A pitched roof has more surface than its horizontal footprint. The Whatson Roof Area Calculator starts with that footprint, adds horizontal overhangs and adjusts the area for the entered pitch angle.
It supports simple flat, symmetric gable and single slope roofs. It does not break a complex roof into dormers, hips, valleys or separate intersecting planes.
Measure horizontally for this tool
Enter building length and width in the selected unit. The overhang is the horizontal projection beyond each edge, in that same unit. It is added on both sides of length and width.
Pitch is an angle in degrees, not a rise to run ratio. Do not enter “4” because a drawing says a 4:12 pitch without converting that ratio to an angle.
For a symmetric gable, the reported rise uses half the overhang adjusted width. For a single slope, it uses the full width. The calculator’s roof rise is a geometric result, not a height taken directly from your building drawing.
A 600 sq ft building with a larger roof
Enter length 30 ft, width 20 ft, horizontal overhang 1 ft at each edge and a symmetric gable pitch of 25 degrees.
The adjusted footprint is 32 × 22 ft, or 704 sq ft. Dividing by the cosine of 25 degrees produces a checked surface area of 776.78 sq ft. With 5% allowance, the covering quantity becomes 815.62 sq ft.
The reported roof rise is about 5.13 ft. A flat roof with the same adjusted footprint would have 704 sq ft of geometric surface before allowance.
What the surface estimate leaves out
No roof openings are deducted. No extra flashings, ridges, valleys or edge details are added. If those details matter to a material order, measure and price them separately.
A simple area can help compare covering quotes, but it cannot decide how sheets or tiles will fit on each plane. Use the Roof Sheet Calculator for rectangular sheet layouts after measuring the actual plane dimensions along the slope.
Read the price unit before entering it
The optional rate is PKR per sq ft of covering quantity. A quote that includes labour and accessories may have a different scope from a material only rate.
Keep the measured surface separate from the allowance figure. That lets you explain to a supplier why the roof geometry is 776.78 sq ft while your planned covering allowance is 815.62 sq ft.