The method
Before using the result
- Single uniform slope, or combined plan area of surfaces with the same pitch. Valleys, dormers, overlaps and wastage are excluded.
Worked examples
These original training examples illustrate the method for InterNACHI Certified Professional Inspector (CPI). Use the matching “Load example” button above to explore a variation; example values are not equipment settings or prescribed criteria.
1. Worked example
Relate a visible rise/run measurement to roof angle and a simple area multiplier. Explain which complex roof features the simple estimate omits.
- Roof rise
- 1 m
- Horizontal run
- 2 m
- Horizontal plan area (optional)
- 100 m²
- Pitch = 1 / 2 = 0.5.
- Area factor = √(1 + 0.5²) = 1.118033988749895.
- Rise per 12 run: 6
- Angle: 0.4636476 rad
- Slope length: 2.236068 m
- Area multiplier: 1.118034
- Simple roof area: 111.8034 m²
2. Comparison example
Compare a different supplied condition using the same method. For Home Inspect Cert, the important check is that the input meaning, units and method assumptions still match the problem.
- Roof rise
- 1.5 m
- Horizontal run
- 2 m
- Horizontal plan area (optional)
- 100 m²
- Pitch = 1.5 / 2 = 0.75.
- Area factor = √(1 + 0.75²) = 1.25.
- Rise per 12 run: 9
- Angle: 0.6435011 rad
- Slope length: 2.5 m
- Area multiplier: 1.25
- Simple roof area: 125 m²
Check your understanding
1. Which relationship is used?
Pitch=rise/run; slope length=hypot(rise,run); simple roof area=plan area × √(1+pitch²).
2. Which assumption should you check first?
Single uniform slope, or combined plan area of surfaces with the same pitch. Valleys, dormers, overlaps and wastage are excluded.
3. How should you verify a different unit system?
Convert every input to the displayed calculation units first. Equivalent physical inputs must give equivalent results; a unit label alone does not convert a number.
Continue with the matching exam topic
This tool connects to InterNACHI Certified Professional Inspector (CPI) free practice. Work through a question independently, then use the calculation steps to identify where your method differs. The tool stays free whether or not you choose paid preparation.