To determine roof rise, make a proportion between the roof slope and the rafter run (roof span + thickness of the ridge board ÷ 2) and solve for the missing variable. This approach involves more steps as you first need to calculate the roof rise – the distance from the center of the span to the top of the ridge. Using the Pythagorean TheoremĪlternatively, you can use the Pythagorean theorem to determine rafter length. To make ample room for cuts, buy lumber at least one foot longer than the total rafter length. Our rafters should measure 12’ 2.75” long. To see our rafter size in feet, we need to divide our inches figure by 12. We can now send these numbers through a calculator following the rafter length calculator formula: 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. You might also be interested in our roofing calculator to find how many squares of shingles you’ll need to finish it off.Before building a gable roof or hip roof, you need to know your common rafter sizes. Remember that things like allowance, lumber size, and roof type can play into the overall length of each rafter. Meaning that your rafter length will equal: The rafter length is equal to the run divided by the cosine of the roof’s angle.Īnother method is to use the rise of the roof using this formula: Since the cosine of an angle in a right triangle is equal to the length of the adjacent side divided by the hypotenuse, we can derive the following formula to calculate the rafter length: Now that you know the run and the angle in degrees, you can use trigonometry to calculate the rafter length. Run = (total width ÷ 2) – (beam width ÷ 2) Step Four: Calculate the Rafter Length Then, account for the beam or ridge board by subtracting half of its width from the run. To calculate the run, divide the total width of the building in half. The rafter’s run is the horizontal distance between the end of the rafter and where it meets the beam, which is known as the rise. Measure the span by measuring the width of the building using a tape measure, then add the overhang on each side to the measurement. You’ll also need to account for the overhang beyond the building on either side. The roof’s span is the total length of the roof. You can also use our rise over run calculator to find this. The run is half the measure of the roof span. You can use the following formula to get the rise: The rise is also the term frequently used for the vertical beam that the rafters will butt against. If you have this figure, you can use it plus the run, to get the rafter length. Pitch can be used to help calculate the rise. Table showing the angle in degrees for various roof pitches. You can also find the angle for your roof pitch in the table below. The angle in degrees is equal to the inverse tangent of the pitch of the roof. But, in order to calculate the rafter length, we need to calculate the pitch in degrees. The roof pitch is the angle of the roof and can be measured in several ways, but is most commonly expressed in rise over a standard 12-inch run.
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