How to measure and calculate how much roofing material you will need.
You will order roofing and underlayment by the "square" -- that is, 100 square feet. To estimate your needs, calculate the total square footage of your roof. For the most accurate measurement, you could actually go on the roof and measure each section, but that is often difficult and time-consuming.
More commonly, square footage is estimated by measuring from the ground and then making some simple calculations. (However, you may need to climb up on the roof to measure dormers or other protrusions.)
Once you have calculated your roof area, do not subtract for small obstructions such as chimneys or skylights. If you have dormers, climb onto the roof and measure them or make a rough estimate. Add up all the roofing sections; add 10 percent for waste.
Arranging for delivery
It's not a good idea to leave bundles of shingles sitting on the roof for days, so delay delivery until you are ready to roof. Watch the weather reports; you can't roof when it is raining. If the roofing will be delivered onto the roof, have it delivered after you have installed the flashings and underlayment.
If it will be delivered to your yard or driveway, have it arrive sooner. If you have to move bundles around, do so safely.
Sketch all the sections. Make rough outline drawings of all the roof's rectangular and triangular sections. These drawings do not have to be accurate. They may include the main roof, overhang sections, and dormers. Divide each section into simple squares, rectangles, and right triangles. For example, if a section is a rectangle with a triangular section at one end (like the roof over this entryway), draw a line to divide it into a rectangle and a right triangle.
Estimate the roof pitch by using a ruler and a combination square. Use the bubble in the combination square to level the straightedge. Move the ruler to the 12-inch mark on the straightedge. Then slide the ruler up until it intersects the roofline. The number of inches up to the roofline is the rise. For instance, if the ruler shows 6 inches, your roof has a 6 in 12 rise (a 6:12 slope).
Once you have measurements from the ground as well as the roof pitch, you can estimate the square footage of the roof. First figure the roof's total rise: Multiply the unit rise (which would be 6 if you have a 6:12 pitch) by half the house's total width to the eaves, which is the base of the triangle. If your house is 24 feet wide, the base of this triangle is 12 feet. Multiply 12 times 6 (the unit rise for a 6:12 pitch) to get 72; this is the total rise in inches, so the total rise is 6 feet.
To determine the rake distance (the distance from the bottom of the eave to the top of the ridge), add the square of the total rise (A) plus the square of the total run (B). If the total rise is 6 feet, square that to get 36; if the total run is 12 feet, square that to get 144. Add them to get 180. Use a calculator to find the square root of 180, which is 13.4 -- the length of the rake in feet.
Now that you know the length of the rake (C), you can start to figure your roof sections by simply multiplying length times width. For instance, if a section of this house is 20 feet long, multiply 13.4 times 20 to get 268 square feet for that section.