Roofing Calculator for Sloped Roof

A roofing calculator is used to estimate the amount of material needed to build the roof. Or to make it waterproof using shingles, steel roofing, or other products such as roll roofing, tile roofing, etc.

Measuring the Roof’s Square Footage

Determine Roof Pitch

The only sure way to get an accurate roofing estimate is to have a professional quote it.

But it is fairly simple to work out an approximate size and price. Having a fair idea of what to expect reduces sticker shock.

If re-roofing is going to be a DIY project, knowing the amount of roofing required eliminates unnecessary extra trips to buy and/or return material. This becomes especially important when ordering metal roofing, which is often ordered in custom lengths. Many tile and rubber roofing products are custom ordered also. Running out before the job is finished may mean long waiting periods and temporary waterproofing.

Start With the House Footprint

The house footprint is the number of square feet of area a house takes up on the lot. Not the number of square feet in the house. One roof usually covers the basement, main floor, and second story. Another roof covers the garage–unless it has part of the house constructed above.

Measure the length and width of the house. Measure the length and width of any bump-outs such as porches, closets, or bay windows. Measure the length and width of the garage. Include any sheds that are going to be re-roofed.

Multiply the length by the width of each section. Add all of the results to get the total building footprint. For example:

  • House. 24 x 40 = 960 square feet
  • Master Bedroom Extension. 12 x 12 = 144 square feet
  • Porch. 16 x 6 = 96 square feet
  • Detached Garage. 24 x 24 = 576 square feet
  • Total Building Foot Print. 1676 square feet

For more accuracy, add the length of roof overhangs to the wall length and width measurements. For instance, if the garage overhang is 24”, the measurement of this garage roof print is 28 x 28 = 784 square feet.

Determine Roof Pitch

Roof pitch is the slope of the roof–determined by dividing the vertical rise by the horizontal run.

Knowing the roof pitch is essential in obtaining an accurate material count of a roof. It is also necessary to determine roof walkability, drainage, and snow loads. Houses in heavy rain or snow areas usually have steeper roofs.

Slope Correction

Use a roof pitch calculator to work out the roof pitch–or slope unless you are certain of the roof pitch. The following chart provides pitch correction factors. Multiplying the total footprint by the appropriate factor will provide the most accurate area of the roof.

Typical Slope Correction Factors:

Roof PitchAngleRise/RunRoof Pitch Factor

This method provides very accurate measurements, but it can be cumbersome if there are multiple roof pitches involved. For instance:

  • Main House. 12/12 pitch – multiply the number of square feet by 1.414
  • Patio Cover. 1/12 pitch – multiply the number of square feet by 1.003
  • Garage. 4/12 pitch – multiply the number of square feet by 1.054

Each individual section must be multiplied by the correct factor from the chart–then added together for a final total.

Add For Roof Complexity

The amount of material needed and the cost of a re-roof is also affected by:

  • Height. The higher the house, the more difficult it is to get men, materials, and equipment onto the roof.
  • Pitch. The steeper the pitch, the more difficult the working conditions. Most jurisdictions require safety harnesses and even safety railings on very steep-pitched roofs. Production is much slower.
  • Complexity. Any roof with non-ordinary complexities–such as roofs terminating into walls, dormers, double gables, multiple pitches, etc. add to costs. The job goes slower and more material is wasted to achieve proper waterproof installation.
  • Material. Most types of roofing materials are heavy. Some, like metal, are heavy and flimsy. Clay tiles are heavy and brittle. One bundle of asphalt or fiberglass shingles is 100 lbs. of dead weight.

Very complex roofs are best left to professional roofers to estimate and to re-roof. Not only can they be dangerous to work on, but proper flashing, underlayment, and waterproofing are subject to the International Residential Code (IRC) to prevent leaking.

Average Roofing Costs

In the US, the average home size is around 2000 square feet. Here are some of the average costs of new roofs–supplied, installed, and warranted. The scope of work should include:

  • All required materials–roofing material, underlayment, flashings, roof edge, and fasteners.
  • Professional installation.
  • Removal of existing roofing materials.
  • Disposal costs.
  • Permits.
  • Material and labor warranties.

Some of the more popular roofing materials include:

  • Asphalt/Fiberglass Roofing $3.50 – $6.70 per square foot
  • Metal Roofing $5.00 – $8.00 per square foot
  • Clay/Concrete Roofing $9.00 – $21.00 per square foot
  • Slate Roofing $6.50 – $10.65 per square foot
  • Tile Roofing $10.25 – $17.50 per square foot

These prices are based on the size of the covered area of the roof plus waste. Complexity, height, pitch, and quality of materials all contribute to the variation in prices.