How to Calculate Roof Sheet Quantity
To calculate the number of roof sheets required, divide the total roof length by the effective cover width of one sheet to find sheets per row, then divide the rafter length by the chosen sheet length (accounting for end overlap) to find the number of rows, and multiply the two results together. Always add 5–10% extra for wastage and cutting at edges and valleys.
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XXXXXXXXXX ca-pub-XXXXXXXXXXQuick Formula Summary
- Sheets per row = Roof length ÷ Effective cover width per sheet
- Number of rows = Rafter length ÷ (Sheet length − End overlap)
- Total sheets = Sheets per row × Number of rows
- With wastage = Total sheets × 1.05 (5%) or × 1.10 (10%)
- Standard effective cover: GC corrugated = 838 mm; Trapezoidal = 1000 mm; Hi-Rib = 760 mm
- Standard end overlap: 150 mm (slopes ≥ 10°), 200 mm (slopes 5°–10°)
What You Need Before You Start
Before calculating, gather these measurements and decisions:
- Roof length — the horizontal dimension running along the ridge (eave to eave).
- Roof width (rafter length) — the slope distance from ridge to eave. If you only have the plan width, convert using: rafter length = plan width ÷ cos(slope angle). For a 15° slope, divide by 0.966.
- Sheet profile — standard corrugated GC, trapezoidal, or tile/Hi-Rib. Each has a different effective cover width.
- Sheet length — standard lengths are 6 ft (1830 mm), 8 ft (2440 mm), 10 ft (3050 mm), 12 ft (3660 mm), and 14 ft (4270 mm). Choose the length closest to your rafter length (or greater, to minimise joints).
- Roof slope — determines the minimum required end overlap.
- Number of roof slopes — a gabled shed has 2 slopes; a hip roof has 4; a lean-to (monopitch) has 1.
Step-by-Step Manual Calculation
Step 1 — Determine Effective Cover Width
The effective cover width is the width of roof each sheet covers after overlapping with its neighbour. It is always less than the sheet's overall width.
| Profile | Overall Width | Side Overlap | Effective Cover Width |
|---|---|---|---|
| Standard GC Corrugated (sinusoidal) | 1000 mm | 1.5 corrugations (~162 mm) | 838 mm |
| Trapezoidal (e.g. JSW Colouron+) | 1130 mm | 1 rib (~130 mm) | 1000 mm |
| Hi-Rib / Tile (e.g. JSW Colouron+) | 900 mm | 1 rib (~140 mm) | 760 mm |
| IBR / Box Profile (industrial) | 925 mm | ~85 mm | 840 mm |
Step 2 — Calculate Sheets Per Row (Across the Slope)
Divide the total roof length by the effective cover width of one sheet. Always round up to the next whole number — you cannot use a fraction of a sheet.
Sheets per row = ⌈ Roof length ÷ Effective cover width ⌉
Example: Roof length = 9000 mm, GC corrugated (838 mm effective cover)
Sheets per row = ⌈ 9000 ÷ 838 ⌉ = ⌈ 10.74 ⌉ = 11 sheets
Step 3 — Determine End Overlap Required
End overlap (also called "end lap" or "head lap") is the overlap between two sheets running up the slope where one sheet ends and the next begins. It prevents wind-driven rain from getting under the sheets.
| Roof Slope | Minimum End Overlap | Notes |
|---|---|---|
| ≥ 15° (1:4 pitch or steeper) | 150 mm | Standard residential |
| 10° to 15° | 150 mm + foam closure strip | Use sealant tape |
| 5° to 10° | 200 mm + foam closure + sealant | Low-slope industrial |
| Below 5° | Not recommended for GC sheets | Use standing seam or flat roof system |
Step 4 — Calculate Number of Rows (Up the Slope)
If the rafter length is less than or equal to one sheet length, you only need one row and no end laps. If the rafter length is longer than one sheet, you need multiple rows with end laps.
Effective sheet length = Sheet length − End overlap (for non-first rows)
Number of rows = ⌈ Rafter length ÷ Effective sheet length ⌉
For a single row (rafter length ≤ sheet length), simply use 1 row.
Step 5 — Calculate Total Sheets (Before Wastage)
Total sheets = Sheets per row × Number of rows × Number of slopes
Step 6 — Add Wastage Factor
Always add a wastage allowance for cuts at hip ends, valleys, around penetrations (vents, pipes), and breakage during installation.
- 5% wastage — simple rectangular roof with no hips or valleys
- 10% wastage — gabled roof with some cutting at verge ends
- 15% wastage — complex hip roof, multiple valleys, or diagonal cutting
Final quantity = ⌈ Total sheets × (1 + wastage%) ⌉
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XXXXXXXXXX ca-pub-XXXXXXXXXXWorked Example — 20 × 30 ft Shed
Let us work through a complete calculation for a common Indian shed: a gabled shed measuring 20 ft × 30 ft (approximately 6.1 m × 9.1 m), using standard corrugated GC sheets (0.47 mm, 12 ft / 3660 mm length), with a 15° roof slope.
Given Information
- Shed plan dimensions: 30 ft (9144 mm) long × 20 ft (6096 mm) wide
- Roof type: Gabled (2 slopes)
- Slope: 15° (plan half-width = 3048 mm per slope)
- Rafter length = 3048 mm ÷ cos(15°) = 3048 ÷ 0.966 = 3155 mm ≈ 3.16 m
- Sheet: Standard GC corrugated, 12 ft (3660 mm) length, 838 mm effective cover
- End overlap: 150 mm (slope ≥ 15°)
- Wastage: 8%
| Step | Calculation | Result |
|---|---|---|
| 1. Effective cover width | Standard GC corrugated profile | 838 mm |
| 2. Sheets per row (along ridge) | ⌈ 9144 mm ÷ 838 mm ⌉ = ⌈ 10.91 ⌉ | 11 sheets |
| 3. Rafter length | 3048 ÷ cos(15°) = 3048 ÷ 0.966 | 3155 mm |
| 4. Sheet length chosen | 12 ft = 3660 mm (covers full rafter in one sheet) | 3660 mm — single row |
| 5. Number of rows per slope | 3155 mm < 3660 mm → single row, no end lap needed | 1 row |
| 6. Sheets per slope | 11 sheets × 1 row | 11 sheets |
| 7. Number of slopes | Gabled roof | 2 slopes |
| 8. Total sheets (before wastage) | 11 × 2 | 22 sheets |
| 9. Wastage factor (8%) | ⌈ 22 × 1.08 ⌉ = ⌈ 23.76 ⌉ | 24 sheets |
Result: Order 24 sheets of 12 ft GC corrugated (0.47 mm) for this shed.
If you had used 10 ft sheets (3050 mm) instead, the rafter length (3155 mm) would exceed one sheet. You would need 2 rows per slope with a 150 mm end lap, giving an effective run of 3050 + (3050 − 150) = 5950 mm for two rows — more than enough. But you would order: 11 sheets × 2 rows × 2 slopes = 44 sheets before wastage (more sheets, more joints, more potential for leaks). Always prefer a sheet length that spans the full rafter in one piece.
Calculator vs Manual Calculation
The manual method above gives you a solid understanding of the logic, but for real projects you should cross-check with a calculator tool — especially when dealing with non-rectangular roofs, multiple pitches, or different sheet profiles in one building.
Our Roof Sheet Calculator handles all the arithmetic automatically — enter your dimensions, select your sheet profile and length, set your slope and wastage percentage, and it returns the exact sheet count with a full breakdown. It supports GC corrugated, trapezoidal (1000 mm), and Hi-Rib (760 mm) profiles.
Try the Roof Sheet Calculator
Enter your shed dimensions and get an exact sheet count in seconds. Supports multiple profiles, wastage factors, and multi-slope roofs.
Open Roof Sheet CalculatorFrequently Asked Questions
How many GC sheets do I need for a 20x30 ft shed?
For a 20×30 ft gabled shed using 12 ft GC corrugated sheets (838 mm effective cover), you need approximately 22–24 sheets including 8% wastage — 11 sheets per slope across 2 slopes. If you choose 10 ft sheets (requiring 2 rows per slope), you will need 44–48 sheets before wastage. Use our Roof Sheet Calculator for an exact count based on your slope angle and sheet profile.
What is effective cover width and why does it matter?
Effective cover width is the width of roof that each sheet actually covers after overlapping with the adjacent sheet. A standard GC corrugated sheet is 1000 mm wide overall, but the side overlap consumes about 162 mm, leaving 838 mm of effective cover. If you calculate using the full 1000 mm width, you will under-order by about 16% and your sheets will not reach the end of the roof.
How much wastage should I add for roof sheets?
Add 5% for a simple lean-to (monopitch) rectangular roof with no cuts. Add 8–10% for a standard gabled roof with some cutting at verge ends. Add 12–15% for hip roofs, roofs with skylights or vents, or any roof requiring diagonal cuts. Wastage covers sheet breakage, cutting for eave overhangs, trimming at ridge and verge, and miscuts during installation.
What is end overlap and how much do I need?
End overlap (also called end lap or head lap) is the vertical overlap where one sheet ends and the next sheet begins going up the slope. A minimum of 150 mm end overlap is required for roof slopes of 10° or greater. For low-pitched roofs between 5° and 10°, increase the end overlap to 200 mm and add foam closure strips with sealant tape. End overlap is not needed if one sheet spans the full rafter length.
Should I use the plan dimension or rafter length for sheet calculation?
Always use the rafter length (actual slope dimension), not the plan dimension (horizontal ground measurement). For a 15° slope, the rafter length is about 3.4% longer than the plan measurement. For steeper slopes the difference is larger — at 30° the rafter is 15% longer than plan. Use the formula: rafter length = plan half-width ÷ cos(slope angle). Getting this wrong will leave your sheets short on the slope.
What length sheet should I choose?
Choose the sheet length that covers the full rafter length in one piece, with 50–75 mm overhang at the eave. A single-sheet span avoids end joints, which are always potential leak points. If no standard length covers the full rafter, use the longest available sheet and add a second row at the top with a 150–200 mm end lap. Avoid mixing two partial rows when one full sheet would do.