How to Estimate Roofing Jobs - Pro Construction Guide
How to Estimate Roofing Jobs

How to Estimate Roofing Jobs

To estimate roofing jobs, follow these steps to quickly and accurately calculate roofing materials, often without getting on the roof.

Contractors often provide estimates and quotes that don’t result in actual work. The homeowner was either simply “doing research” or made decisions with a different quote. In the case of roofing jobs, this can amount to countless hours of calculation, driving back and forth to potential jobsites, and even climbing onto and down from roofs where you never actually do any work.

Finding roof slope with a level and tape measure.

Finding roof slope with a level and tape measure. In this photo, divide the measurement in half to determine slope, because the measurement is taken off the end of a 2-foot level.

You can accomplish to estimate roofing jobs quite easily from the ground, over the phone or via email, if the homeowner can supply you with a few simple dimensions and pictures. Complicated roofs still may require you to climb up and take exact measurements of features, such as valleys, sidewalls, chimneys and dormers, or to inspect for rot or damage. However, most of the times you can estimate roofing jobs from the ground using the following steps.

Measure the home and calculate roof slope

Draw a simple roof diagram, marking all pitch changes, valleys, ridge and hip lines, chimneys, vent pipes and other penetrations. Then measure all exterior wall sections from corner to corner, adding in any roof overhangs or bump outs. Put these dimensions in place on the diagram.

Next, calculate the roof slope, which is typically defined in inches. Some customers will know the roof slope and can supply it to you, but determining the slope yourself is easy. Unless there is a low-hanging roof eave, you may have to get on a ladder and work with a 1- or 2-foot level. Place one end of the level on the roof and level it horizontally. Then measure straight down from the end of the level in the air to the roof surface. The measurement in inches is the roof slope if you are using a 1-foot level. With a 2-foot level, divide that measurement in half to determine slope (14 inches from the level to the roof with a 2-foot level results in a 7-inch slope).

Calculate roof size

With the roof dimensions and slope, you now need to use a pitch multiplier to accurately calculate roof size. See the Conversion Factors table (below) to find the appropriate multiplier for various roof slopes.

Simple gable roofs like those found on colonial or ranch homes are easy to calculate. Simply take the ground dimensions, multiply length times width (include overhangs) and multiply by the roof pitch multiplier. For example, a 24- x 48-foot home with a 6:12 roof would calculate as follows: 24 x 48 = 1,152 x 1.12 = 1,290.24, or just over 1,290 square feet of roof.

For more complicated roofs with different sections, hips, valleys, dormers or other details, it is best to break up the roof into sections and calculate each one separately, and then add the sections together. Don’t subtract for skylights, chimneys or other large penetrations (unless there are a large number of them). Any amount they remove from the total square footage is usually accounted for in waste material resulting from working around them.

Calculate materials

Taking your total square footage and dividing it by 100 will give you the total number of “squares.” To partially account for waste, round up to the nearest whole number. So 1,290 square feet of roof ÷ 100 = 12.9 squares, rounded up to 13.

Rounding up only partially accounts for waste incurred when cutting around penetrations and at the end of a run. Most roofing materials and applications will have a waste factor of 5 percent to 10 percent, but simple roofs can be as low as 2 percent to 3 percent. Roofs with several valleys, dormers and sidewalls can bump up waste to 15 percent or more. Some materials, such as cedar shingles and clay tiles, carry even higher waste factors. If you aren’t sure how much waste to add in, consult your materials supplier or the manufacturer.

Also, don’t forget to account for material used on starter courses (total length of eaves) and cap material (total length of ridge/ridges).

In addition to final roofing material, you’ll need to calculate how much underlayment (including any self-adhering membranes needed on eaves, valleys, etc.) drip edge, ridge vent, flashing, vent boots, fasteners and other accessories you’ll need. This will vary widely depending on material/type used, as well as overlap/exposure required. Amounts required are often printed clearly on material packaging, but your supplier or the manufacturer can help you get an accurate estimate based on roof dimensions.

An accurate, profitable price estimate

Knowing a total amount of material and accessories will get you to a fairly close quote, but to refine that number and assure both you and the customer are happy at the end of the job, look for other factors.

  • Second layer tear offs can add $50 to $100 more per square
  • For complete or partial sheathing replacement, add $80 to $100 per square
  • Roofs with several valleys or dormers can incur a per-instance upcharge
  • Roofs that have several different sections often require additional charges of $100 or more per square.

Look for anything that will add time and expense to your job, such as replacing skylight flashing (which is often reused but may be damaged) or repairing damage from the previous roof’s failure, and account for that in your estimate.

Note: When calculating materials, a division by 100 is made only if the shingles cover 100 square feet. Some three-tab shingles cover more area, and some architectural shingles cover less.

Did you know?

“Slope” and “pitch” are often – incorrectly – used interchangeably when referring to roofs. Slope is defined as vertical rise (in inches) per foot of horizontal run. Pitch is the ratio of rise to total span. For example, a roof that rises 6 inches for every 12 inches of run is a “6-in-12,” or 6:12 roof, with a pitch of 1⁄4.

Conversion Factors

Slope Pitch

Multiplier

Hip/Valley factor
4 1.06 1.45
5 1.08 1.47
6 1.12 1.5
7 1.16 1.52
8 1.2 1.56
9 1.25 1.6
10 1.3 1.64
11 1.36 1.68
12 1.41 1.73

Rake length = run x pitch multiplier

Hip or valley length = run x hip/valley factor

Material Estimation

________ Length of roof (including overhangs)

x ________ Width of roof (including overhangs)

= ________ Square footage of your roof

(without slope)

x ________ Pitch multiplier (see chart)

= ________Total square footage of roof

+ ________ Waste factor

= ________Total

÷ 100

= ________Total number of squares needed



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