Roofing Construction & Estimating
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Introduction | Table of Contents | Back Cover

1. Measuring and Calculating Roofs

If you're like some roofing contractors, you estimate roofing quantities by calculating the area of a roof, then adding 10 percent for waste. That might be OK in a fat building market, but in a tight market you'll need a sharper pencil to compete successfully for the good jobs, and then make money on them. In this book, I'm going to show you how to make a quick and accurate takeoff for any kind of roof.

You'll also learn the latest and most acceptable roofing methods in an industry where installation practices are closely related to warranties. That's because material warranties may be invalid if you don't follow the manufacturer's recommendations for installation. Look here for general guidelines, but always follow the manufacturer's instructions to the letter.

New products come on the market every day to solve the complex roof covering requirements presented by modern building technology. Your job is to know as much as you can about those products. You also have to know how to install them so the job passes inspection and presents no future repair and maintenance problems. Callbacks are hard on your profit margin and they don't do your reputation any good either. Know as much as you can about your roofing business, and you'll avoid them.

This book is more than an estimating book for roofing contractors. It develops a system, beginning with Chapter 1, for all types of roofing materials and installation methods. We'll cover the entire roofing trade, including how to manage your crews and keep them safe. So let's get started.

Before you can bid any job, you have to figure your costs. And before you can figure the costs, you have to know the size of the job. So you have to do two things: First, measure the roof and calculate the total area. Then find the lengths of the eaves, gables (or rakes), ridges, hips and valleys.

When you construct a roof on a new building, you can get these measurements from the plans. On repair or replacement jobs, you'll probably have to take your pencil, clipboard and tape measure, haul out your ladder, climb onto the roof, and start measuring. 

To avoid mistakes, or a second trip to the job site, develop a system for taking measurements. Use a 100-foot flexible tape which has a 1/2-inch grout hook at the "stupid" end of the tape. Flexible tapes are made of metal, or fiberglass-reinforced nylon fabric. Find a tape that's marked with highlights at 5-inch intervals to match the exposure of most composition shingles.

There is no cardinal rule for the sequence you use to measure a roof, as long as you don't miss anything. Here's a system that works for me:

Start by measuring the length of the eaves. On a gable roof, you only have to measure in one direction. On a hip roof, you'll have to measure the eaves in two directions.

Next, measure the width of the roof. On a gable roof, hook the tape over one of the eaves, and run it over the ridge to the opposite eave. On a hip roof, measure the width the same way. To measure the length, hook the tape to the eaves at the ridge rafter (look ahead to Figure 1-16 on page 13 for an illustration of the parts of a roof), run the tape the length of the ridge and down the opposite ridge rafter. Measure the ridge at the same time.

Now, measure the hips and valleys by hooking the tape to a building comer and running the tape to the ridge. You use these measurements to calculate material requirements such as valley flashing and hip-covering material.

When you measure, some dimensions need to be more accurate than others. For instance, you could miss the length of ridge, hip or valley by a foot or more, and the error wouldn't affect your total bid price too much. But don't make a mistake in the length and width, because that error could be substantial. For example, assume you measure a roof at 100 feet by 200 feet, while the actual measurements are 100'6" by 200'6". The difference between the two measurements is 150 square feet, or 11/2 squares of material.

Always make a sketch of the roof layout, including dimensions, roof slopes, location of penetrations and any unusual circumstances such as rotten deck areas, ventilation problems, or overhanging tree branches or other obstructions.

Once you have the measurements, you'll use them to calculate areas, slopes, angles, and allowance factors. Lets begin with an easy example.

Level Roofs

The dimensions on the plans give you the actual measurements for a level roof. To get the area of a rectangular roof, multiply its length by its width.

Area of a level rectangular roof = L x W

where L is the length and W is the width.

Of course, not every roof you work on will be a single rectangle. You may need to figure the area of a roof like the one in Figure 1-1. There are two ways to calculate this area:

1. The positive method

2. The negative method

In the positive method, you divide the roof into rectangular areas, then add the parts to get the total area. See Figure 1-2.

With the negative method, you extend the roof lines to form a single rectangle. Calculate the area of this rectangle, and subtract the areas of the rectangular spaces which lie outside the actual roof. Figure 1-3 illustrates this.

Left: Figure 1-1 Roof Plan of Level Roof
Right: Figure 1-2 The Positive Method

Example 1-1: The Positive Method

Divide the roof into rectangles as shown in Figure 1-2. Calculate the area of each rectangle, then add them together:

• Area A = 20 feet by 60 feet, or 1,200 square feet

• Area B = 20 feet by 40 feet, or 800 square feet

• Area C = 20 feet by 20 feet, or 400 square feet

Then, the total area = 

• 1,200 SF + 800 SF + 400 SF, or 2,400 SF

Figure 1-3 The Negative Method

Example 1-2: The Negative Method

Extend the roof lines to form one rectangle, as in Figure 1-3. Calculate the total area of that rectangle, then subtract the areas of any rectangles which aren't in the actual roof:

Extended rectangle = 60' x 60' = 3,600 SF

• Area A = 40 feet by 20 feet, or 800 square feet

• Area B = 20 feet by 20 feet, or 400 square feet

Total area outside the roof layout is 800 SF + 400 SF, or 1,200 SF. Subtract that from the extended area to get the total area:

• 3,600 SF - 1,200 SF = 2,400 SF

You get the same answer both ways. So you might as well use the easiest method - the one that requires the fewest calculations. For example, in Figure 1-4 you'd have to calculate three areas, then add them together. But in Figure 1-5 you only have to calculate two areas, and then subtract one from the other

Introduction | Table of Contents | Back Cover