Calculate Roof Area

Installing a new roof is a capital intensive affair that requires you to establish your budget in advance. Thankfully, it is easy to estimate the costs by calculating the roof area and then using the sum to determine the cost of shingles per square foot.

How to calculate the roof area

1. Begin with calculating the area of your house in a plane parallel to the ground. If your house is rectangular, all you need to do is multiply the length and width of the building. If the shape of your house is more complex, simply enter the total area (after measuring the exterior dimensions) into the appropriate box.
2. Determine the roof pitch of your house. Roof pitch is the slope created by the rafters. If you don’t know how to calculate it, head to our roof pitch calculator. You can input it either in degrees, as a percentage or as a ratio of x:12.
3. Once you know these values, you will be able to calculate the roof area. Begin by converting the roof pitch to an angle expressed in degrees, using the following formulas:

pitch(%) = x / 12 * 100%

pitch(deg) = arctan [pitch(%)]

4. Then, use the following equation to find the total area of the roof:

roof area = base area / cos[pitch(deg)]

How to Estimate Roofing Materials

Although roofing projects are estimated and sold by the square, roofing material is often not sold in 1 square increments.

How Many Bundles of Shingles Will You Need

The average bundle of shingles covers 33.3 ft², so three bundles of shingles are needed per square. Asphalt shingles range in price from about $25 per bundle for a standard 3-tab style to around $50-$75 or more for an upgraded architectural style.

Since transportation is a major cost factor, expect prices to vary by location. A typical bundle of shingles weighs about 75 pounds, while some premium products can weigh over 100 pounds per bundle, so consider delivery of materials.

How Many Rolls of Felt Will You Need

Roofing felt is sold by the roll. The average roll of 15-pound roofing felt covers about 400 ft², or 4 squares, while the average roll of 30-pound roofing felt covers about 200 ft², or 2 squares.

Rolls of felt are 36″ wide x 144′ long for 15# and 72′ long for 30#. Consider other underlayment options as needed, such as rubber or tar products.

How Many Nails Will You Need

Nails will also be required for any roofing project. The average 3-tab shingle needs four nails per shingle. High wind areas and other types of shingles may need more. 320 nails will be needed to install a square of standard 3-tab shingles, assuming four nails per shingle and 80 shingles per square.

Remember, be wary of too-good-to-be-true estimates that promise cheap and fast results. Investing in name brand materials and quality service will save you time and money in the long run.

How to measure your roof area from the ground

A more accurate way to calculate the area of the roof than using Google Earth is to get outside to estimate the roof pitch and the base area of the property. Using these two figures, we can get a good idea of the shingle roof’s square footage and estimate your asphalt shingles’ needs and costs. This DIY technique can be helpful, especially if you are not comfortable getting on your roof, or if you have limited access to it.

Ground measures only work well for a gabled roof, since there are usually just two main rectangular pitches to calculate area for. For more complex roof shapes, like hip roofs, you’ll need to work with a roofer or measure from on top of the roof itself to calculate its total area.