2. Basic Measurement Terms You Must Know

• Span: The full width of the building (wall-to-wall).
• Run: Half the span (distance from wall plate to ridge horizontally).
• Pitch / slope: Roof angle (degrees) or ratio (rise:run).
• Rafter length: Slope length from wall plate to ridge (includes overhang).
• Sheet cover width: The effective width a roofing sheet covers after overlaps (varies by profile).
• Spacing: Distance between rafters (commonly 600mm or 900mm) and between battens (300–400mm).

Common cover widths (typical examples): IBR ≈ 0.686m, Corrugated ≈ 0.762m. Confirm with your supplier.

3. Calculate Roofing Sheets (Step-by-Step)

We’ll use a practical example so you can follow the math. Example house:

• Plan size: 10m (length) × 7m (span)
• Roof type: Gable
• Pitch: 25°
• Sheet profile: IBR (cover width 0.686m)

Step 1 — Calculate rafter/slope length

Run = Span ÷ 2 = 7 ÷ 2 = 3.5m

Rafter length (un-edged) = Run ÷ cos(pitch) → cos(25°) ≈ 0.906

Rafter length = 3.5 ÷ 0.906 ≈ 3.86m

Add overhang (eaves) typically 0.3–0.4m each side → Use sheet length = 4.2m in this example.

Step 2 — Sheets per side

Sheets per side = Roof length ÷ sheet cover width

10 ÷ 0.686 ≈ 14.58 → round up to 15 sheets per side.

Total sheets (two sides) = 15 × 2 = 30 sheets.

Step 3 — Wastage & extras

Always add 5–10% for cutting, overlaps, damaged sheets, and non-standard lengths. For 30 sheets add 10% → 3 extra sheets → 33 sheets total.

4. Calculate Timber — Rafters, Ridge, Wall Plates & Battens

Rafters — how many and length

Rafter spacing commonly 0.9m (900mm) or 0.6m (600mm) depending on timber size and roof load. Use:

Number of rafters per side = Roof length ÷ spacing

Example (spacing 0.9m): 10 ÷ 0.9 ≈ 11 rafters per side → 11 × 2 = 22 rafters.

Rafter length (cut length) = slope length + overhang ≈ 4.2m (from earlier).

Ridge board

Ridge length = house length (10m). Use solid timber or engineered product sized per engineer’s advice (e.g., 38 × 200mm).

Wall plate

Wall plate runs continuously along top of walls. Perimeter = 2 × (length + span) → 2 × (10 + 7) = 34m. If timber comes in 6m lengths you will need 34 ÷ 6 ≈ 6 → round to 7 pieces.

Battens / purlins

Batten spacing depends on sheet type (commonly 300–400mm). Calculate battens per rafter:

Battens per rafter = Rafter length ÷ batten spacing

Example: 3.86 ÷ 0.3 ≈ 13 battens per rafter → 13 × 22 rafters = 286 battens. Convert to linear lengths and order standard timber lengths (6m) — round up for wastage.

Fasteners & fittings

Order galvanised screws, washers, ridge flashings, barge boards and fascia. Always check supplier packs (screws per box) and estimate by sheet and rafter counts.

5. Material List — Example (10m × 7m gable roof)

Note: This is a simplified take-off to get a quick quote. For tendering, request a detailed bill of quantities from your contractor or use a professional take-off service.

6. Quick Formulas (Copy & Save)

• Rafter length = (Span ÷ 2) ÷ cos(pitch)
• Sheets per side = Roof length ÷ sheet cover width
• Total sheets = Sheets per side × number of slopes (typically 2)
• Rafters per side = Roof length ÷ rafter spacing
• Battens per rafter = Rafter length ÷ batten spacing

7. Practical Tips & Waste Allowance

• Always add 5–10% to sheets and timber for waste, offcuts and mistakes.
• Confirm available sheet lengths from local suppliers — ordering uncommon lengths increases cost.
• Use galvanized fasteners and corrosion-resistant accessories to extend roof life.
• For hip roofs or complex shapes, increase quantities by 15–25%.
• Order battens and common timbers in standard sizes (6m) and plan joinery to reduce laps.
• Keep a small contingency stock on site to avoid delays.

8. Final Notes

Accurate roofing and timber calculations save time and money on site. Use the step-by-step method above for quick estimates and always validate with supplier data and contractor quotes before ordering. For complex roofs, or if you plan to use heavy roofing finishes (tiles, concrete), consult a structural engineer.