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Factors in Roofing Calculations

Before starting any roofing job, you must do some calculating. This includes how much roofing to obtain, what kind of roofing and what the local building requirements are for roofs. On the surface, this may seem easy. Once you actually start the calculation process, things can get tricky. You have to account for slope, overhangs and valleys on dormers. Furthermore, you have to calculate in the "goof factor," since odds are great slight mistakes will be made.

  1. Simple Rectangular Roofs

    • A simple rectangle roof is the easiest of all to calculate. This is the kind of roof found on a shed, with just one flat rectangular surface. You simply multiply the length by the width of the roof line, and this will give you the square footage. For a gable roof, measure the surface from the wall line to the peak. Multiply width times length, and multiply the result by two.

    Triangular Sections

    • Triangles in a roof get tricky to calculate. The formula for area of a triangle is one half of the base times the height. You have to look at your roof sections, and lay out the triangles formed by where the dormers meet the main roof. Iko Roofing recommends you draw your roof on a flat piece of paper with all the measurements you took, and calculate the roofing area. A qualified architect or experienced roofer can assist you with this.

    Overhangs

    • Overhangs are another tricky area to calculate. Your original measurements may have been from the wall, but an overhang adds significant area to the roof line. The University of Central Florida specifies how overhangs on passive solar homes are designed. You have to calculate the square footage of the overhang. Again, a qualified roofer can assist you with this.

    Goof Factor

    • No human being is perfect, and in the process of installing a roof, mistakes will be made. When a mistake is made, the shingle will have to be torn out, and a new shingle installed. Allow 5 percent extra for the goof factor so you don't run short on roofing. Ask the building supply company if they allow returns. It is better to buy too much and return the unused shingles than to run out.

    Square Feet to Bundles

    • All roofing shingles are sold by the "square." A square is 100 square ft, or an area that is 10 ft by 10 ft. Shingles are sold in bundles. A bundle is 33 1/3 square feet. To calculate how many bundles you need, first calculate out the total square feet of roofing needed. Divide this figure by 33 1/3. This will give you the number of bundles needed. Round up the leftover fraction to compensate for the goof factor.

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Remodeling Plans
  1. Roofing Structure
  2. Roof Area Slope Factors
  3. Building a Rooftop
  4. DIY Aluminum Roofing
  5. 4 V Metal Roof Materials
  6. Roof Diaphragm Calculations
  7. DIY Membrane Roof
  8. Gambrel Roof Calculations
  9. Hip Roof Calculations
  10. Do-It-Yourself Roofing
  11. Roofing 101
  12. Roofing Calculations

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