Multi-Section Roof Area Calculator
Last reviewed How we calculate
Few houses have one simple roof. A garage, a porch or a back extension often has its own pitch, and working the whole roof out as one rectangle gets the order wrong. This adds up to four sections, each at its own pitch.
Roof area for a roof with several parts
Add up a roof with several parts at different pitches, such as a house, garage and porch, into one order.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Squares to order
- Section 1, roof area
- Section 2, roof area
- Section 3, roof area
- Section 4, roof area
- Total roof area
- Total including waste
Split the roof wherever the pitch changes or another part joins on: the main house, a garage, a porch, a dormer. Measure each footprint separately at ground level, and where one part overlaps another, count the overlap once only.
How the calculation works
Each section’s footprint is multiplied by the pitch multiplier for its own pitch, which turns the ground area into the sloped roof area. The sections are added together, a waste allowance goes on top, and the total is given in roofing squares, the unit roofing is ordered in.
A worked example
A house with a 1,000 sq ft footprint at 6 in 12 has 1,118 sq ft of roof. An attached garage with a 300 sq ft footprint at 4 in 12 adds 316 sq ft. Together that is 1,434 sq ft, and with 10 per cent waste the order is 15.78 squares. Working the whole 1,300 sq ft footprint at 6 in 12 would have overstated the garage.
Splitting a roof into sections
Draw the roof from above and break it wherever the pitch changes or another part joins on. Measure each footprint at ground level, including its overhang. Where two parts overlap, such as a porch roof tucked under the main eaves, count the overlap once only. Hip, valley and dormer cuts are what the waste allowance is for.
Frequently asked questions
When do I need the multi-section calculator?
Whenever parts of the roof have different pitches, such as a main house at 6 in 12 and a garage or porch at 4 in 12. Working them out as one roof at one pitch gets the order wrong.
How do I deal with parts of the roof that overlap?
Count the overlapping area once only. Measure each footprint at ground level, and where a porch roof tucks under the main eaves, leave the tucked-in part out of one of the two sections.
How do I include a dormer?
Treat the dormer roof as its own section, with the footprint it covers and its own pitch. The main roof area it sits on is usually still covered with underlayment, so leave the main section as it is.