# How to Estimate Material for a Roofing Job

••• Jupiterimages/Photos.com/Getty Images
Share It

Spare yourself expense and headaches by getting an accurate estimate of roofing materials before you begin your project. This prevents over-purchasing expensive materials and will save you from last-minute trips back to the supply store to pick up materials you lack. Whether you are estimating for a do-it-yourself project or just getting started in your roofing business, you can create a fairly accurate estimate with just a few measurements and a little bit of math.

Measure the length of the roof at its base along one side of your house. Use a ladder, if necessary, to get the most accurate measurement.

Measure along the roof's angle to get the width of your first roof area. Multiply the length by the width to get the area in square feet.

Repeat the measurements for each section of roof you need to cover. If you have odd shapes such as triangles over dormer windows, you can find the area of a triangle by multiplying the base by the height, then dividing your answer by two.

Add all of your areas together for the total roof square footage.

Divide a piece of paper into six columns. Write the headings "Component," "Area," "Coverage" "Needs," "Price" and "Cost" in the headers of the columns. In the "Component" column, list the following items: "Underlayment," "Decking" and "Ridge."

Write the number of square feet you determined for your roof in the "Area" column after the first three items.

Add together all of your roof length (not width) measurements and write that figure in the "Area" column after "Ridge." This represents the materials that go along the peak of your roof.

Check how many square feet each roll of tar paper will cover if you are adding an underlayment of tar paper. If you are shingling directly over existing asphalt or composite shingles, you can cross off this column.

Write the tar paper's square foot coverage in the "Coverage" column on the "Underlayment" row. Write your roofing material's square foot coverage per package in the "Coverage" column after "Decking." Add the coverage for each package of ridge material in the "Ridge" row.

Divide each "Area" column by each "Coverage" column to get the amount of material you need for each type. For example, if your roof is 10,000 square feet and each case of shingles covers 100 square feet with the proper overlap, you will need 100 cases of shingles. Your "Needs" column now represents the estimated amount of materials you will need to complete your roof project.

Add the price per package of each item to the "Price" column in each row, and then multiply your "Needs" by the "Price" entries to get the total cost for each item.

Divide your total roof square footage by 100 to determine how many 100-square-foot sections your roof has. Multiply that number by 320 to get the number of nails or staples you need to secure the roofing material properly as it takes 320 nails, on average, to secure 100 square feet of shingles. For metal or concrete roofing, check with the manufacturer for fastener requirements, and then calculate these based on your square footage.