The actual length of the wood will be longer because it has to sit on something to support itself. “Span” refers to the distance between the inside of the two bearing points of a piece of horizontal lumber. It is important to know the definition of the term “span” when talking about lumber and framing. A single 2×4 is never used for a girder because it cannot bear the loads required when framing a structure. In a span table, you can only find information for a doubled 2×4 beam – also called a girder. Other commonly available species of dimensional lumber span slightly less distance, such as SPF, which can only span 6’ 1”. How Far Can You Span a 2×4 Ceiling Joist?Ī 2×4 floor joist can span 6’ 7” when spaced at 16” and using Southern Pine.How Far Can a 2×4 Span Without Support?.
Rafter length table how to#
Read this roof framing article for step-by-step, DIY instructions on how to outline and cut rafters. With your cuts drawn out, you can then rev up your circular saw and put your carpentry skills to work. 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. Seasoned framers know how to calculate this crucial measurement no matter the project requirements thanks to millennia-old trigonometry. Before building a gable roof or hip roof, you need to know your common rafter sizes.
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