Estimating the cost of a roof replacement is simply a matter of choosing the type of roofing material you want and knowing how much that material costs per square foot. Once you have that figure, all that's left is determining how many square feet of material it will take to cover your roof. Now multiply the two figures together and you have a good estimate of how much it is going to cost to replace your roof.
Choose the type of roofing material you need and divide the cost of the material by the number of square feet the material will cover. Roofing material packages will tell you how many square feet each package/bundle will cover. Once you have divided the cost of the material by the number of square feet it will cover, you will know how much it will cost per square foot to replace your roof.
Determine how many square feet your roof is. To do this you will have to see your roof as a combination of squares, rectangles, triangles and possibly parallelograms. Use your tape measure or your laser measuring device and measure the length of each side of each square, rectangle, triangle and parallelogram in feet. Write the figures down.
Multiply any two sides of each square area of your roof. The result will be the area of the square in square feet. Write down your answer for the square footage of each square. For example, a square that is 18 feet on each side would require 324 square feet of roofing material (18 feet x 18 feet = 324 square feet).
Multiply the length times the height of each rectangle to determine the number of square feet of roofing material you will need per rectangle. Write this figure down on the same paper where you noted the area of each square on your roof. As an example, a rectangle that is 23 feet long by 13 feet wide requires 299 square feet of roofing material (13 feet x 23 feet = 299 square feet).
Determine the area of each triangle on your roof. In some cases this can take a little extra work. The formula for determining the square footage of a triangle is one-half the base times the height. The base is any edge of the triangle. The height is the length of a line measured from the point of the triangle straight down to the edge that you have chosen as the base. The line you measure must meet the base at a 90-degree angle. Multiply the length of the base by the height of the triangle and then divide that answer by two. The result will be the area of the triangle in square feet. Write the answer down along with your other square footage answers.
Find the square footage of any parallelograms that might be part of your roof. To find the area of a parallelogram, multiply the length of the base by the height. The base is either of the two long sides. The height is a straight line from one of the long sides to the other long side, with the height line meeting the sides at a 90-degree angle. Write down the number of square feet in any parallelograms along with you other square footage answers.
Add together all of the square footage answers you have been keeping track of. The result will be the total square footage of your roof. Now simply multiply the total square footage of your roof by the cost of your roofing materials per square foot to determine the total cost of replacing your roof. For example, if your roof is 953 square feet and your roofing materials cost £2.70 per square foot, then the cost of replacing your roof would be £2,570.7 ($4.15 x 953).
Add any labour costs to the cost of materials from Step 7 to determine the full cost of replacing your roof.
Add at least a 7 per cent contingency to your estimate for things that you have missed, for wastage, and for incidentals.