We use inches in this example.Īdditionally, we need to calculate the roof angle by plugging arctangent (6/12) into a calculator.įollowing these recalculations, we have the following: Let’s say you’re building a roof with the following dimensions:įirst, let’s convert all measurements to the same unit. Rafter overhang – The horizontal distance the rafter projects past the outside wall. Roof slope – The roof rise and run, expressed by a ratio that indicates roof steepness.Ĥ. Ridge boards are usually made of 2-by board, so 1.5 inches is typical.ģ. Ridge board thickness – The thickness of the board where the top ends of the opposite rafters meet. Roof span – Also called the building width or rafter span, this measurement refers to the distance between the outside opposite walls.Ģ. To determine roof rafter length, you need to know the following four measurements:ġ. It might trigger traumatic memories of high school math class, but the sheer bliss in knowing that you sized your rafters correctly is well worth it. To put you on par with the experts, we provide the formula carpenters use to figure out common rafter sizing and where to mark cuts.Īll you need is paper, a pencil, and a scientific calculator. Seasoned framers know how to calculate this crucial measurement no matter the project requirements thanks to millennia-old trigonometry. As such, while it can be cumbersome, measuring the area and pitch of each part of the roof and multiplying by the corresponding correction factor will result in the most accurate estimate of necessary roofing materials.Before building a gable roof or hip roof, you need to know your common rafter sizes. While it is possible to estimate the amount of necessary materials using only the total roof area measurement, as can be seen from the table, depending on how large the pitch of the roof, the actual area of the roof can differ by up to 2.236 from the measured total area at a pitch of 24/12. Given pitch and a horizontal area measurement, multiply the horizontal area by a correction factor corresponding to pitch, provided in the table below, to determine the actual area of the roof to be used in the Roofing Material Calculator. Depending on whether the roof area is measured horizontally (possibly from a drawing or photograph), a correction factor is necessary to determine the actual area of the roof. Roof pitch affects the actual area of the roof. It affects walkability as well as drainage, and roofs in areas of high rain or snowfall tend to have steeper pitches. Roof pitch is a determining factor for the cost of the roof, as well as the roof area, and the type of materials used. Outside of the U.S., a degree angle is typically used. For instance, a 7/12 roof pitch means that the roof rises 7 inches for every 12 horizontal inches. In the United States, a run of 12 inches (1 foot) is used, and pitch is measured as the rise of the roof over 12 inches. It is often compared to slope, but is not exactly the same. Roof pitch is the measurement of a roof's vertical rise divided by its horizontal run. Although ceramic tile roofs are expensive, they can have a life span of over 100 years. Shingle roofs typically have a life span of 15-30 years, while membrane roofs usually last 5-15 years. The most common roofing materials used in the United States include shingles, membrane roofing, and ceramic tile, all of which have different life spans. Using the aggregate area of these simple shapes can yield a more accurate roof area to be used with the Roofing Material Calculator. The Area Calculator can be used to calculate the area of a variety of simple shapes that together can comprise the area of the roof. The calculator cannot account for complex shapes based on a measurement of square footage alone. In cases where a roof has a complex shape, such as in the image to the right, measuring the dimensions and areas of each part of the roof to calculate the total area will result in a more accurate measurement of area. The calculated area is only an estimation.
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