With a framing square, pencil, and tape measure, mark outlines for the plumb cut, tail cut, heel cut, seat cut, and fascia cuts on your first rafter according to your rafter length measurements. Make sure all measurements read the same units, as in all inches or feet. To calculate the number of rafters you need, plug these measurements into this formula. Gable overhang – the distance the end rafters project past the gable wall. Typical rafter spacing ranges between 12 inches to 24 inches on center.ģ. Rafter spacing – how much space separates each rafter course on the center. Roof length – the distance from one gable-end to the other.Ģ. The following three measurements dictate the number of rafters you need:ġ. How Do I Figure Out How Many Rafters I Need? You can then plug the numbers into the Pythagorean theorem formula to solve for the hypotenuse, i.e., the length of the rafter. 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. Plug Your Measurements Into the Rafter Length Formula 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.
#Rafter length table how to
Seasoned framers know how to calculate this crucial measurement no matter the project requirements thanks to millennia-old trigonometry. You can calculate the rafter length with the measurements using pitch or using the angle, which are given above.Before building a gable roof or hip roof, you need to know your common rafter sizes. Given here are the formulae to calculate the length of a shed roof rafter to make your roofings stronger and look beautiful. While making roofing for your house with the roof rafters, it is important to know the rafter length to make the roof seat perfectly on the rafter. In home construction, rafters are normally made of wood.
#Rafter length table series
It is one of a series of sloped structural members that extend from the ridge or hip to the wall plate designed to support the roof deck and its associated loads. There could be many rafters supporting a roof. Rafter is a sloping beam (usually wood) that supports a pitched roof. This calculator helps you make the rafters of the suitable sizes required for your roofing. Use our online Rafter Length Calculator to quickly compute the length of the shed roof rafter by entering the measurements of overhead, length of roof span, pitch or angle.
Rafter Length using Pitch = V + (R/2)sqrt(1+P 2)
Rafter Length using Angle = V + R/(2cos(A))
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