How to do roof estimates quickly and accurately
Contractors are used to providing roof estimates and quotes that don’t result in actual work – the homeowner was either simply “doing research” or they went with a less expensive quote.
In the case of roof estimates, this can amount to countless hours of calculation, driving back and forth to potential jobsites and even climbing up and down roofs that you never work on.
Accurate roof estimates can be accomplished quite simply from the ground, or even over the phone or via e-mail if the homeowner can supply you with a few simple dimensions and pictures.
Complicated roofs may still require you to climb up and take exact measurements of things like valleys, sidewalls, chimneys, and dormers, or to inspect for rot or damage, but most roofs can be easily estimated from the ground using the following steps.
Start roof estimates with this
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 their 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 slope).
Note: “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 “six-in-twelve,” or 6:12 roof, with a pitch of ¼.
Calculate roof size
With the roof dimensions and slope, you now need to use a pitch multiplier to accurately calculate roof size. Use 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 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, 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.
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 very simple roofs can be as low as 2 percent to 3 percent while 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 any 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.
Arrive at accurate, profitable roof estimates
Knowing the total amount of material and accessories will get you to a pretty close quote, but to refine that number and make sure both you and the customer are happy at the end of the job you can 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 up-charge
- 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.
Conversion factors for roof estimates
|Slope||Pitch Multiplier||Hip/Valley factor|
Hip or valley length = run x hip/valley factorRake length = run x pitch multiplier
__________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 number of squares needed