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NOTE: This works for hip and gable roofs.
Calculating Roof Area from Plans
- Add the overhang to dimensions.
- Multiply Length by Width.
- Multiply the area by the rafter multiplier that corresponds with the roof slope of your plans.
- Divide by 100 to calculate the number of squares.
Calculating Common Rafters on Hip / Valley
- Measure the run (Flat dimension).
- Identify the multiplier that corresponds with the roofs slope.
- Multiply the run by the multiplier.
- Round to the next "even" number.
RAFTERS
Example: What is Square Feet in the Gable Area of a 50' wide building with a 12' Gable Height?
- GA = Gable Height x Run ÷ 2
- GA = 12 x 25 ÷ 2
- GA = 150 SF
Example: What is the run of a building that has a 60' run? The Slope is 6:12
- Run = 60 ÷ 2
- Run = 30
- GH = Run x Vertical Rise
- GH = 30 x 6
- GH = 180" (or 15')
Example: What is the run of a building that has a 50' span?
- Run = Span ÷ 2
- Run = 50 ÷ 2
- Run = 25
Calculating the Gable Area of a Building
GA = Gable Height x Run
Calculating the Gable Height of a Building
GH = Run x Vertical Rise
Calculating the Run of a Building
Run = Span ÷ 2
Gable Calculations
Gable Height
Coverage Area per Roll of Felt (Underlayment)
- 15# Felt Underlayment 400 SF
- 30# Felt Underlayment 200 SF
Waste Factors
- Simple roofs (Gable) ---1 - 5%
- Complex roofs -----------5 - 10%
- Measure Height from the bottom of the roofline to the ridge board.
- Measure the Building Length ( include the overhang )
- Multiply Length by Height for the area.
- Divide total area by 100 ( this is the number of squares required )
- Multiply the squares by the number of bundles per square ( or see the table Below, or manufactures instructions for quantity )
Calculating Bundles of Roof Covering