Roof Measuring Made Easy

Measuring Roof Pitch

• The roof pitch is the measurement that tells you how much the roof slopes from the edge to the ridge, or the angle on the roof. For instance, a 4:12 roof pitch means the roof rises 4 inches for every 12 inch span. To measure the roof pitch, place a ladder against the front of your house and lay a 2-foot level so one end is on the edge of the roof and the other end is pointing toward the peak. Lift the end of the level that is at the roof edge until the bubble in the vial is in the middle. Measure the distance from the bottom of the lifted edge of the level to the surface of the roof. Calculate roof pitch by converting the measurements to a fraction and dividing by two. For instance, a measurement of 10 inches, from level to roof, becomes 10 inches over 24 inches or a roof pitch of 5:12.

Roof Sections

• Sketch the roof onto a piece of graph paper as though you were looking down from above the house. A scale of one square equals one foot works well when figuring roof measurements. Draw one section at a time and use basic shapes, such as squares, rectangles and triangles to represent each roof section. For instance, represent the two main sections of the roof with rectangles and then add triangles for the gable ends or dormers. Most of the measurements can be gathered without climbing a ladder. Use a 100 foot tape measure to measure the exterior walls of your home. This gives you the general length and width of your roof. You must add the overhang, usually 6 to 12 inches wide. For instance, if your house is 40 feet long and there is a 6-inch overhang on each side, then the total measurement for the length is 41 feet. If the width of the house is 20 feet, then the total measurement for width is 21 feet. The total area is 861 square feet.

Dormers and Porches

• Get the approximate measurements for a porch from the underside, running end to end and inside wall to the outer edge of the porch. Multiply the measurements together to calculate the total square footage for the porch. For dormers, use geometry to figure the measurements. Measure the length and width of the dormer and measure the length of the valley. Draw the dormer as a rectangle topped with a triangle. The formula for the area of a triangle is base times height divided by two, or B x H / 2 = A.

Calculating For Materials

• Roofing materials are sold in squares (10 square feet) or square footage. For instance, three bundles of shingles generally covers one square of roofing surface. Brand new roofs and replacement roofs require an underlay, a heavy tar paper sometimes referred to as felt paper. The underlay is generally sold in four squares per roll. You must account for pitch, waste and protrusions, such as the chimney or soil pipe, when calculating for roofing material. This is done by multiplying the square footage of an area by the slope factor. The standard slope factor for a 4:12 roof is 1.05. Once you have calculated the square footage with the slope factor, add the total square footage of the roof and divide the sum by 100. This will give you the size of your roof in squares. You can then calculate how much material you need for the job. For instance, if your total square footage is 2,564 then the number of squares is 25.64.