How to Estimate My Re-Roof

Inspect the roof to determine how the sections will be divided up. Sections should be in rectangles and triangles.

Draw a rough sketch of the roof that shows every eave, valley, rake, hip and ridge. Sketch out every rectangle and triangle formed on the roof and number each section.

Measure the length and the width on simple, rectangular shaped areas using a 100-foot tape measure. Multiply the length by the width to get the square footage. Write the total down on the sketch.

Measure the length of the eave or ridge in sections shaped like a triangle. Measure the perpendicular line from the peak to the eave or ridge. Multiply the length by the perpendicular line. Divide the total by two to get the number of square feet in this section. Write the total down on the sketch.

Divide up sections that have a rectangular eave or ridge with a hip or valley on the end to form a triangle. Measure the length of the rectangular section. Start from the beginning of the rectangle and measure to the point where the triangular begins. This is the length of the eave or ridge. Write the measurement down. Measure the length of the eave or ridge which forms the triangular end. Begin the measurement where the measurement for the rectangle ended and the triangle begins. Measure out to the end of the the hip or valley which forms the point of the triangle. Measure the perpendicular line from top to bottom which forms the hip or valley. Multiply the length of the triangle by the perpendicular line. Divide by two. To obtain the total square footage of this type of area multiply the measurement for the length of the first rectangular section by the length of the perpendicular line. Write this total down. Multiply the length of the triangle by the length of the perpendicular line and divide by two. Write this total down. Add these two totals together to get the total square footage of this section.

Repeat this process until you have covered every area on the rough sketch. Double check your sketch to ensure that every area has been measured.

Measure the length of the ridge line of the roof using a tape measure. Three-tab shingles are cut into 3 pieces at the cut-out to make ridge cap shingles. Approximately 35 linear feet of ridge line can be capped with a bundle of three-tab shingles. Three-tab shingles are sold three bundles to the square. Divide the total length of the ridge line by 35 to get the number of bundles of regular shingles needed to cap the ridge line. Write the number down on the sketch.

Add the sum of all the measurements. Divide the total number of square feet by 100 to get the total number of squares of the roof. For example 5000 square feet divided by 100 equals 50 roofing squares.The total number of shingle bundles needed is determined by multiplying the number of squares by the number of shingle bundles in a square. Add 5 per cent to this figure to cover starter shingles and waste.