A sheet’s labelled size is not the area it adds after overlapping the previous sheet. The Whatson Roof Sheet Calculator accounts for that difference by counting rows and columns on each rectangular plane.
The first sheet covers its full entered dimension. Every following sheet extends the layout by its dimension minus the relevant overlap. That is why dividing total roof area by sheet area can understate the purchase.
Use slope length, not the horizontal run
Plane length is measured along the roof slope. Plane width is measured across it. Both follow the selected measurement unit.
Sheet length and width are always feet. End overlap and side overlap are inches. Keep the sheet’s effective width aligned with the across plane direction; rotating a product or ignoring its profile can produce the wrong layout.
Planes counts identical rectangles. Different roof planes need separate runs because one set of dimensions cannot describe all of them.
Two roof planes need 40 base sheets
For the checked layout, use a plane 15 ft long and 30 ft wide, two identical planes, 10 ft sheet length, 3.5 ft sheet width, 6 inch end overlap and 3 inch side overlap.
Along the slope, one sheet covers 10 ft and the next adds 9.5 ft, so two rows cover the required 15 ft. Across the plane, each extra sheet adds 3.25 ft after the first 3.5 ft sheet. Ten columns are needed.
That gives 20 sheets per plane and 40 base sheets overall. A 5% spare allowance rounds the checked order to 42 sheets.
The spare allowance does not replace overlap
Overlaps are already part of the row and column calculation. The allowance is added to the resulting sheet count. Increasing waste to “cover the overlaps” would count that effect twice.
The tool rejects overlaps as large as the associated sheet dimension. Smaller values still need to come from the product instructions and roof specification; a calculator accepting an input does not certify that overlap.
Details to price outside the sheet count
Ridge pieces, flashing, fixings, sealants, edge trims and labour are not counted. The layout does not optimise cut sheets, pool offcuts between planes or handle hips and valleys.
The optional rate is PKR per whole sheet and prices the 42 sheet order in this example. Check sheet length, profile and thickness against the quoted product before using that total.
If you only have a horizontal building footprint, start with the Roof Area Calculator and the drawing. Do not paste the horizontal run directly into this tool’s slope length field.