Roofing sheet (CGI/jasta) calculator
Work out the true sloped roof area from your plan dimensions and pitch, then the number of CGI/jasta sheets needed, with a wastage allowance and cost estimate.
Pick a pitch preset or enter a custom angle, set your sheet's effective cover width and length, and see every step of the maths, from plan area to sheets across, sheets up the slope and total cost.
Roof plan
Runs up the rafter direction.
Sets sheets side by side.
Sheet size
Usable width per sheet after the side overlap, commonly 26 in.
Wastage & cost
Extra for cuts and overlap at hips/valleys, typically 5%.
Enter your local sheet price to estimate cost.
Sheets needed (with wastage)
53 sheets
10 across × 5 up the slope, before wastage
Estimated cost
Rs 45,050
Plan area
600 sqft
Sloped area
649.4 sqft
Sheets (no wastage)
50
With wastage
53
| Sloped area formula | plan area ÷ cos(pitch in radians) |
| Plan area | 30 ft × 20 ft = 600 sqft |
| Sloped area | 600 ÷ cos(22.5°) = 649.4 sqft |
| Sheets across (lengthwise) | ceil( 20 ft ÷ 2.17 ft ) = 10 |
| Sheets up the slope | ceil( 32.5 ft ÷ 8 ft ) = 5 |
| Total, with wastage | 10 × 5 × (1 + 5%) = 53 sheets |
| Estimated cost | 53 × Rs 850 = Rs 45,050 |
A planning-level estimate for corrugated GI (CGI/jasta) roofing sheets. It assumes a simple single-slope or gable roof with straight runs; hips, valleys, dormers and complex roof shapes need extra cutting and are not modelled here. Effective cover width and sheet length vary by supplier and gauge, always confirm the exact sizes and price on the invoice before ordering.
From plan dimensions to a sheet order
The pitch turns a flat footprint into the true sloped surface a sheet has to cover, then the sheet's effective cover width and length turn that surface into a sheet count.
Sloped area
Sloped area = plan area ÷ cos(pitch), converting the pitch from degrees to radians before taking the cosine.
Sheet count
Sheets across = ceil(width ÷ effective cover width). Sheets up the slope = ceil(sloped length ÷ sheet length).
Wastage & cost
Total sheets × (1 + wastage %), rounded up, then × price per sheet for the estimated material cost.
Roofing sheets, answered
Why is the sloped roof area bigger than the plan (footprint) area?+
The plan area is the flat, bird's-eye footprint you'd measure on a site drawing. A pitched roof is tilted, so its actual surface, the part sheets have to cover, is longer than the flat footprint by a factor of 1 ÷ cos(pitch). A steeper pitch means a bigger gap between plan area and true sloped area: at 22.5° the sloped area is about 8% more than the plan area, at 30° about 15% more.
What does "effective cover width" mean?+
CGI/jasta sheets are manufactured wider than their usable coverage because adjoining sheets overlap by one or more corrugations along their long edge to keep the joint watertight. The effective cover width is the width one sheet actually contributes to the roof after that overlap, commonly around 26 inches for standard corrugated sheets, even though the sheet itself may be wider. Always check the effective width your supplier quotes, since profile and overlap vary.
What are typical CGI/jasta sheet sizes used in Nepal?+
Sheet length commonly comes in 6, 8, 10 and 12 ft lengths (custom lengths are also cut to order), with an effective cover width around 26 inches for standard corrugated profiles, though some profiles vary. Thickness (gauge) affects strength and price but not the area covered, choose gauge based on wind load and budget, and confirm exact dimensions with your supplier before ordering.
Why does the calculator add a wastage allowance?+
Real roofs need overlapping cuts at the ridge, eaves, hips, valleys and around any openings, and a few sheets are usually damaged in handling or cutting. A 5% wastage allowance (editable) covers this for a simple roof; more complex roof shapes with hips, valleys or multiple sections may need a higher allowance.
Should I use my roof's actual pitch or a preset?+
Use your actual design pitch when you know it, most Nepali residential roofs fall between 15° and 30°. The presets (Flat 0°, Low 15°, Standard 22.5°, Steep 30°) are common reference points; switch to Custom and enter the exact degrees from your drawing or a pitch gauge for an accurate sloped area.
Sources & data note
Based on standard CGI/jasta corrugated roofing sheet sizing and overlap conventions used in Nepali construction practice; this is informational, not a regulatory figure. Sheet dimensions, overlap and price vary by supplier and gauge, always confirm the exact sizes and current price before ordering.