Calculate Roof Area
To calculate the total roofing area, you need to consider the footprint of the roof along with any additional areas that might need covering, such as overhangs or slopes.
Calculate the footprint area of the roof: This is the area covered by the main part of the roof. It's typically calculated by multiplying the length and width of the roof.
Footprint area = Roof length × Roof width
If there are overhangs, slopes, or other features that need roofing material, you'll need to add these areas to the footprint area
Here's an example calculation:
- Roof length: 30 feet
- Roof width: 20 feet
Footprint area = 30 × 20 = 600 square feet.
If the roof has an angle, slope, or pitch, it will typically increase the area that needs to be covered with roofing material. This increase in area is due to the additional surface area created by the incline of the roof.
Find the total roof area by using roof pitch
Imagine that the roof has a pitch of 4/12. This signifies that for every 12 horizontal inches, there is a rise of 4 inches vertically. In simpler terms, the roof slope ascends 4 inches for every 12 inches of horizontal run. Roof pitch tells you the steepness of the roof. We can use special multipliers based on the pitch to find the roof area more easily.
Check out the table below to see these multipliers and their corresponding angles.
| Pitch |
Angle |
Multiplier |
| 1/12 |
4.8° |
1.003 |
| 2/12 |
9.5° |
1.014 |
| 3/12 |
14.0° |
1.031 |
| 4/12 |
18.4° |
1.054 |
| 5/12 |
22.6° |
1.083 |
| 6/12 |
26.6° |
1.118 |
| 7/12 |
30.3° |
1.158 |
| 8/12 |
33.7° |
1.202 |
| 9/12 |
36.9° |
1.25 |
| 10/12 |
39.8° |
1.302 |
| 11/12 |
42.5° |
1.357 |
| 12/12 |
45.0° |
1.414 |
The corresponding multiplier of the ratio will be multiplied by the base area of the roof. In our example, the multiplier for a roof pitch 4/12 is 1.054. That means that the roof surface area is 1.054 * 600 = 632.456 square meters.
Calculate the number of plywood sheets needed
To calculate the required total plywood for covering an roof area with plywood sheets, you need to follow these steps:
Divide the total area to be covered by the area of one plywood sheet to find the number of sheets required:
Example:
Total Area = 600 sq ft
Plywood Length = 8 ft
Plywood Width = 4 ft
Number of plywood sheets = 18.75 sheets
Since you can't have a fraction of a plywood sheet, you'll need to round up to the nearest whole number. Therefore, you would need approximately 19 sheets of plywood to cover an area of 600 square feet.