Sri Lanka Roofing Sheet Calculator

Roofing quantities come from one piece of geometry and a handful of catalogued sheet dimensions. Every length you type is converted to metres first, so the metric and feet inputs always give the same answer.

1. Rafter run. For a gable or hip roof the horizontal run of one slope is span ÷ 2 + eave overhang. A single-slope lean-to uses the full span plus the eave.
2. Slope (rafter) length. The sheet runs up the slope, not across the plan, so slope = run ÷ cos(pitch). This is standard roof trigonometry — the same measurement CIDA/ICTAD BOQs use for roof area.
3. Columns across the ridge. columns = ⌈ridge length ÷ effective cover width⌉. Cover width is the net figure after the side-lap: about 0.98 m for UPVC/ASA and 1.0 m for metal tile-profile and fibre-cement, per SLS standards and manufacturer spec sheets.
4. Rows up the slope. rows = ⌈slope length ÷ (sheet length − end-lap)⌉. The end-lap (150 mm for fibre-cement per SLS 9, ~200 mm for profiled sheets) is the vertical overlap between stacked courses.
5. Sheets. Multiply columns × rows × the number of slopes, then add the wastage allowance for cuts and breakage — about 7% for a gable, 10% for a hip (more diagonal cuts) and 5% for a lean-to. The result is rounded up to whole sheets.
6. Ridge caps, purlins and screws. Ridge caps = ⌈ridge ÷ 1.8 m⌉ (cover per cap). Purlin rows per slope = ⌊slope ÷ spacing⌋ + 1 at the CIDA 0.6–0.9 m spacing. Fasteners = roof area × 6 per m² — one per corrugation crest at every purlin line.

A hip roof has the same total slope area as a gable over the same footprint, so its sheets are estimated by area (roof area ÷ one sheet's coverage) with the higher hip wastage, because the diagonal cuts across its triangular faces make a plain column-by-row layout meaningless. As a credibility check the calculator always shows an independent area-based sheet count beside the layout count. Sheet cover widths and end-laps come from manufacturer catalogues (Amano, Melwa/Lanwa, Rhino/Ceylon) and the SLS dimensional standards; the spacing, wastage and fastener norms come from CIDA/ICTAD roofing practice.

• CIDA / ICTAD — Construction Industry Development Authority (Schedule of Rates & roofing norms)
• Sri Lanka Standards Institution — SLS 9 & SLS 723 (roofing sheet dimensional standards)
• SLSI standards catalogue — profiled & fibre-cement sheeting

Sheet cover widths, standard lengths and end-laps are catalogued dimensions from Sri Lankan manufacturer/importer spec sheets (Amano ASA/UPVC; Melwa/Lanwa colour-coated metal tile-profile; Rhino/Ceylon fibre-cement) cross-checked against the SLS standards. Purlin spacing, wastage allowances and the ~6 fasteners/m² norm follow CIDA/ICTAD roofing practice. Last verified 2026-07-11. This is a quantity and costing estimator, not a structural design tool — consult a qualified engineer for rafter sizing, wind load and spans.