How to Calculate Materials for a 20X14 Garage Roof

Accurately estimating the materials needed for a 20-foot-by-14-foot garage roof is critical to successfully completing your project. Ordering too much ties up money and wastes time storing and returning unused materials. Ordering too little increases labor costs as workers sit around waiting for the materials needed to finish the job. Calculating your material needs requires an understanding of basic mathematics.

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Instructions

- 1
  Determine which dimension is the width and which is the length. This is important because it will determine the length of your rafters and ridge beam, or, if you decide to use them, your trusses.
- 2
  Divide the length of the building by two to determine your rafter or truss needs. For example, if the building is 20 feet long, you'll need 10 pairs of rafters -- one rafter for each side of the roof -- and one 20-foot ridge beam or 10 trusses. You'll also need approximately 50 feet of lumber to brace the rafters or trusses until you're finished framing.
- 3
  Determine the pitch you want for the roof. Roof pitch is expressed as a fraction with in which the numerator is the vertical distance and the denominator is the horizontal distance -- rise over run. The denominator is always 12. To illustrate: on a 4/12 roof, which is a relatively flat roof, the roof goes up 4 inches for every 12 inches across.
- 4
  Multiply the area of the roof, 280 square feet, or 14 times 20, by the appropriate roofing multiplier for the pitch of your roof to determine the surface area of your roof. Pitches less than 4/12 use a multiplier between 1.15 and 1.25; pitches between 5/12 and 9/12 use a multiplier between 1.25 and 1.4; pitches greater than 9/12 use a multiplier between 1.4 and 1.7.
- 5
  Divide the surface area of the roof by 100 to determine how many squares of decking and how many squares of shingles and roofing felt you'll need to cover your roof. A square is a 10-foot-by-10-foot area.
- 6
  Add one box of framing nails and one box of framing nails to complete your materials' list.

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