To calculate roof covering, you need to determine the total surface area of your roof, taking into account both its flat dimensions and its slope. This calculation is essential for accurately estimating the amount of material required for a roofing project.
How to Calculate Roof Covering Area
The fundamental method for calculating the area of your roof involves determining the base footprint and then adjusting it for the roof's slope. Here's how it's done:
- Calculate the Base Area: First, multiply the length by the width of each individual roof plane (e.g., a single side of a gable roof, or one section of a hip roof). If you have a simple gable roof, you might initially multiply the total length of the house by its width to get the roof's footprint, but for accuracy, it's often better to consider each sloped plane.
- Apply the Pitch Multiplier: Then, take the product of these two dimensions (the base area) and multiply it by your pitch multiplier. The pitch multiplier is a factor that accounts for the increase in surface area due to the roof's slope. A steeper roof has a larger surface area than a flat roof with the same footprint.
- Sum for Total Area: The number you get for each plane, when summed together, will be an accurate estimate of how much area you have to cover for your roofing project.
This method provides the actual sloped surface area of your roof, which is what you need for material ordering.
Understanding Roof Pitch and Pitch Multipliers
Roof pitch describes the steepness of your roof. It's typically expressed as a ratio, such as "4/12," meaning the roof rises 4 inches vertically for every 12 inches it extends horizontally.
The pitch multiplier is a decimal value derived from the roof pitch that translates the flat footprint area into the actual sloped area.
Here's a table of common roof pitches and their corresponding multipliers:
| Roof Pitch (Rise/Run) | Pitch Multiplier |
|---|---|
| 1/12 | 1.003 |
| 2/12 | 1.014 |
| 3/12 | 1.031 |
| 4/12 | 1.054 |
| 5/12 | 1.083 |
| 6/12 | 1.118 |
| 7/12 | 1.158 |
| 8/12 | 1.202 |
| 9/12 | 1.250 |
| 10/12 | 1.302 |
| 11/12 | 1.357 |
| 12/12 | 1.414 |
For a deeper dive into roof pitch, explore resources like Understanding Roof Pitch or a comprehensive guide from the National Roofing Contractors Association (NRCA).
Step-by-Step Roof Covering Calculation
Follow these steps to accurately calculate your roof covering area:
- Measure Each Roof Plane:
- Measure the length and width (from eave to ridge, including overhangs) of each distinct sloped section of your roof. For complex roofs, you might need to divide the roof into simpler geometric shapes (rectangles, triangles).
- Determine the Roof Pitch:
- You can often find this information from original blueprints, or by measuring the rise and run directly from your attic or an exposed rafter. Place a level horizontally against a rafter and measure 12 inches. Then, measure the vertical distance (rise) from the end of the 12-inch mark down to the rafter.
- Find the Corresponding Pitch Multiplier:
- Use the table above or an online roof pitch calculator to find the multiplier for your specific pitch.
- Calculate Area for Each Plane:
- For each roof plane, multiply its measured length by its measured width to get its flat footprint area.
- Then, multiply this flat footprint area by the pitch multiplier to get the sloped surface area for that plane.
- Example: If a roof plane is 20 feet long by 15 feet wide (300 sq ft) and has a 6/12 pitch (multiplier 1.118), its sloped area is 300 sq ft * 1.118 = 335.4 sq ft.
- Sum All Plane Areas:
- Add up the sloped surface areas of all individual roof planes to get the total calculated roof area.
- Add for Waste Factor and Overhangs:
- Waste Factor: It's crucial to add a percentage for waste due to cutting, trimming, and potential errors. A typical waste factor for standard roofs is 10-15%, but for complex roofs with many dormers, valleys, or hips, it could be 20% or more.
- Overhangs: Ensure your initial measurements for length and width include any eaves and rake overhangs.
- Example: If your total calculated sloped area is 2000 sq ft, and you add a 10% waste factor, you'd need 2000 sq ft * 1.10 = 2200 sq ft of material.
Important Considerations for Material Ordering
- Roofing Squares: Roofing materials are often sold in "squares." One roofing square covers 100 square feet. So, if you need 2200 sq ft, you would order 22 squares.
- Complexity of Roof: Roofs with multiple dormers, hips, valleys, or skylights will require more intricate measurements and a higher waste factor due to increased cutting.
- Type of Material: Different roofing materials (asphalt shingles, metal panels, tiles) have different coverage rates and installation requirements. Always consult the manufacturer's specifications for the specific product you plan to use. You can research various roofing material types from manufacturers like GAF or Owens Corning.
By following these steps, you can arrive at an accurate estimate for your roof covering needs, ensuring you purchase sufficient materials without excessive waste.
