More than 90 percent of roofers underbid for the job, according to "Roofing Contractor" magazine. The problem, it says, is that many contractors price a job per roofing square -- industry terminology for a 100-square-foot block -- without taking into account the vagaries of the job. Hips, valleys, roof height and other details expand the time necessary to complete the job and distort the per-square average rate. By relating overhead costs to time, rather than squares, you can accurately price a job and win bids that make you money.

Measure the Roof

Measure the exterior dimensions of the house in square feet. Divide the measurement by 100 to give you the number of ground level squares. Next, consider the roof pitch. For low roof pitches -- those with a 5:12 ratio or less, which means the roof rises 5 feet for every 12 feet of horizontal length -- multiply your ground level squares by 1.15 to 1.25. For medium-pitch roofs in the 6:12 to 9:12 slope range, multiple your squares by 1.24 to 1.4. For high-pitched slopes, multiply your squares by 1.4 to 1.7. This gives you the number of squares you will install.

Survey the Roof

Examine the roof and note the number and condition of the eaves, ridges, vent stacks, valleys and flashing where the roof deck meets chimneys or walls. Roofs that require a complete tear off and cleanup of existing materials will require more materials and take more time to complete than jobs in which materials can be reused. Check building codes before you begin. Many codes restrict the number and type of materials, such as asphalt shingles, that you can overlay and reuse.

Calculate Material and Labor Costs

Contact your supplier for current materials pricing. Add up the cost of all the materials you need. Include other equipment, such as a dumpster, and add sales tax and delivery costs. Next, add labor costs. If you pay your contractors by the square, multiply the number of squares by the pay rate. If you pay by the hour or day, estimate the number of hours or days the job will take and multiply this by the pay rate. Use your experience to determine the factors that make the job difficult and, therefore, more time consuming, such as steep pitches and roof height. Add the labor cost to the cost of materials.

Add the Extras

Include overheads in your pricing. For example, include workers' compensation insurance based on your expected payroll and workers' comp rate. Include a portion of your general overheads, such as office rent and utilities. Finally, add in your markup. Either multiply your total costs by the percentage markup you require, or add in a dollar amount of profit. Divide the final bid price by the number of squares the roof has to give you a per-square bid rate.

Write Your Bid

Write a letter to the client that includes your bid estimate. Your aim is to convince the customer that you are the best roofer for the job. List the industry-proven materials you will use and show that they are high quality. Include photos of any problems with the roof and explain in detail how you will fix them. Mention any warranties you offer and include details of your state contractor certification and liability insurance. Testimonials are a nice touch, as they prove that you are capable of finishing a job on time and on budget. Finally, mention your start and completion dates.