CostBuildrRoofing Calculators
Roofing sheets have specific overlap requirements that make simple area division unreliable — every sheet loses usable width to side laps and effective length to end laps. This calculator accounts for those overlaps so you get an accurate sheet count the first time, avoiding the costly mistake of being one or two sheets short once your crew is already on the roof.
Common questions
How much overlap should I allow for corrugated roofing sheets?
Standard practice for corrugated galvanized iron (CGI) sheets is one-and-a-half corrugations of side overlap (about 75–100 mm) and 150–200 mm of end overlap where sheets meet along the slope. For roof pitches below 15°, increase the end overlap to 200–250 mm or use sealant tape to prevent wind-driven rain from entering. The effective covering width of a standard 1070 mm CGI sheet after side lap is approximately 920–940 mm.
What is the difference between CGI sheets and polycarbonate roofing sheets?
CGI (corrugated galvanized iron) sheets are opaque steel sheets coated with a zinc layer for corrosion resistance. They are strong, inexpensive, and widely used for main roof coverage. Polycarbonate sheets are translucent or transparent plastic panels that let in natural daylight — commonly used for skylights, carports, and covered walkways. Polycarbonate costs 2–4 × more per sheet but offers UV filtering and natural lighting. For structural roofing, CGI or pre-painted profiled steel is the go-to; for light-transmitting areas, polycarbonate fills the gap.
How do I calculate ridge cap quantity for a gable roof?
Measure the total ridge length (the horizontal line at the peak of the roof). Standard ridge cap pieces are 1.8 m or 2.4 m long with a 150 mm overlap at each joint. Divide the ridge length by the effective length of one cap (total length minus overlap), then round up. For a 10 m ridge using 2.4 m caps with 150 mm overlaps: effective length = 2.25 m, so 10 / 2.25 = 4.44 → order 5 ridge caps.
Does roof pitch affect the number of sheets I need?
Yes. A steeper pitch increases the slope length (the actual distance from eave to ridge) compared to the horizontal span. The slope length equals the horizontal span divided by cos(pitch angle). For example, a 6 m horizontal span at a 20° pitch has a slope length of 6 / cos(20°) ≈ 6.38 m — about 6 % more sheeting than a flat calculation would suggest. Steeper pitches also allow smaller end overlaps because water runs off faster, partially offsetting the extra length.