
Angle of the roof slope, from 0° (flat) to 45°.
Optional — price per square metre of roofing material.
Roofing material needed
101.62 m²
Includes 10% waste allowance
Sloped roof area
92.38 m²
Ground area 80 m² × slope factor 1.15
In square feet
1,094 ft²
Slope factor
1.15×
Estimated material cost
₹60,972
101.62 m² × ₹600/m²
Measures the footprint on the ground, not the sloped surface. Add 5–10% extra for cuts, overlaps and ridge waste. Cost covers material only, not labour.
Slope correction factors
Multiply the ground (horizontal) area by the factor for your pitch to get the sloped roof area. A 6/12 pitch (26.6°) adds about 12% to the area.
| Pitch | Angle | Multiply by |
|---|---|---|
| 1/12 | 4.76° | 1 |
| 2/12 | 9.46° | 1.01 |
| 3/12 | 14.04° | 1.03 |
| 4/12 | 18.43° | 1.05 |
| 5/12 | 22.62° | 1.08 |
| 6/12 | 26.57° | 1.12 |
| 7/12 | 30.26° | 1.16 |
| 8/12 | 33.69° | 1.2 |
| 9/12 | 36.87° | 1.25 |
| 10/12 | 39.81° | 1.3 |
| 11/12 | 42.51° | 1.36 |
| 12/12 | 45° | 1.41 |
| 16/12 | 53.13° | 1.67 |
| 20/12 | 59.04° | 1.94 |
| 24/12 | 63.43° | 2.24 |
Last updated: May 2026
How this calculator is verified
Checked by True Calculator automated test suite on
- Formula verified against a published worked example in the automated test suite
- Edge cases (zero, negative, boundary and unit-mismatch inputs) covered by unit tests
The full verification method is on our how we verify page. Found an error? Tell us and we will re-check it.
When to Use This Calculator
Use this calculator whenever you need to buy roofing sheets, tiles or thatch for a new house, an extension, a shed, a garage or a veranda. It is especially useful in India's construction market, where materials are sold by the square metre or square foot and every sheet costs real money. Contractors often quote only the footprint, but you pay for the sloped surface, so knowing the true area helps you compare quotes fairly. It also helps when planning rainwater collection, solar panel layouts or ceiling insulation — all of which depend on the actual roof area. Run it once before you visit the dealer, and again if you change the roof design or pitch.
How to Use This Calculator
- Step 1: Measure the building footprint — the length and width of the roof as seen from directly above the ground, not the sloped surface.
- Step 2: Enter the roof pitch in degrees (0 for a flat roof, 30–45 for most pitched roofs). Steeper pitches increase the material area.
- Step 3: Set a waste allowance, typically 10% for cutting, overlaps and ridge trimming.
- Step 4: Read the material needed in m² and sq ft, then add any sheets or tiles you plan to keep as spares.
Worked Example
A house with a 12 m × 9 m footprint has a ground area of 108 m². At a 45° pitch the slope factor is 1.41, so the sloped roof measures 152.73 m². Adding 10% waste for cuts and overlaps gives about 168 m² of roofing material to order, which is roughly 1,808 sq ft. The same footprint with a flat roof would need only 108 m² plus waste, showing how much a steep pitch adds to the material bill.
Tips and Common Mistakes
- •Tip 1: Measure the footprint wall to wall — eaves overhangs add area you may want to include.
- •Tip 2: For corrugated sheets, check the effective cover width, which is less than the sheet width due to overlaps.
- •Tip 3: Add 5% extra if your roof has valleys, hips or skylights where cutting waste is higher.
- ✗Mistake 1: Do not measure the sloped surface directly with a tape — that double-counts the pitch factor.
- ✗Mistake 2: Do not skip the waste allowance for thatched or tile roofs, which lose a lot of material at edges and ridges.
Slope Factor by Roof Pitch
| Item | Value |
|---|---|
| Flat (0°) | 1.00 |
| Low pitch (15°) | 1.04 |
| Standard (30°) | 1.15 |
| Steep (45°) | 1.41 |
| Very steep (60°) | 2.00 |
Expert Note
The waste allowance is critical: corrugated sheets lose 10-15% to overlaps, while clay tiles can lose 20% at ridges and valleys. In India, roofing materials are sold per sheet or per square metre — always order based on the sloped area, not the footprint. For pitched roofs above 30 degrees, the material cost increase is significant (slope factor > 1.15).
Sources and References
- •IS 875 (Part 3): Code of Practice for Design Loads for Buildings — Bureau of Indian Standards
- •Roof pitch and slope factor tables — National Building Code of India 2016
Official sources are linked so you can confirm the current rate yourself. Check the linked page for the latest notification before relying on these figures.
Frequently Asked Questions
How is roofing material area calculated?
The ground (footprint) area is multiplied by a slope factor of 1 ÷ cos(pitch). A 30° pitch has a factor of about 1.15, so a 100 m² footprint becomes roughly 115 m² of sloped roof. A waste allowance of 5–10% is then added for cuts and overlaps.
Why do I need more material than the ground area?
Because a pitched roof is larger than the floor it covers. The steeper the pitch, the bigger the sloped surface. At 45°, for example, the slope factor is about 1.41, so the roof surface is 41% larger than the footprint.
What waste percentage should I use in India?
For standard corrugated sheets or tiles, 5–10% is typical. Allow more for complex hip and valley roofs, or when sheets are cut around chimneys and skylights. For thatched or traditional tiled roofs, 10–15% is safer.
Does this calculator work for flat roofs?
Yes — set the pitch to 0°, which gives a slope factor of 1. The material area then equals the ground area plus the waste allowance you choose.
Is the result in square metres or square feet?
The primary result is in square metres (m²), which is standard for construction in India, with a square feet equivalent shown alongside. 1 m² equals 10.76 sq ft.
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