Calculating Roof Area

Gable Roof Area

To find the length of a rake on a gable roof, multiply its horizontal distance (the roof’s run) by the slope conversion factor in Slope, Hip/Valley Conversion, below, then multiply by the length of the ridge to find the area.

Rake Length = Run x Slope Factor 

Gable Roof Area = Rake Length x Ridge Length

Hip/Valley Length

To find the length of a hip or a valley, multiply its horizontal distance (run) by the hip/valley factor in Slope, Hip/Valley Conversion, below.

Hip or Valley Length = Run x Hip/Valley Factor

Figure: Slope, Hip/Valley Conversion
Slope (in. per ft.) Conversion Factor
Slope Hip/Valley
4:12 1.054 1.452
5:12 1.083 1.474
6:12 1.118 1.500
7:12 1.157 1.524
8:12 1.202 1.564
9:12 1.250 1.600
10:12 1.302 1.642
11:12 1.356 1.684
12:12 1.414 1.732
Multiply a roof’s run by the correct factor to get the length of its rake. To find the length of hips and valleys, multiply the correct factor by the horizontal distance of a hip or valley.

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